Transistor Application Cookbook - Jan Zumwalt

7/24/2019 Transistor Application Cookbook - Jan Zumwalt

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Ver - November 14, 2015

There are two types of basic transistor out there: bi-polar junction(BJT) and metal-oxide field-effect (MOSFET). In this tutorial wellfocus on the BJT, because its slightly easier to understand. Diggingeven deeper into transistor types, there are actually two versions of

the BJT: NPNand PNP. Well turn our focus even sharper by limitingour early discussion to the NPN. By narrowing our focus down getting a solid understanding of the NPN itll be easier tounderstand the PNP (or MOSFETS, even) by comparing how it differsfrom the NPN...


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Transistor Data Sheet


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Contents

Transistor Data Sheet .................................. 2

INTRODUCTION ........................................ 5

Symbols, Pins, and Construction ................... 5

NPN: Not Pointing iN ................................... 5

Transistor Construction .................................... 6

Extending the Water Analogy ....................... 7

1) On Short Circuit ........................................ 8

2) Off Open Circuit ....................................... 8

3) Linear Flow Control...................................... 8

Amplifying Power ............................................. 9

Operation Modes......................................... 9

Relating to the PNP ....................................... 13

Applications I: Switches .............................. 14

Transistor Switch ........................................... 14

Base Resistors! .............................................. 16

Digital Logic .................................................. 16Inverter ........................................................ 17

AND Gate ...................................................... 17

OR Gate ........................................................ 18

H-Bridge ....................................................... 18

Oscillators ..................................................... 19

Applications II: Amplifiers ...........................21

Common Configurations ................................. 21

Common Emitter ........................................... 21

Common Collector (Emitter Follower) .............. 22

Common Base ............................................... 23

Multistage Amplifiers ...................................... 24Darlington ..................................................... 24

Differential Amplifier ...................................... 25

Push-Pull Amplifier ......................................... 26

multi-stage transistor (Op Amp) ..................... 27

BIPOLAR TRANSISTOR BASICS .................29

TRANSISTOR CHARACTERISTICS ................ 31

PRACTICAL APPLICATIONS ......................... 32

DIODE AND SWITCHING CIRCUITS .............32

LINEAR AMPLIFIER CIRCUITS .....................36

THE DIFFERENTIAL AMPLIFIER ...................37

THE DARLINGTON CONNECTION ................ 38

MULTIVIBRATOR CIRCUITS ........................38

COMMON-COLLECTOR AMPLIFIERS ..........41

DIGITAL AMPLIFIERS .................................41

RELAY DRIVERS .........................................42

CONSTANT-CURRENT GENERATORS ........... 44

LINEAR AMPLIFIERS.................................. 48

BOOTSTRAPPING ...................................... 49

COMPLEMENTARY EMITTER FOLLOWERS ... 50

THE AMPLIFIED DIODE ............................. 51

COMMON-EMITTER AMPLIFIERS ...............53DIGITAL CIRCUITS ................................... 53

RELAY DRIVERS ........................................ 55

LINEAR BIASING CIRCUITS ....................... 56

CIRCUIT VARIATIONS ............................... 58

HIGH-GAIN CIRCUITS ............................... 60

COMMON-BASE AMPLIFIER CIRCUITS ........ 61

AUDIO AMPLIFIER BASICS .......................64

SIMPLE PRE-AMPS .................................... 64

RIAA PRE-AMP CIRCUITS........................... 67

A UNIVERSAL PRE-AMP ............................. 68VOLUME CONTROL ................................... 69

TONE CONTROL CIRCUITS ........................ 70

AUDIO MIXER CIRCUITS ........................... 72

SCRATCH/RUMBLE FILTERS ....................... 73

OSCILLATOR BASICS ................................75

C-R OSCILLATORS .................................... 75

L-C OSCILLATORS ..................................... 78


MODULATION ........................................... 81

CRYSTAL OSCILLATORS ............................ 82WHITE NOISE GENERATORS ..................... 83

MULTIVIBRATOR CIRCUIT TYPES .............85

ASTABLE MULTIVIBRATOR BASICS............. 85

ASTABLE CIRCUIT VARIATIONS ................. 87

MONOSTABLE BASICS ............................... 88

LONG DELAYS .......................................... 89

ELECTRONIC TRIGGERING ........................ 90

BISTABLE CIRCUITS .................................. 90

THE SCHMITT TRIGGER ............................ 92

POWER AMPLIFIER BASICS ......................94

CLASS-AB BASICS ..................................... 96


PRACTICAL CLASS AB AMPLIFIERS ............ 100

ALTERNATIVE DRIVERS ............................ 101

SCRATCH/RUMBLE FILTERS ...................... 101


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USEFUL CIRCUITS & GADGETS ............... 105

A NOISE LIMITER CIRCUIT ....................... 105

ASTABLE MULTIVIBRATOR CIRCUITS ........ 105

LIE DETECTOR ........................................ 106

CURRENT MIRRORS ................................. 106

AN ADJUSTABLE ZENER ........................... 107

L-C OSCILLATORS .................................... 107

FM TRANSMITTERS .................................. 109

TRANSISTOR AC VOLTMETERS ................. 109

AC MILLIVOLTMETER CIRCUITS ................ 111

Resources and Going Further .................. 114

Going Further .......................................... 114

Suggested Reading .................................. 114


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INTRODUCTION

Transistors make our electronics world go round. Theyre critical as a control source injust about every modern circuit. Sometimes you see them, but more-often-than-nottheyre hidden deep within the die of an integrated circuit. In this tutorial well introduceyou to the basics of the most common transistor around: the bi-polar junction transistor

(BJT).

In small, discrete quantities, transistors can be used to create simple electronicswitches, digital logic, and signal amplifying circuits. In quantities of thousands,millions, and even billions, transistors are interconnected and embedded into tiny chipsto create computer memories, microprocessors, and other complex ICs.

Symbols, Pins, and Construction

Transistors are fundamentally three-terminal devices. On a bi-polar junction transistor(BJT), those pins are labeled collector(C), base(B), and emitter(E). The circuitsymbols for both the NPN and PNP BJT are below:

The only difference between an NPN and PNP is the direction of the arrow on theemitter. The arrow on an NPN points out, and on the PNP it points in. A usefulmnemonic for remembering which is which is:

NPN: Not Pointing iN


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Backwards logic, but it works!

TRANSISTOR CONSTRUCTION

Transistors rely on semiconductors to work their magic. A semiconductor is a materialthats not quite a pure conductor (like copper wire) but also not an insulator (like air).The conductivity of a semiconductor how easily it allows electrons to flow depends

on variables like temperature or the presence of more or less electrons. Lets lookbriefly under the hood of a transistor. Dont worry, we wont dig too deeply intoquantum physics.

A Transistor as Two Diodes

Transistors are kind of like an extension of another semiconductor component: diodes.In a way transistors are just two diodes with their cathodes (or anodes) tied together:

The diode connecting base to emitter is the important one here; it matches thedirection of the arrow on the schematic symbol, and shows you which way current isintended to flowthrough the transistor.

The diode representation is a good place to start, but its far from accurate. Dont baseyour understanding of a transistors operation on that model (and definitely dont try toreplicate it on a breadboard, it wont work). Theres a whole lot of weird quantumphysics level stuff controlling the interactions between the three terminals.

(This model isuseful if you need to test a transistor. Using the diode (or resistance) testfunction on a multimeter, you can measure across the BE and BC terminals to check forthe presence of those diodes.)

Transistor Structure and Operation

Transistors are built by stacking three different layers ofsemiconductor material together. Some of those layers have extraelectrons added to them (a process called doping), and othershave electrons removed (doped with holes the absence ofelectrons). A semiconductor material with extraelectrons is calledan n-type(nfor negative because electrons have a negativecharge) and a material with electrons removed is called a p-type


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(for positive). Transistors are created by either stacking an non top of apon top of ann, orpover noverp.

With some hand waving, we can say electrons can easily flow from nregions topregions, as long as they have a little force (voltage) to push them. But flowing from a

pregion to an nregion is really hard (requires a lotofvoltage). But the special thing about a transistor the partthat makes our two-diode model obsolete is the fact thatelectrons caneasily flow from the p-type base to then-type collector as long as the base-emitter junction isforward biased(meaning the base is at a higher voltagethan the emitter).

The NPN transistor is designed to pass electrons from theemitter to the collector (so conventional current flows fromcollector to emitter). The emitter emits electrons into thebase, which controls the number of electrons the emitter

emits. Most of the electrons emitted are collected by thecollector, which sends them along to the next part of thecircuit.

A PNP works in a same but opposite fashion. The base still controls current flow, butthat current flows in the opposite direction from emitter to collector. Instead ofelectrons, the emitter emits holes (a conceptual absence of electrons) which arecollected by the collector.

The transistor is kind of like an electron valve. The base pin is like a handle you mightadjust to allow more or less electrons to flow from emitter to collector. Lets investigatethis analogy further

Extending the Water Analogy

If youve been reading a lot of electricity concept tutorials lately, youre probably usedto water analogies. We say that current is analogous to the flow rate of water, voltageis the pressure pushing that water through a pipe, and resistance is the width of thepipe.


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Unsurprisingly, the water analogy can be extended to transistors as well: a transistor islike a water valve a mechanism we can use to control the flow rate.

There are three states we can use a valve in, each of which has a different effect onthe flow rate in a system.

1) ON SHORT CIRCUIT

A valve can be completely opened, allowing water to flow freely passing through asif the valve wasnt even present.

Likewise, under the right circumstances, a transistor can look like a short circuitbetween the collector and emitter pins. Current is free to flow through the collector,and out the emitter.

2) OFF OPEN CIRCUIT

When its closed, a valve can completely stop the flowof water.

In the same way, a transistor can be used to create an open circuitbetween thecollector and emitter pins.

3) LINEAR FLOW CONTROL

With some precise tuning, a valve can be adjusted to finely control the flow ratetosome point between fully open and closed.


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A transistor can do the same thing linearly controlling the currentthrough acircuit at some point between fully off (an open circuit) and fully on (a short circuit).

From our water analogy, the width of a pipe is similar to the resistance in a circuit. If avalve can finely adjust the width of a pipe, then a transistor can finely adjust theresistance between collector and emitter. So, in a way, a transistor is like a variable,adjustable resistor.

AMPLIFYING POWER

Theres another analogy we can wrench into this. Imagine if, with the slight turn of avalve, you could control the flow rate of the Hoover Dams flow gates. The measlyamount of force you might put into twisting that knob has the potential to create aforce thousands of times stronger. Were stretching the analogy to its limits, but thisidea carries over to transistors too. Transistors are special because they can amplifyelectrical signals, turning a low-power signal into a similar signal of much higher power.

Kind of. Theres a lot more to it, but thats a good place to start! Check out the next

section for a more detailed explanation of the operation of a transistor.

Operation Modes

Unlike resistors, which enforce a linear relationship between voltage and current,transistors are non-linear devices. They have four distinct modes of operation, whichdescribe the current flowing through them. (When we talk about current flow through atransistor, we usually mean current flowing from collector to emitter of an NPN.)

The four transistor operation modes are:

Saturation The transistor acts like a short circuit. Current freely flows fromcollector to emitter.

Cut-off The transistor acts like an open circuit. No current flows fromcollector to emitter.

Active The current from collector to emitter is proportionalto the currentflowing into the base.


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Reverse-Active Like active mode, the current is proportional to the basecurrent, but it flows in reverse. Current flows from emitter to collector (not,exactly, the purpose transistors were designed for).

To determine which mode a transistor is in, we need to look at the voltages on each ofthe three pins, and how they relate to each other. The voltages from base to emitter(VBE), and the from base to collector (VBC) set the transistors mode:

The simplified quadrant graph above shows how positive and negative voltages at thoseterminals affect the mode. In reality its a bit more complicated than that.

Lets look at all four transistor modes individually; well investigate how to put thedevice into that mode, and what effect it has on current flow.

Note:The majority of this page focuses on NPN transistors. To understand how aPNP transistor works, simply flip the polarity or > and < signs.

Saturation Mode

Saturation is the on modeof a transistor. A transistor in saturation mode acts like ashort circuit between collector and emitter.


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In saturation mode both of the diodes in the transistor are forward biased. Thatmeans VBEmust be greater than 0, andso must VBC. In other words, VBmust be higherthan both VEand VC.

Because the junction from base to emitter looks just like a diode, in reality, V BEmust begreater than a threshold voltageto enter saturation. There are many abbreviationsfor this voltage drop Vth, V, and Vdare a few and the actual value varies betweentransistors (and even further by temperature). For a lot of transistors (at roomtemperature) we can estimate this drop to be about 0.6V.

Another reality bummer: there wont be perfect conduction between emitter andcollector. A small voltage drop will form between those nodes. Transistor datasheetswill define this voltage as CE saturation voltage VCE(sat) a voltage from collector to

emitter required for saturation. This value is usually around 0.05-0.2V. This valuemeans that VCmust be slightly greater than VE(but both still less than VB) to get thetransistor in saturation mode.

Cutoff Mode

Cutoff mode is the opposite of saturation. A transistor in cutoff mode is off there isno collector current, and therefore no emitter current. It almost looks like an opencircuit.

To get a transistor into cutoff mode, the base voltage must be less than both theemitter and collector voltages. VBCand VBEmust both be negative.

In reality, VBEcan be anywhere between 0V and Vth(~0.6V) to achieve cutoff mode.


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Active Mode

To operate in active mode, a transistors VBEmust be greater than zero and VBCmust benegative. Thus, the base voltage must be less than the collector, but greater than theemitter. That also means the collector must be greater than the emitter.

In reality, we need a non-zero forward voltage drop(abbreviated either Vth, V, orVd) from base to emitter (VBE) to turn on the transistor. Usually this voltage is usuallyaround 0.6V.

Amplifying in Active Mode

Active mode is the most powerful mode of the transistor because it turns the deviceinto an amplifier. Current going into the base pin amplifies current going into thecollector and out the emitter.

Our shorthand notation for the gain(amplification factor) of a transistor is (you mayalso see it as F, or hFE). linearly relates the collector current (IC) to the base current(IB):

The actual value of varies by transistor. Its usually around 100, but can range from50 to 200even 2000, depending on which transistor youre using and how muchcurrent is running through it. If your transistor had a of 100, for example, thatd

mean an input current of 1mA into the base could produce 100mA current through thecollector.

Active mode model. VBE= Vth, and IC= IB.

What about the emitter current, IE? In active mode, the collector and base currents gointothe device, and the IEcomes out. To relate the emitter current to collector current,


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we have another constant value: . is the common-base current gain, it relates thosecurrents as such:

is usually veryclose to, but less than, 1. That means ICis very close to, but less

than IEin active mode.

You can use to calculate , or vice-versa:

If is 100, for example, that means is 0.99. So, if ICis 100mA, for example, then IEis101mA.

Reverse Active

Just as saturation is the opposite of cutoff, reverse active mode is the opposite of activemode. A transistor in reverse active mode conducts, even amplifies, but current flows inthe opposite direction, from emitter to collector. The downside to reverse active modeis the (Rin this case) is muchsmaller.

To put a transistor in reverse active mode, the emitter voltage must be greater than thebase, which must be greater than the collector (VBE0).

Reverse active mode isnt usually a state in which you want to drive a transistor. Itsgood to know its there, but its rarely designed into an application.

RELATING TO THE PNP

After everything weve talked about on this page, weve still only covered half of theBJT spectrum. What about PNP transistors? PNPs work a lot like the NPNs they have

the same four modes but everything is turned around. To find out which mode a PNPtransistor is in, reverse all of the < and > signs.

For example, to put a PNP into saturation VCand VEmust be higher than VB. You pullthe base low to turn the PNP on, and make it higher than the collector and emitter toturn it off. And, to put a PNP into active mode, VEmust be at a higher voltage than VB,which must be higher than VC.

In summary:


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Voltage relations NPN Mode PNP Mode

VE< VB< VC Active Reverse

VE< VB> VC Saturation Cutoff

VE> VB< VC Cutoff Saturation

VE> VB> VC Reverse Active

Another opposing characteristic of the NPNs and PNPs is the direction of current flow.In active and saturation modes, current in a PNP flows from emitter to collector.This means the emitter must generally be at a higher voltage than the collector.

If youre burnt out on conceptual stuff, take a trip to the next section. The best way tolearn how a transistor works is to examine it in real-life circuits. Lets look at someapplications!

Applications I: Switches

One of the most fundamental applications of a transistor is using it to control the flowof power to another part of the circuit using it as an electric switch. Driving it in eithercutoff or saturation mode, the transistor can create the binary on/off effect of a switch.

Transistor switches are critical circuit-building blocks; theyre used to make logic gates,which go on to create microcontrollers, microprocessors, and other integrated circuits.Below are a few example circuits.

TRANSISTOR SWITCH

Lets look at the most fundamental transistor-switch circuit: an NPN switch. Here weuse an NPN to control a high-power LED:


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Our control input flows into the base, the output is tied to the collector, and the emitteris kept at a fixed voltage.

While a normal switch would require an actuator to be physically flipped, this switch is

controlled by the voltage at the base pin. A microcontroller I/O pin, like those on anArduino, can be programmed to go high or low to turn the LED on or off.

When the voltage at the base is greater than 0.6V (or whatever your transistors V thmight be), the transistor starts saturating and looks like a short circuit between collectorand emitter. When the voltage at the base is less than 0.6V the transistor is in cutoffmode no current flows because it looks like an open circuit between C and E.

The circuit above is called a low-side switch, because the switch our transistor ison the low (ground) side of the circuit. Alternatively, we can use a PNP transistor to

create a high-side switch:

Similar to the NPN circuit, the base is our input, and the emitter is tied to a constantvoltage. This time however, the emitter is tied high, and the load is connected to thetransistor on the ground side.


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This circuit works just as well as the NPN-based switch, but theres one hugedifference: to turn the load on the base must be low. This can cause complications,especially if the loads high voltage (VCCin this picture) is higher than our control inputshigh voltage. For example, this circuit wouldnt work if you were trying to use a 5V-operating Arduino to switch on a 12V motor. In that case itd be impossible to turn theswitch off because VBwould always be less than VE.

BASE RESISTORS!

Youll notice that each of those circuits uses a series resistor between the control inputand the base of the transistor. Dont forget to add this resistor! A transistor without aresistor on the base is like an LED with no current-limiting resistor.

Recall that, in a way, a transistor is just a pair of interconnected diodes. Were forward-biasing the base-emitter diode to turn the load on. The diode only needs 0.6V to turnon, more voltage than that means more current. Some transistors may only be rated fora maximum of 10-100mA of current to flow through them. If you supply a current over

the maximum rating, the transistor might blow up.

The series resistor between our control source and the base limits current into thebase. The base-emitter node can get its happy voltage drop of 0.6V, and the resistorcan drop the remaining voltage. The value of the resistor, and voltage across it, will setthe current.

The resistor needs to be large enough to effectively limitthe current, but small enoughto feed the base enoughcurrent. 1mA to 10mA will usually be enough, but check yourtransistors datasheet to make sure.

DIGITAL LOGIC


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Transistors can be combined to create all our fundamental logic gates: AND, OR, andNOT.

(Note: These days MOSFETS are more likely to be used to create logic gates than BJTs.MOSFETs are more power-efficient, which makes them the better choice.)

INVERTER

Heres a transistor circuit that implements an inverter, or NOT gate:

An inverter built out of transistors.

Here a high voltage into the base will turn the transistor on, which will effectivelyconnect the collector to the emitter. Since the emitter is connected directly to ground,the collector will be as well (though it will be slightly higher, somewhere around VCE(sat)

~ 0.05-0.2V). If the input is low, on the other hand, the transistor looks like an opencircuit, and the output is pulled up to VCC

(This is actually a fundamental transistor configuration called common emitter. Moreon that later.)

AND GATE

Here are a pair of transistors used to create a 2-input AND gate:


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2-input AND gate built out of transistors.

If either transistor is turned off, then the output at the second transistors collector willbe pulled low. If both transistors are on (bases both high), then the output of thecircuit is also high.

OR GATE

And, finally, heres a 2-input OR gate:

2-input OR gate built out of transistors.

In this circuit, if either (or both) A or B are high, that respective transistor will turn on,and pull the output high. If both transistors are off, then the output is pulled lowthrough the resistor.

H-BRIDGE


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An H-bridge is a transistor-based circuit capable of driving motors both clockwiseand counter-clockwise. Its an incredibly popular circuit the driving force behindcountless robots that must be able to move both forward andbackward.

Fundamentally, an H-bridge is a combination of four transistors with two inputs linesand two outputs:

Can you guess why its called an H bridge?

(Note: theres usually quite a bit more to a well-designed H-bridge including flybackdiodes, base resistors and Schmidt triggers.)

If both inputs are the same voltage, the outputs to the motor will be the same voltage,and the motor wont be able to spin. But if the two inputs are opposite, the motor will

spin in one direction or the other.

The H-bridge has a truth table that looks a little like this:

Input A Input B Output A Output B Motor Direction

0 0 1 1 Stopped (braking)

0 1 1 0 Clockwise

1 0 0 1 Counter-clockwise

1 1 0 0 Stopped (braking)

OSCILLATORS


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An oscillator is a circuit that produces a periodic signal that swings between a high andlow voltage. Oscillators are used in all sorts of circuits: from simply blinking an LED tothe producing a clock signal to drive a microcontroller. There are lots of ways to createan oscillator circuit including quartz crystals, op amps, and, of course, transistors.

Heres an example oscillating circuit, which we call an astable multivibrator. By usingfeedbackwe can use a pair of transistors to create two complementing, oscillatingsignals.

Aside from the two transistors, the capacitors are the real key to this circuit. The capsalternatively charge and discharge, which causes the two transistors to alternativelyturn on and off.

Analyzing this circuits operation is an excellent study in the operation of both caps andtransistors. To begin, assume C1 is fully charged (storing a voltage of about VCC), C2 isdischarged, Q1 is on, and Q2 is off. Heres what happens after that:

If Q1 is on, then C1s left plate (on the schematic) is connected to about 0V. Thiswill allow C1 to discharge through Q1s collector.

While C1 is discharging, C2 quickly charges through the lower value resistor R4.

Once C1 fully discharges, its right plate will be pulled up to about 0.6V, which

will turn on Q2. At this point weve swapped states: C1 is discharged, C2 is charged, Q1 is off,

and Q2 is on. Now we do the same dance the other way. Q2 being on allows C2 to discharge through Q2s collector. While Q1 is off, C1 can charge, relatively quickly through R1. Once C2 fully discharges, Q1 will be turn back on and were back in the state we

started in.


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It can be hard to wrap your head around. You can find another excellent demo of thiscircuit here.

By picking specific values for C1, C2, R2, and R3 (and keeping R1 and R4 relativelylow), we can set the speed of our multivibrator circuit:

So, with the values for caps and resistors set to 10F and 47krespectively, ouroscillator frequency is about 1.5 Hz. That means each LED will blink about 1.5 times persecond.

As you can probably already see, there are tonsof circuits out there that make use oftransistors. But weve barely scratched the surface. These examples mostly show how

the transistor can be used in saturation and cut-off modes as a switch, but what aboutamplification? Time for more examples!

Applications II: Amplifiers

Some of the most powerful transistor applications involve amplification: turning a lowpower signal into one of higher power. Amplifiers can increase the voltage of a signal,taking something from the V range and converting it to a more useful mV or V level.Or they can amplify current, useful for turning the A of current produced by aphotodiode into a current of much higher magnitude. There are even amplifiers that

take a current in, and produce a higher voltage, or vice-versa (called transresistanceand transconductance respectively).

Transistors are a key component to many amplifying circuits. There are a seeminglyinfinite variety of transistor amplifiers out there, but fortunately a lot of them are basedon some of these more primitive circuits. Remember these circuits, and, hopefully, witha bit of pattern-matching, you can make sense of more complex amplifiers.

COMMON CONFIGURATIONS

Three of the most fundamental transistor amplifiers are: common emitter, common

collector and common base. In each of the three configurations one of the three nodesis permanently tied to a common voltage (usually ground), and the other two nodes areeither an input or output of the amplifier.

COMMON EMITTER

Common emitter is one of the more popular transistor arrangements. In this circuit theemitter is tied to a voltage common to both the base and emitter (usually ground). Thebase becomes the signal input, and the collector becomes the output.


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The common emitter circuit is popular because its well-suited for voltage

amplification, especially at low frequencies. Theyre great for amplifying audio signals,for example. If you have a small 1.5V peak-to-peak input signal, you could amplify thatto a much higher voltage using a slightly more complicated circuit, like:

One quirk of the common emitter, though, is that it invertsthe input signal (compareit to the inverter from the last page!).

COMMON COLLECTOR (EMITTER FOLLOWER)

If we tie the collector pin to a common voltage, use the base as an input, and theemitter as an output, we have a common collector. This configuration is also known asan emitter follower.

The common collector doesnt do any voltage amplification (in fact, the voltage out willbe 0.6V lower than the voltage in). For that reason, this circuit is sometimes called avoltage follower.


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This circuit does have great potential as a current amplifier. In addition to that, thehigh current gain combined with near unity voltage gain makes this circuit a greatvoltage buffer. A voltage buffer prevents a load circuit from undesirably interferingwith the circuit driving it.

For example, if you wanted to deliver 1V to a load, you could go the easy way and usea voltage divider, or you could use an emitter follower.

As the load gets larger (which, conversely, means the resistance is lower) the output ofthe voltage divider circuit drops. But the voltage output of the emitter follower remainssteady, regardless of what the load is. Bigger loads cant load down an emitterfollower, like they can circuits with larger output impedances.

COMMON BASE

Well talk about common base to provide some closure to this section, but this is theleast popular of the three fundamental configurations. In a common base amplifier, theemitter is an input and the collector an output. The base is common to both.

Common base is like the anti-emitter-follower. Its a decent voltage amplifier, andcurrent in is about equal to current out (actually current in is slightly greater thancurrent out).


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The common base circuit works best as a current buffer. It can take an input currentat a low input impedance, and deliver nearly that same current to a higher impedanceoutput.

Summary

These three amplifier configurations are at the heart of many more complicated

transistor amplifiers. They each have applications where they shine, whether theyreamplifying current, voltage, or buffering.

Common Emitter Common Collector Common Base

Voltage Gain Medium Low High

Current Gain Medium High Low

Input Impedance Medium High Low

Output Impedance Medium Low High

MULTISTAGE AMPLIFIERS

We could go on and on about the great variety of transistor amplifiers out there. Hereare a few quick examples to show off what happens when you combine the single-stageamplifiers above:

DARLINGTON

The Darlington amplifier runs one common collector into another to create a highcurrent gainamplifier.


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Voltage out is aboutthe same as voltage in (minus about 1.2V-1.4V), but the currentgain is the product of twotransistor gains. Thats 2, upwards of 1000!

The Darlington pair is a great tool if you need to drive a large load with a very smallinput current.

DIFFERENTIAL AMPLIFIER

A differential amplifier subtracts two input signals and amplifies that difference. Its acritical part of feedback circuits, where the input is compared against the output, toproduce a future output.

Heres the foundation of the differential amp:


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This circuit is also called a long tailed pair. Its a pair of common-emitter circuits thatare compared against each other to produce a differential output. Two inputs areapplied to the bases of the transistors; the output is a differential voltage across thetwo collectors.

PUSH-PULL AMPLIFIER

A push-pull amplifier is a useful final stage in many multi-stage amplifiers. Its an

energy efficient power amplifier, often used to drive loudspeakers.

The fundamental push-pull amp uses an NPN and PNP transistor, both configured ascommon collectors:

The push-pull amp doesnt really amplify voltage (voltage out will be slightly less thanthat in), but it does amplify current. Its especially useful in bi-polar circuits (those with


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positive and negative supplies), because it can both push current into the load fromthe positive supply, and pull current out and sink it into the negative supply.

If you have a bi-polar supply (or even if you dont), the push-pull is a great final stageto an amplifier, acting as a buffer for the load.

MULTI-STAGE TRANSISTOR (OP AMP)

Lets look at a classic example of a multi-stage transistor circuit: an Op Amp. Being ableto recognize common transistor circuits, and understanding their purpose can get you along way! Here is the circuit inside an LM3558, a really simple op amp:

The internals of an LM358 operational amplifier. Recognize some amplifiers?

Theres certainly more complexity here than you may be prepared to digest, howeveryou might see some familiar topologies:

Q1, Q2, Q3, and Q4 form the input stage. Looks a lot like an common collector(Q1 and Q4) into a differential amplifier, right? It just looks upside down,because its using PNPs. These transistors help to form the input differentialstage of the amplifier.

Q11 and Q12 are part of the second stage. Q11 is a common collector and Q12

is a common emitter. This pair of transistors will buffer the signal from Q3scollector, and provide a high gain as the signal goes to the final stage.

Q6 and Q13 are part of the final stage, and they should look familiar as well(especially if you ignore RSC) its a push-pull! This stage buffers the output,allowing it to drive larger loads.

There are a variety of other common configurations in there that we haventtalked about. Q8 and Q9 are configured as a current mirror, which simplycopies the amount of current through one transistor into the other.


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After this crash course in transistors, we wouldnt expect you to understand whatsgoing on in this circuit, but if you can begin to identify common transistor circuits youreon the right track!


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BIPOLAR TRANSISTOR BASICS

A bipolar transistor (first invented in 1948) is a three-terminal (base, emitter, andcollector), current-amplifying device in which a small input current can control themagnitude of a much larger output current. The term bipolar means that the device ismade from semiconductor materials in which conduction relies on both positive and

negative (majority and minority) charge carriers.

A normal transistor is made from a three-layer sandwich of n-type and p-typesemiconductor material, with the base or control terminal connected to the central

layer, and the collector and emitter terminals connected to the outer layers. If it usesan n-p-n construction sandwich, as in Figure 1(a), it is known as an npn transistor and

uses the standard symbol in Figure 1(b). If it uses a p-n-p structure, as in Figure 2(a), it

is known as a pnp transistor and uses the symbol in Figure 2(b).

An easy way to remember whether a transistor is NPN or PNP is to look at he arrow.Remember, NPNmeans the arrow is Not Pointing in. Also a PNPis Pointing iN

Figure 1. Basic construction (a) and symbol

(b) of npn transistor.

Figure 2. Basic construction (a) and symbol

(b) of pnp transistor.


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In use, npn and pnp transistors each need a power supply of the appropriate polarity,as shown in Figure 3. An npn device needs a supply that makes the collector positive tothe emitter its output or main-terminal signal current (Ic) flows from collector toemitter, and its amplitude is controlled by an input control current (Ib) that flows from

base to emitter via an external current-limiting resistor (Rb) and a positive bias voltage.

A pnp transistor needs a negative supply its main terminal current flows from emitterto collector, and is controlled by an emitter-to-base input current that flows to anegative bias voltage.

Note that the negative collector-emitter voltage tells you that the transistor is PNP.

Also that the output current increases with input or base current and varies very littlewith collector-emitter voltage.

Primary considerations when selecting a transistor

(a)Voltage ratings of all three junctions(b)Power rating and thermal resistance(c)Current handling capability and the transistor case size(d)Leakage currents, mainly Icbo and Iebo(e)Frequency response and /or switching times.(f)Current gain (HFEand hfe)

(g)Temperature parameter variation.(h)Saturation resistance(I)h-parameters for linear applications

The first bipolar transistors were made from germanium materials. These devices werefragile, excessively temperature-sensitive, electronically noisy, and had very poorpower-handling capacities. Germanium transistors are now obsolete. Virtually allmodern bipolar transistors are made from silicon semiconductor materials. Such devices

Figure 3. Polarity

connections to (a) npn and (b) pnp transistors.


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are robust, have good power-handling capacities, are not excessively temperaturesensitive, and generate negligible electronic noise.

Today, a very wide variety of excellent silicon bipolar transistor types are readilyavailable. Figure 4lists the basic characteristics of two typical general-purpose, low-power types the 2N3904 (npn) and the 2N3906 (pnp) which are each housed in a

TO-92 plastic case and have the under-side pin connections shown in the diagram.Note, when reading the Figure 4list, that VCEO(max)is the maximum voltage that maybe applied between the collector and emitter when the base is open-circuit, and

VCBO(max)is the maximum voltage that may be applied between the collector and basewhen the emitter is open-circuit. IC(max)is the maximum mean current that can beallowed to flow through the collector terminal of the device, and PT(max)is the maximummean power that the device can dissipate, without the use of an external heatsink, atnormal room temperature.

One of the most important parameters of the transistor is its forward current transferratio, or hfe this is the current-gain or output/input current ratio of the device

(typically 100 to 300 in the two devices listed). Finally, the fTfigure indicates theavailable gain/bandwidth product frequency of the device, i.e., if the transistor is usedin a voltage feedback configuration that provides a voltage gain of x100, the bandwidthis 1/100 of the fTfigure, but if the voltage gain is reduced to x10, the bandwidthincreases to fT/10, etc.

TRANSISTORCHARACTERISTICS

Figure 4. General characteristics and outlines of the 2N3904 and 2N3906 low-power silicon

transistors.

Figure 5.


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To get themaximumvalue from a transistor, the user must understand both its static (DC) and dynamic (AC)characteristics. Figure 5shows the static equivalent circuits of npn and pnp transistors.

A zener diode is inevitably formed by each of the transistors n-p or p-n junctions, andthe transistor is thus (in static terms) equal to a pair of reverse-connected zener diodeswired between the collector and emitter terminals, with the base terminal wired to their

common point. In most low-power, general-purpose transistors, the base-to-emitterjunction has a typical zener value in the range 5V to 10V the base-to-collectorjunctions typical zener value is in the range 20V to 100V.

Thus, the transistors base-emitter junction acts like an ordinary diode when forward-biased and as a zener when reverse-biased. In silicon transistors, a forward-biased

junction passes little current until the bias voltage rises to about 600mV, but beyondthis value, the current increases rapidly. When forward-biased by a fixed current, the

junctions forward voltage has a thermal coefficient of about -2mV/0C. When thetransistor is used with the emitter open-circuit, the base-to-collector junction acts likethat just described, but has a greater zener value. If the transistor is used with its base

open-circuit, the collector-to-emitter path acts like a zener diode wired in series with anordinary diode.

The transistors dynamiccharacteristics can be understood withthe aid of Figure 6, which shows thetypical forward transfer characteristicsof a low-power npn silicon transistorwith a nominal hfe(current gain) value

of 100. Thus, when the base current(Ib) is zero, the transistor passes only

a slight leakage current. When thecollector voltage is greater than a few

hundred millivolts, the collectorcurrent is almost directly proportionalto the base currents, and is littleinfluenced by the collector voltagevalue. The device can thus be used asa constant-current generator byfeeding a fixed bias current into the

base, or can be used as a linear amplifier by superimposing the input signal on anominal input current.

PRACTICAL APPLICATIONS

A transistor can be used in a variety of different basic circuit configurations, and theremainder of this opening episode presents a brief summary of the most important ofthese. Note that although all circuits are shown using npn transistor types, they can beused with pnp types by simply changing circuit polarities, etc.

DIODE AND SWITCHING CIRCUITS

Static equivalent circuits of npn and pnp transistors.

Figure 6. Typical transfer characteristics of low-power npn transistors with hfevalue of 100

nominal.


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The base-emitteror base-collector

junction of asilicon transistorcan be used as asimple diode orrectifier, or as azener diode byusing it in theappropriatepolarity. Figure 7

shows twoalternative ways of using an npn transistor as a simple diode clamp that converts an

AC-coupled rectangular input waveform into a rectangular output that swings betweenzero and a positive voltage value, i.e., which clamps the output signal to the zero-volts reference point via either the transistors internal base-emitter or base-collector

diode junction.

Figure 7. Clamping diode circuit, using an npn transistor as a diode.

Figure 8. A transistor used as a zener diode.

Figure 9. Transistor switch or digital inverter.


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Figure 10. Transistor switch (digital inverter) driving a relay coil(or other inductive load).

Figure 11. Common-emitter linear amplifier.

Figure 12. Common-base linear amplifier.


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Figure 13. DC common-collector linear amplifier or voltage

follower.

Figure 14. AC common-collector amplifier or voltage follower.


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Figure 8shows an npn

transistor used as azener diode that convertsan unregulated supplyvoltage into a fixed-valueregulated output with atypical value in the range5V to 10V, depending onthe individual transistor.Only the reverse-biasedbase-emitter junction ofthe transistor is suitablefor use in thisapplication. If thereverse-biased base-collector junction is used,the zener value typically

rises into the 30V-100Vrange, and the transistor

may self-destruct (due to over-heating) at fairly low zener current levels.

Figure 9shows a transistor used as a simple electronic switch or digital inverter. Itsbase is driven (via Rb) by a digital input that is at either zero volts or at a positive value,and load RLis connected between the collector and the positive supply rail. When theinput voltage is zero, the transistor is cut off and zero current flows through the load,so the full supply voltage appears between the collector and emitter. When the input ishigh, the transistor switch is driven fully on (saturated) and maximum current flows inthe load, and only a few hundred millivolts are developed between the collector and

emitter. The output voltage is thus an inverted form of the input signal.

The basic Figure 9circuit is intended for use as a simple digital switch or inverter,driving a purely resistive load. It can be used as an electronic switch that drives a relaycoil or other highly inductive load (such as a DC motor) by connecting it as shown inFigure 10, in which diodes D1 and D2 protect the transistor from high-value switch-off-induced back EMFs from the inductive load at the moment of power switch-off.

LINEAR AMPLIFIER CIRCUITS

A transistor can be used as a linear current or voltage amplifier by feeding a suitablebias current into its base and then applying the input signal between an appropriatepair of terminals. The transistor can, in this case, be used in any one of three basicoperating modes, each of which provides a unique set of characteristics. These threemodes are known as common-emitter (Figure 11), common-base (Figure 12),and common-collector (Figures 13 and 14).

In the common-emitter circuit (which is shown in very basic form in Figure 11),resistive load RLis wired between the transistors collector and the positive supply line,and a bias current is fed into the base via resistor Rb, whose value is chosen to set thecollector at a quiescent half-supply voltage value (to provide maximum undistorted

Figure 15. Comparative performances of the three basic circuit

configurations.


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output signal swings). The input signal is applied between the transistors base andemitter via capacitor C, and the output signal (which is phase-inverted relative to theinput) is taken between the collector and emitter. This circuit gives a medium-valueinput impedance and a fairly high overall voltage gain.

In the common-base circuit in Figure 12, the base is biased via Rband is AC-decoupled(or AC-grounded) via capacitor Cb. The input signal is effectively applied between the

emitter and base via C1, and the amplified but non-inverted output signal is effectivelytaken from between the collector and base. This circuit features good voltage gain,near-unity current gain, and a very low input impedance.

In the DC common-collector circuit in Figure 13, the collector is shorted to the low-impedance positive supply rail and is thus effectively at virtual ground impedancelevel. The input signal is applied between base and ground (virtual collector), and thenon-inverted output is taken from between emitter and ground (virtual collector). Thiscircuit gives near-unity overall voltage gain, and its output follows the input signal. Itis thus known as a DC-voltage follower (or emitter follower) and it has a very high-inputimpedance (equal to the product of the RLand hfevalues).

Note that the above circuit can be modified for AC use by simply biasing the transistorto half-supply volts and AC-coupling the input signal to the base, as shown in the basiccircuit in Figure 14, in which potential divider R1-R2 provides the half-supply-voltagebiasing.

The chart in Figure 15summarizes the performances of the three basic amplifierconfigurations. Thus, the common-collector amplifier gives near-unity overall voltagegain and a high input impedance, while the common-emitter and common-baseamplifiers both give high values of voltage gain, but have medium to low values ofinput impedance.

THE DIFFERENTIAL

AMPLIFIER

Figure 16shows in basic

form how a pair of amplifiersof the basic Figure 11type canbe coupled together to make a

differential amplifier or long-tailed pair that produces anoutput signal that isproportional to the differencebetween the two input signals.

In this case, Q1 and Q2 share acommon emitter resistor (the

tail), and the circuit is biased(via R1-R2 and R3-R4) so that the two transistors pass identical collector currents (thusgiving zero difference between the two collector voltages) under quiescent zero-inputconditions.

Figure 16. Differential amplifier or long-tailed pair.


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If, in the above circuit, a rising input voltage is applied to the input of one transistoronly, it makes the output voltage of that transistor fall and (as a result of emitter-coupling action) makes the output voltage of the other transistor rise by a similaramount, thus giving a large differential output voltage between the two collectors. Ifidentical signals are applied to the inputs of both transistors, on the other hand, bothcollectors will move by identical amounts, and the circuit will thus produce a zerodifferential output signal. The circuit therefore produces an output signal that isproportional to the difference between the two input signals.

THE DARLINGTON CONNECTION

The input impedance of the Figure 13emitterfollower circuit equals the product of RLand

the transistors hfevalues if an ultra-high

input impedance is wanted, it can beobtained by replacing the single transistorwith a pair of transistors connected in the

Darlington or Super-Alpha configuration, asshown in Figure 17. Here, the emitter current

of the input transistor feeds directly into thebase of the output transistor, and the pair actlike a single transistor with an overall hfe

value equal to the product of the twoindividual hfe values, i.e., if each transistorhas an hfevalue of 100, the pair act like asingle transistor with an hfeof 10,000, and

the overall circuit presents an input impedance of 10,000 x RL.

MULTIVIBRATOR CIRCUITS

A multivibrator is, in essence, a two-state digital circuit that can be switched from theoutput-high to the output-low state, or vice versa, via a trigger signal that may bederived from an external source or via an automatic or triggered timing mechanism.Transistors can be used in four basic types of multivibrator circuits, as shown in Figures18 to 21.

Figure 17. Darlington or Super-Alpha DC

emitter follower.


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The Figure 18circuit is a simple, manually-triggered, cross-coupled bistablemultivibrator, in which the base bias of each transistor is derived from the collector of

the other, so that one transistor automatically turns off when the other turns on, andvice versa.

Thus, the output can be driven low by briefly turning Q2 off via S2, thus shorting Q2sbase-emitter path. As Q2 turns off R2-R4 feed base drive to Q1 base, the circuitautomatically locks into this state until Q1 is similarly turned off via S1, at which pointthe output locks into the high state again, and so on ad infinitum.

Figure 19shows in basic form a monostable multivibrator or one-shot pulse

generator circuit. Its output (from Q1 collector) is normally low, since Q1 is normallybiased on via R5, but switches high for a preset period (determined by the C1-R5

component values) if Q1 is briefly turned off by momentarily closing push-button Startswitch S1.

The actual monostable timing period starts as the push-button Start switch isreleased, and has a period (P) of approximately 0.7 x C1 x R5, where P is in S, C is inF, and R is in kilohms.

Figure 18. Manually-triggered bistablemultivibrator.

Figure 19. Manually-triggered monostablemultivibrator.


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Figure 20. Astable multivibrator or free-

running squarewave generator.

Figure 21. Schmitt trigger or sine-to-square

waveform converter.

Figure 20shows an astable multivibrator, or free-running squarewave generator, inwhich the on and off periods of the squarewave are determined by the C1-R4 and C2-R3 component values. Basically, this circuit acts like a pair of cross-coupled monostablecircuits, which automatically trigger each other sequentially. If the C1-R4 and C2-R3timing periods are identical, the circuit generates a free-running squarewave outputwaveform. If the two timing periods are not identical, the circuit generates anasymmetrical output waveform.

Finally, Figure 21shows a basic Schmitt trigger or sine-to-square waveform convertercircuit. The circuit action here is such that Q2 switches abruptly from the on state tothe off state, or vice versa, as Q1 base goes above or below pre-determined triggervoltage levels.

If the circuits input is fed with a reasonable-amplitude sinewave input, the circuit thusgenerates a sympathetic squarewave output waveform.


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COMMON-COLLECTOR AMPLIFIERS

The common-collector amplifier (also known as the grounded-collector amplifier,emitter follower, or voltage follower) can be used in a wide variety of digital and analogamplifier and constant-current generator applications. This month we start off bylooking at practical digital amplifier circuits.

DIGITAL AMPLIFIERS

Figure 1shows a simple npncommon-collector digitalamplifier in which the input iseither low (at zero volts) or high(at a Vpeakvalue not greater thanthe supply rail value). When theinput is low, Q1 is cut off andthe output is at zero volts. When

the input is high, Q1 is driven onand current ILflows in RL, thusgenerating an output voltageacross RL intrinsic negativefeedback makes this outputvoltage take up a value onebase-emitter junction volt-drop(about 600mV) below the input Vpeakvalue. Thus, the output voltage follows (but is600mV less than) the input voltage.

This circuits input (base) current equals the ILvalue divided by Q1s hfevalue(nominally 200 in the 2N3904), and its input impedance equals h

fex R

L, i.e., nominally

660k in the example shown. The circuits output impedance equals the input signalsource impedance (Rs) value divided by hfe. Thus, the circuit has a high input and lowoutput impedance, and acts as a unity-voltage-gain buffer circuit.

Figure 1. Common-collector digital amplifier basic details.


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If this buffer circuit is fedwith a fast input pulse,its output may have a

deteriorated falling edge, as shown in Figure 2. This deterioration is caused by thepresence of stray capacitance (Cs) across RL. When the input pulse switches high, Q1

turns on and rapidly sources (feeds) a charge current into Cs, thus giving an output

pulse with a sharp leading edge. However, when the input signal switches low again,Q1 switches off and is thus unable to sink (absorb) the charge current of C s, which

thus discharges via RLand makes the output pulses trailing edge decay exponentially,with a time constant equal to the Cs-RLproduct.

Note from the above description that an npn emitter follower can efficiently source (but not sink)

high currents a pnp emitter follower gives the opposite action, and can efficiently sink (but not

source) high currents.

RELAY DRIVERS

If the basic Figure 1switching circuit is used to drive inductive loads such as coils orloudspeakers, etc., it must be fitted with a diode protection network to limit inductiveswitch-off back-EMFs to safe values. One very useful inductor-driving circuit is the relaydriver, and a number of examples of this are shown in Figures 3 to 7.

Figure 2. Effects of Cson the circuits output pulses in Figure

1.

Figure 3. Simple emitter-follower relay driver.


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Figure 4. Pnp version of the relay driver.

Figure 5. Darlington version of the

npn relay driver.

Figure 6. Delayed switch-on relay driver.


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The relay in thenpn driver circuitin Figure 3canbe activated via adigital input or via

switch SW1 itturns on when theinput signal is highor SW1 is closed,and turns offwhen the inputsignal is low orSW1 is open.Relay contacts

RLA/1 are available for external use, and the circuit can be made self-latching by wiringa spare set of normally-open relay contacts (RLA/2) between Q1s collector and emitter,

as shown dotted. Figure 4is a pnp version of the same circuit; in this case, the relaycan be turned on by closing SW1 or by applying a zero input signal. Note in Figure 3that D1 damps relay switch-off back-emfs by preventing this voltage from swingingbelow the zero-volts rail value. Optional diode D2 can be used to stop this voltageswinging above the positive rail.

The circuits shown in Figures 3 and 4effectively increase the relay current sensitivityby a factor of about 200 (the h fevalue of Q1), e.g., if the relay has a coil resistance of120R and needs an activating current of 100mA, the circuits input impedance is 24kand the input operating current requirement is 0.5mA. Sensitivity can be furtherincreased by using a Darlington pair of transistors in place of Q1 (as shown in Figure

5), but the emitter following voltage of Q2 will be 1.2V (two base-emitter volt drops)below the base input voltage of Q1. This circuit has an input impedance of 500k andneeds an input operating current of 24A C1 protects the circuit against activationvia high-impedance transient voltages, such as those induced by lightening flashes, RFI,etc. The Darlington buffer is useful in relay-driving C-R time-delay designs such asthose shown in Figures 6 and 7, in which C1-R1 generate an exponential waveformthat is fed to the relay via Q1-Q2, thus making the relay change state some delayedtime after the supply is initially connected. With an R1 value of 120k, the circuits giveoperating delays of roughly 0.1 seconds per F of C1 value, i.e., a 10 second delay if C1= 100F, etc. The Figure 6circuit makes the relay turn on some delayed time after itspower supply is connected. The Figure 7 circuit makes the relay turn on as soon as the

supply is connected, but turn off again after a fixed delay.

CONSTANT-CURRENT GENERATORS

A constant-current generator (CCG) is a circuit that generates a constant load currentirrespective of wide variations in load resistance. A bipolar transistor can be used as aCCG by using it in the common-collector mode shown in Figure 8. Here, R1-ZD1 appliesa fixed 5.6V reference to Q1 base, making 5V appear across R2, which thus passes

Figure 7. Auto-turn-off time-delay circuit.


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5mA via Q1s emitter. A transistors emitter and collector currents are inherently almostidentical, so a 5mA current also flows in any load connected between Q1s collector andthe positive supply rail, provided that its resistance is not so high that Q1 is driven intosaturation.


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Figure 8. Simple 5mA

constant-current generator.

Figure 9. Ground-

referenced variable (1mA-10mA) constant-current generator.

Figure 10. Precision constant-current

generator.

Figure 11. Thermally-stabilized constant-

current generator, using

an LED as a voltagereference.


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These two points thus act as 5mA constant-current terminals. This circuits constant-current value is set by Q1s base voltage and the R2 value, and can be altered byvarying either of these values. Figure 9shows how the basic circuit can be invertedto give a ground-referenced, constant-current output that can be varied from about1mA to 10mA via RV1.

In many practical CCG applications, the circuits most important feature is its highdynamic output impedance or current constancy the precise current magnitudebeing of minor importance in such cases the basic Figure 8 and 9circuits can beused. If greater precision is needed, the reference voltage accuracy must beimproved. One way of doing this is to replace R1 with a 5mA constant-currentgenerator, as indicated in Figure 10by the double circle symbol, so that the zenercurrent (and thus voltage) is independent of supply voltage variations. A red LED acts

as an excellent reference voltage generator, and has a very low temperature coefficient,and can be used in place of a zener, as shown in Figure 11. In this case, the LEDgenerates roughly 2.0V, so only 1.4V appears across R1, which has its value reduced to270R to give a constant-current output of 5mA. The CCG (constant-current generator)circuits in Figures 8 through 11are all three-terminal designs that need bothsupply and output connections. Figure 12shows a two-terminal CCG that consumes afixed 2mA when wired in series with an external load. Here, ZD1 applies 5.6V to thebase of Q1, which (via R1) generates a constant collector current of 1mA this current

Figure 12. Two-terminal 2mA constant-

current generator.

Figure 13. Two-terminal

variable (1mA-10mA) constant-current

generator.


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drives ZD2, which thus develops a very stable 5.6V on the base of Q2 which, in turn,generates a constant collector current of about 1mA, which drives ZD1. The circuit thusacts as a closed-loop current regulator that consumes a total of 2mA. R3 acts as a start-up resistor that provides the transistor with initial base current. Figure 13shows aversion of the two-terminal CCG in which the operating current is fully variable from1mA to 10mA via dual ganged variable resistor RV1. Note that these two circuits eachneed a minimum operating voltage, between their two main terminals of about 12V, butcan operate with maximum ones of 40V.

LINEAR AMPLIFIERS

A common-collector circuit can be used as an AC-coupled linear amplifier by biasing itsbase to a quiescent half-supply voltage value (to accommodate maximal signal swings)and AC-coupling the input signal to its base and taking the output signal from itsemitter, as shown in the basic circuits in Figures 14 and 15. Figure 14shows the

simplest possible version of the linear emitter follower, with Q1 biased via a singleresistor (R1). To achieve half-supply biasing, R1s value must (ideally) equal Q1s input

resistance the biasing level is thus dependent on Q1s h fevalue.

Figure 14. Simple emitter follower. Figure 15. High-stability emitter follower.

Figure 15shows an improved version of the basic circuit in which R1-R2 applies aquiescent half-supply voltage to Q1 base, irrespective of variations in Q1s h fevalues.

Ideally, R1 should equal the paralleled values or R2 and RIN, but in practice, it isadequate to simply make R1 low relative to RIN, and to make R2 slightly larger thanR1.

In these two circuits, the input impedance looking directly into Q1 base equals h fexZload, where (in the basic Figure 14circuit) Zloadis the parallel impedance of R2 andexternal output load ZX. Thus, the base impedance value is roughly 1M0 when ZXisinfinite. The input impedance of the complete circuit equals the parallel impedances ofthe base impedance and the bias network. The circuit in Figure 14gives an input


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impedance of about 500k, and the circuit in Figure 15is about 50k. Both circuits givea voltage gain (AV) that is slightly below unity, the actual gain being given by:

AV= Zload/(Zb+ Zload)

where Zb= 25/Icohms, and where Icis the collector current (which is the same as theemitter current) in mA. Thus, at an operating current of 1mA these circuits give a gainof 0.995 when Zload= 4k7, or 0.975 when Zload= 1k0.

BOOTSTRAPPING

Figure 16. Bootstrapped emitter follower.

Figure 17. Bootstrapped Darlington emitter

follower.


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The Figure 15circuits input impedancecan easily be boosted by using the basic

bootstrapping technique shown inFigure 16. Here, 47k resistor R3 is wired between the R1-R2 biasing network junctionand Q1 base, and the input signal is fed to Q1 base via C1. Note, however, that Q1soutput is fed back to the R2-R2 junction via C2, and near-identical signal voltages thusappear at both ends of R3 very little signal current flows in R3, which appears (to theinput signal) to have a far greater impedance than its true resistance value.

All practical emitter followers give an AVof less than unity, and this value determinesthe resistor amplification factor, or ARof the circuit, as follows:

AR= 1/(1 - AV)

Thus, if the circuit has an AVor 0.995, ARequals 200 and the R3 impedance is almost10M. This impedance is in parallel with RIN, so the Figure 16circuit has an inputimpedance of roughly 900k. The input impedance of the circuit in Figure 16can beincreased even more by using a pair of Darlington-connected transistors in place of Q1,and increasing the value of R3, as shown in Figure 17, which gives a measured inputimpedance of about 3M3.

An even greater input impedance can be obtained by using the bootstrappedcomplementary feedback pair circuit in Figure 18, which gives an input impedance ofabout 10M. In this case, Q1 and Q2 are, in fact, both wired as common emitteramplifiers, but they operate with virtually 100 percent negative feedback and give anoverall voltage gain of almost exactly unity this pair of transistors thus acts like anear-perfect Darlington emitter follower.

COMPLEMENTARY EMITTER FOLLOWERS

Figure 18. Bootstrapped complementaryfeedback pair.

Figure 19. Complementary


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It was pointed out earlier thatan npn emitter follower cansource current, but cannot sinkit, and that a pnp emitterfollower can sink current, butcannot source it; i.e., thesecircuits can handleunidirectional output currentsonly. In many applications, a

bidirectional emitter followercircuit (that can source andsink currents with equal ease)is required, and this action canbe obtained by using acomplementary emitterfollower configuration in whichnpn and pnp emitter followers

are effectively wired in series.Figures 19 to 21show basiccircuits of this type.

The circuit in Figure 19uses adual (split) power supply andhas its output direct-coupled toa grounded load (RL). Theseries-connected npn and pnptransistors are biased at a

quiescent zero volts value via the R1-D1-D2-R2 potential divider, with each transistorslightly forward-biased via silicon diodes D1 and D2, which have characteristicsinherently similar to those of the transistor base-emitter junctions. C2 ensures thatidentical input signals are applied to the transistor bases, and R3 and R4 protect thetransistors against excessive output currents. The circuits action is such that Q1sources current into the load when the input goes positive, and Q2 sinks load currentwhen the input goes negative. Note that input capacitor C1 is a non-polarized type.Figure 20 shows an alternative version of the above circuit, designed for use with asingle-ended power supply and an AC-coupled output load note in this case, that C1is a polarized type.

THE AMPLIFIED DIODE

The Q1 and Q2 circuits in Figures 19 and 20are slightly forward-biased (to minimizecross-over distortion problems) via silicon diodes D1 and D2 in practice, the diodecurrents (and thus the transistor forward-bias voltages) are usually adjustable over alimited range. If these basic circuits are modified for use with Darlington transistorsstages, a total of four biasing diodes are needed in such cases the diodes are usuallyreplaced by a transistor amplified diode stage, as shown by the Q5 circuit in Figure21.

emitter follower using split supply and direct-coupled output

load.

Figure 20. Complementary emitter follower using single-

ended supply and AC-coupled load.


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In the Figure 21circuit, Q5s collector-to-emitter voltage equals the Q5 base-emittervolt drop (about 600mV) multiplied by(RV1+R3)/R3 so, if RV1 is set to zeroohms, 600mV are developed across Q5,which thus acts like a single silicon diode.However, if RV1 is set to 47k, about 3.6V isdeveloped across Q5, which thus acts like sixseries-connected silicon diodes. RV1 can beused to precisely set the Q5 volt drop andthus adjust the quiescent current values ofthe Q2-Q3 output stages.

High-power versions of the basic Figure 21circuit are widely used as the basis of manymodern Hi-Fi audio power amplifiercircuits. Some simple circuits of this type willbe described later in this Bipolar Tran-sistor

Cookbook series.

Figure 21. Darlington complementary

emitter follower, with biasing via an

amplified diode (Q5).


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COMMON-EMITTER AMPLIFIERS

The common-emitter amplifier (also known as the common-earth or grounded-emittercircuit) has a medium value of input impedance and provides substantial voltage gainbetween input and output. The circuits input is applied to the transistors base, and theoutput is taken from its collector the circuits basic operating principles were briefly

described in the opening installment of this eight-part series. The common-emitteramplifier can be used in a wide variety of digital and analog voltage amplifierapplications. This section of the Cookbook series starts off by looking at digital

application circuits.

DIGITAL CIRCUITS

Figure 1shows a simple npncommon-emitter digital amplifier,inverter, or switch, in which theinput signal is at either zero volts or

a substantial positive value, and isapplied to the transistors base viaseries resistor Rb, and the outputsignal is taken from the transistorscollector. When the input is zero,the transistor is cut off and theoutput is at full positive supply railvalue. When the input is high, thetransistor is biased on and acollector current flows via RL, thuspulling the output low. If the input

voltage is large enough, Q1 is driven fully on and the output drops to a saturationvalue of a few hundred mV. Thus, the output signal is an amplified and inverted versionof the input signal.

In Figure 1, resistor Rblimits the input base-drive current to a safe value. The circuitsinput impedance is slightly greater than the Rbvalue, which also influences the rise andfall times of the output signal the greater the Rbvalue, the worse these become. Thissnag can be overcome by shunting Rbwith a speed-up capacitor (typically about 1n0),as shown dotted in the diagram. In practice, Rbshould be as small as possible,consistent with safety and input-impedance requirements, and must not exceed RLxhfe.

FIGURE 1. Digital inverter/switch (npn)


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Figure 2shows a pnp version of thedigital inverter/switch circuit. Q1switches fully on, with its output a fewhundred mV below the positive supplyvalue, when the input is at zero volts,and turns off (with its output at zerovolts) when the input rises to withinless than 600mV of the positive supplyrail value.

The sensitivity of the Figure 1 and 2circuits can be increased by replacingQ1 with a pair of Darlington- or Super-

Alpha-connected transistors.Alternatively, a very-high-gain non-inverting digital amplifier/switch can be

made by using a pair of transistors wired in either of the ways shown in Figures 3 or4.

The Figure 3circuit uses two npn transistors. When the input is at zero volts, Q1 is cutoff, so Q2 is driven fully on via R2, and the output is low (saturated). When the input is

high, Q1 is driven to saturation and pulls Q2 base to less than 600mV, so Q2 is cut offand the output is high (at V+).

FIGURE 3. Very-high-gain non-inverting

digital amplifier/switch using npn

transistors

FIGURE 4. Alternative non-inverting

digital amplifier/switch using an npn-pnp

pair of transistors

The Figure 4circuit uses one npn and one pnp transistor. When the input is at zero

volts, Q1 is cut off, so Q2 is also cut off (via R2-R3) and the output is at zero volts.When the input is high, Q1 is driven on and pulls Q2 into saturation via R3. Under thiscondition, the output takes up a value a few hundred mV below the positive supply railvalue.

FIGURE 2. Digital inverter/switch (pnp)


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Figure 5shows (in basic form)

how a complementary pair ofthe Figure 4circuits can beused to make a DC-motordirection-control network,using a dual power supply. Thecircuit operates as follows.

When SW1 is set to Forward,Q1 is driven on via R1, andpulls Q2 on via R3, but Q3 andQ4 are cut off. The live sideof the motor is thus connected(via Q2) to the positive supplyrail under this condition, andthe motor runs in the forward direction.

When SW1 is set to Off, all four transistors are cut off, and the motor is inoperative.

When SW1 is set to Reverse, Q3 is biased on via R4, and pulls Q4 on via R6, but Q1and Q2 are cut off. The live side of the motor is thus connected (via Q4) to thenegative supply rail under this condition, and the motor runs in the reverse direction.

RELAY DRIVERS

The basic digital circuits of Figures 1 through 4can be used as efficient relay drivers if

fitted with suitable diode protection networks. Figures 6 through 8show examples ofsuch circuits.

FIGURE 5. DC-motor direction-control circuit

FIGURE 6. Simple relay-driving circuit


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The Figure 6circuit raisesa relays current sensitivityby a factor of about 200 (=the current gain oftransistor Q1), and greatlyincreases its voltagesensitivity. R1 gives basedrive protection, and canbe larger than 1k0, ifdesired. The relay is turnedon by a positive inputvoltage.

The current sensitivity ofthe relay can be raised by afactor of about 20,000 byreplacing Q1 with aDarlington-connected pair

of transistors. Figure 7shows this technique usedto make a circuit that canbe activated by placing aresistance of less than 2M0across a pair of stainlessmetal probes. Water,steam, and skin contactshave resistances below thisvalue, so this simple littlecircuit can be used as awater, steam, or touch-

activated relay switch.

Figure 8shows another ultra-sensitive relay driver, based on the Figure 4circuit, that

needs an input of only 700mV at 40A to activate the relay. R2 ensures that Q1 and Q2turn completely off when the input terminals are open circuit.

LINEAR BIASING CIRCUITS

A common-emitter circuit can be used as a linear AC amplifier by applying a DC biascurrent to its base so that its collector takes up a quiescent half-supply voltage value(to accommodate maximal undistorted output signal swings), and by then feeding the

AC input signal to its base and taking the AC output from its collector (as shown inFigure 9).

The first step in designing a circuit of the basic Figure 9type is to select the value ofload resistor R2. The lower this is, the higher the amplifiers upper cut-off frequency willbe (due to the smaller shunting effects of stray capacitance on the effective impedanceof the load), but the higher Q1s quiescent operating current will be. In the diagram, R2

FIGURE 7. Touch, water, or steam-activated relay switch

FIGURE 8. Ultra-sensitive relay driver

(needs an input of 700mV at 40A)


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has a compromise value of 5k6, which gives an upper 3dB down frequency of about120kHz and a quiescent current consumption of 1mA from a 12V supply.

FIGURE 9. Simple npn common-emitter amplifier

FIGURE 10. Common-emitter amplifier with feedback biasing


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To biastheFigure 9circuitsoutput tohalf-supplyvolts, R1needs avalue ofR2 x 2hfe,and(assuming anominal

hfeof 200) this works out at about 2M2 in the example shown. The formula for thecircuits input impedance (looking into Q1 base) and voltage gain are both given in the

diagram. In the example shown, the input impedance is roughly 5k0, and is shunted byR1 the voltage gain works out at about x200, or 46dB.

The quiescent biasing point of the Figure 9circuit depends on Q1s hfevalue. Thisweakness can be overcome by modifying the circuit as shown in Figure 10, wherebiasing resistor R1 is wired in a DC feedback mode between Q1s collector and base,and has a value of R2 x hfe. The feedback action is such that any shift in the outputlevel (due to variations in hfe, temperature, or component values) causes a counter-change in the base-current biasing level, thus tending to cancel the original shift.

The Figure 10circuit has the same values of bandwidth and voltage gain as theFigure 9design, but has a lower total value of input impedance. This is because the

AC feedback action reduces the apparent impedance of R1 (which shunts the 5k0 baseimpedance of Q1) by a factor of 200 (= AV), thus giving a total input impedance of 2k7.If desired, the shunting effects of the biasing network can be eliminated by using twofeedback resistors and AC-decoupling them as shown in Figure 11.

Finally, the ultimate in biasing stability is given by the potential-divider biasing circuitof Figure 12. Here, potential divider R1-R2 sets a quiescent voltage slightly greaterthan V+/3 on Q1 base, and voltage follower action causes 600mV less than this toappear on Q1 emitter. V+/3 is thus developed across 5k6 emitter resistor R3, and(since Q1s emitter and collector currents are almost identical) a similar voltage isdropped across R4, which also has a value of 5k6, thus setting the collector at aquiescent value of 2V+/3. R3 is AC-decoupled via C2, and the circuit gives an ACvoltage gain of 46dB.

CIRCUIT VARIATIONS

Figures 13 to 16show some useful common-emitter amplifier variations. Figure 13

shows the basic Figure 12 design modified to give an AC voltage gain of x10 the gainactually equals the R4 collector load value divided by the effective emitter impedancevalue, which in this case (since R3 is decoupled by series-connected C2-R5) equals the

FIGURE 11. Amplifier with AC-decoupled feedback biasing

FIGURE 12. Amplifier with voltage-divider biasing


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value of the base-emitter junction impedance in series with the paralleled values of R3and R5, and works out at roughly 560R, thus giving a voltage gain of x10. Alternativegain values can be obtained by altering the R5 value.

FIGURE 13. Fixed-gain (x10) common-

emitter amplifierFIGURE 14. Unity-gain phase splitter

Figure 14shows a useful variation of the above design. In this case, R3 equals R4, and

is not decoupled, so the circuit gives unity voltage gain. Note, however, that this circuitgives two unity-gain output signals, with the emitter output in phase with the input andthe collector signal in anti-phase. This circuit thus acts as a unity-gain phase splitter.

FIGURE 15. Alternative fixed-gain

(x10) amplifierFIGURE 16. Wide-band amplifier

Figure 15shows another way of varying circuit gain. This design gives high voltage gain


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between Q1 collector and base, but R2 gives AC feedback to the base, and R1 is wiredin series between the input signal and Q1 base the net effect is that the circuitsvoltage gain (between input and output) equals R2/R1, and works out at x10 in thisparticular case. Finally, Figure 16shows how the Figure 10design can be modified to

give a wide-band performance by wiring DC-coupled emitter follower buffer Q2between Q1 collector and the output terminal, to minimize the shunting effects of straycapacitance on R2, and thus extending the upper bandwidth to several hundred kHz.

HIGH-GAIN CIRCUITS

A single-stage common-emitter amplifier circuit cannot give a voltage gain muchgreater that 46dB when using a resistive collector load a multi-stage circuit must beused if higher gain is needed. Figures 17 to 19show three useful high-gain, two-transistor voltage amplifier designs.

FIGURE 17. High-gain two-stage amplifier FIGURE 18. Alternative high-gaintwo-stage amplifier

The Figure 17circuit acts like a direct-coupled pair of common-emitter amplifiers, with

Q1s output feeding directly into Q2 base, and gives an overall voltage gain of 76dB(about x6150) and an upper -3dB frequency of 35kHz. Note that feedback biasingresistor R4 is fed from Q2s AC-decoupled emitter (which follows the quiescentcollector voltage of Q1), rather than directly from Q1 collector, and that the bias circuitis thus effectively AC-decoupled. Figure 18shows an alternative version of the above

design, using a pnp output stage its performance is the same as that of Figure 17.


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The Figure 19circuit gives a

voltage gain of about 66dB. Q1 isa common-emitter amplifier with asplit collector load (R2-R3), andQ2 is an emitter follower andfeeds its AC output signal back tothe R2-R3 junction via C3, thus

bootstrapping the R3 value (asdescribed in last monthsinstallment) so that it acts as ahigh AC impedance. Q1 thus givesa very high voltage gain. Thiscircuits bandwidth extends up toabout 32kHz, but its input impedance is only 330R.

COMMON-BASE AMPLIFIER CIRCUITS

In a so-called common-base transistor amplifier, the input signal is applied to thetransistors emitter, and the output is taken from the transistors collector. Thecommon-base amplifier has a very low input impedance, gives near-unity current gainand a high voltage gain, and is used mainly in wide-band or high-frequency voltageamplifier applications. Figure 20shows an example of a common-base amplifier thatgives a good wide-band response.

The Figure 20circuit is biased in the same way as Figure 12. Note, however, that thebase is AC-decoupled via C1, and the input signal is applied to the emitter via C3. Thecircuit has a very low input impedan

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Transistor Application Cookbook - Jan Zumwalt