Amplifier Basics How Amps Work

7/29/2019 Amplifier Basics How Amps Work

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Amplifier Basics How Amps Work

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Introduction

The term 'amplifier' has become generic, and is often thought (at least by some) to mean a power

amplifier for driving loudspeakers. This is not the case (well, it is, but it is not the onlycase), andthis article will attempt to explain some of the basics of amplification - what it means and how it isachieved. This article is not intended for the designer (although designers are more than welcometo read it if they wish), and is not meant to cover all possibilities. It is a primer, and gives fairlybasic explanations (although some will no doubt dispute this) of each of the major points.

I will explain the basic amplifying elements, namely valves (vacuum tubes), bipolar transistors andFETs, all of which work towards the same end, but do it differently. This article is based on theprinciples of audio amplification - radio frequency (RF) amplifiers are designed differently becauseof the special requirements when working with high frequencies.

Not to be left out, the opamp is also featured, because although it is not a single "component" inthe strict sense, it is now accepted as a building block in its own right.

This article is not intended for the complete novice (although they, too, are more than welcome),but for the intermediate electronics or audio enthusiast, who will have the most to gain from theexplanations given.

Contents Basic Terminology

o Impedance

o Units

Amplification Basicso Input Impedance

o Output Impedance

o Feedback

o Signal Inversion

o Design Phase

Types Of Amplifier Devices

Common Limiting Ratings

Essential Electronics Formulae

Part 1 - Valves (Vacuum Tubes)

o Valve Characteristics

o Valve Current Amplifiero Valve Power Amplifiers

o Summary

Part 2 - Bipolar Transistors

o Transistor Characteristics

o Transistor Current Amplifier

o Transistor Power Amplifiers

o Summary

Part 3 - Field Effect Transistors and MOSFETs

o FET Characteristics

Junction FETs
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MOSFETso FET Current Amplifier

o FET Power Amplifiers

o Summary

Junction FETs MOSFETs

Part 4 - Operational Amplifiers (Opamps)

o Power Opamps Part 5 - Some Basic Linear Circuit Building Blocks

o Current Sources and Sinks

o Current Mirror

o Long Tailed Pair

o Grounded Grid (Gate or Base) Circuits

o Cascode

Part 6 - Conclusions

References Copyright & Update Info

Basic Terminology

Before we continue, I must explain some of the terms that are used. Without knowledge of these,you will be unable to follow the discussion that follows.

Electrical Units

Name Measurement of A.K.A. Symbol

Volt electrical "pressure" voltage V

Ampere the flow of electrons current A, I

Watt power W, P

Ohm resistance to current flow , R

Ohm impedance, reactance , X

Farad capacitance F, C

Henry inductance H, L

Hertz frequency Hz

Note: A.K.A. means "Also Known As". Although the Greek letter omega () is the symbol forOhms, I shall use the word Ohm or the letter "R" to denote Ohms. Any resistance of greater than1,000 Ohms will be shown as (for example) 1k5, meaning 1,500 Ohms, or 1M for 1,000,000Ohms. The second symbol shown in the table is that commonly used in a formula.

When it comes to Volts and Amperes (Amps), we have alternating current and direct current (ACand DC respectively). The power from a wall outlet is AC, as is the output from a CD or tapemachine. The mains from the wall outlet is at a high voltage and is capable of high current, and isused to power the amplifying circuits. The signal from your audio source is at a low voltage andcan supply only a small current, and must be amplified so that it can drive a loudspeaker.

ImpedanceA derived unit of resistance, capacitance and inductance in combination is called impedance,although it is not a requirement that all three be included. Impedance is also measured in Ohms,but is a complex figure, and often fails completely to give you any useful information. The
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impedance of a speaker is a case in point. Although the brochure may state that a speaker has animpedance of 8 Ohms, in reality it will vary depending on frequency, the type of enclosure, andeven nearby walls or furnishings.

UnitsIn all areas of electronics, there are smaller and larger amounts of many things that would be veryinconvenient to have to write in full. For example, a capacitor might have a value of 0.000001F or

a resistor a value of 150,000 Ohms. Because of this, there are conventional units that are appliedto make our lives easier (well, once we are used to using them, anyway). It is much easier to say1uF or 150k (the same as above, but using standard units). These units are described below.

Conventional Metric Units

Symbol Name Multiplication

p pico 1 x 10-12

n nano 1 x 10-9

micro 1 x 10-6

m milli 1 x 10-3

k kilo 1 x 103

M Mega 1 x 106

G Giga 1 x 109

T Tera 1 x 1012

Although commonly written as the letter "u", the symbol for micro is actually the Greek letter mu() as shown. Because this would require the continuing use of a small graphic or unicode, I shalluse the letter "u".

In audio, Giga and Tera are not commonly found (not at all so far - except for specifying the input

impedance of some opamps!). There are also others (such as femto - 1x10 -15) that are extremelyrare and were not included. Of the standard electrical units, only the Farad is so large that thedefacto standard is the microFarad (uF). Most of the others are reasonably sensible in their basicform.

Amplification Basics

The term "amplify" basically means to make stronger. The strength of a signal (in terms ofvoltage) is referred to as amplitude, but there is no equivalent for current (curritude?, nah, soundssilly). This in itself is confusing, because although "amplitude" refers to voltage, it contains theword "amp", as in ampere. Maybe we should introduce "voltitude" - No? Just live with it.

To understand how any amplifier works, you need to understand the two major types ofamplification, and a third "derived" type:

Voltage Amplifier - an amp that boosts the voltage of an input signal

Current Amplifier - an amp that boosts the current of a signal

Power Amplifier - the combination of the above two amplifiers

In the case of a voltage amplifier, a small input voltage will be increased, so that for example a10mV (0.01V) input signal might be amplified so that the output is 1 Volt. This represents a "gain"of 100 - the output voltage is 100 times as great as the input voltage. This is called the voltage

gain of the amplifier.


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In the case of a current amplifier, an input current of 10mA (0.01A) might be amplified to give anoutput of 1A. Again, this is a gain of 100, and is the current gain of the amplifier.

If we now combine the two amplifiers, then calculate the input power and the output power, wewill measure the power gain:

P = V x I (where I = current, note that the symbol changes in a formula)

The input and output power can now be calculated:

Pin = 0.01 x 0.01 (0.01V and 0.01A, or 10mV and 10mA)

Pin = 100uW

Pout = 1 x 1 (1V and 1A)

Pout = 1W

The power gain is therefore 10,000, which is the voltage gain multiplied by the current gain.Somewhat surprisingly perhaps, we are not interested in power gain with audio amplifiers. There

are good reasons for this, as shall be explained in the remainder of this page. Having said this, inreality all amplifiers are power amplifiers, since a voltage cannot exist without power unless theimpedance is infinite or zero. This is never achieved, so some power is always present. It isconvenient to classify amplifiers as above, and no harm is done by the small error of terminology.

Note that a voltage or current gain of 100 is 40dB, and a power gain of 10,000 is also 40dB.

Input ImpedanceAmplifiers will be quoted as having a specific input impedance. This only tells us the sort of load itwill place on preceding equipment, such as a preamplifier. It is neither practical nor useful tomatch the impedance of a preamp to a power amp, or a power amp to a speaker. This will be

discussed in more detail later in this article.

The load is that resistance or impedance placed on the output of an amplifier. In the case of apower amplifier, the load is most commonly a loudspeaker. Any load will require that the source(the preceding amplifier) is capable of providing it with sufficient voltage and current to be able toperform its task. In the case of a speaker, the power amplifier must be capable of providing avoltage and current sufficient to cause the speaker cone(s) to move the distance required. Thismovement is converted to sound by the speaker.

Even though an amplifier might be able to make the voltage great enough to drive a speakercone, it will be unable to do so if it cannot provide enough current. This has nothing to do with its

output impedance. An amplifier can have a very low output impedance, but only be capable of asmall current (an operational amplifier, or opamp is a case in point). This is very important, andneeds to be fully understood before you will be able to fully appreciate the complexity of theamplification process.

Output ImpedanceThe output impedance of an amplifier is a measure of the impedance or resistance "looking" backinto the amplifier. It has nothing to do with the actual loading that may be placed at the output.

For example, an amplifier has an output impedance of 10 Ohms. This is verified by placing a loadof 10 Ohms across the output, and the voltage can be seen to decrease to that with no load.However, unless this amplifier is capable of substantial output current, we might have to make


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this measurement at a very low output voltage indeed, or the amplifier will be unable to drive theload.

Another amplifier might have an output impedance of 100 Ohms, but be capable of driving 10Ainto the load. Impedance and current are completely separate, and must not be seen to be in anyway equivalent. Both of these possibilities will be demonstrated later in this series.

FeedbackFeedback is a term that creates more and bloodier battles between audio enthusiasts than almostany other. Without it, we would not have the levels of performance we enjoy today, and manyamplifier types would be unlistenable without it.

Feedback in its broadest sense means that a certain amount of the output signal is "fed back" intothe input. An amplifier - or an element of an amplifying device - is presented with the input signal,and compares it to a "small scale replica" of the output signal. If there is any difference, the ampcorrects this, and ideally ensures that the output is an exact replica of the input, but with a greateramplitude. Feedback may be as a voltage or current, and has a similar effect in either case.

In many designs, one part of the complete amplifier circuit (usually the input stage) acts as anerror amplifier, and supplies exactly the right amount of signal to the rest of the amp to ensurethat there is no difference between the input and output signals, other than amplitude. This is (ofcourse) an ideal state, and is never achieved in practice. There will always be some difference,however slight.

Signal InversionWhen used as voltage amplifiers, all the standard active devices invert the signal. This meansthat if a positive-going signal goes in, it emerges as a larger - but now negative-going - signal.This does not actually matter for the most part, but it is convenient (and conventional) to try tomake amplifiers non-inverting. To achieve this, two stages must be used (or a transformer) to

make the phase of the amplified signal the same as the input signal.

The exact mechanism as to how and why this happens will be explained as we go along.

Design PhaseThe design phase of an amplifier is not remarkably different, regardless of the type of componentsused in the design itself. There is a sequence that will be used most of the time to finalise thedesign, and this will (or should) follow a pattern.

Power Output vs. Impedance

The power output is determined by the load impedance and the available voltage and

current of the amplifier. An amplifier that is capable of a maximum of 2A output current willbe unable to provide more, just because you want it to. Such an amp will be limited to 16W"RMS" into 8 ohms, regardless of the supply voltage. Likewise, an amp with a supplyvoltage of +/-16V will be unable to provide more than 16W RMS into 8 ohms, regardless ofthe available current. Having more current available will allow the amp to provide (forexample) 32W into 4 ohms (4A peak current) or 64W into 2 ohms (8A peak current), butwill give no more power into 8 ohms than the supply voltage will allow.

Driver CurrentEspecially in the case of bipolar transistors, the driver stage must be able to supply enoughcurrent to the output transistors - with MOSFETs, the driver must be able to charge and

discharge the gate-source capacitance quickly enough to allow you to get the needed


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power at the highest frequencies of interest. With valves, the driver needs to be able tosupply enough current to supply the bias resistors only, since the valve grid draws little orno current (except for the special case of Class-AB2).

For the sake of simplicity, if bipolar output transistors have a gain of 20 at the maximumcurrent into the load, the drivers must be able to supply enough base current to allow this.If the maximum current is 4A, then the drivers must be able to supply 200mA of base

current to the output devices. Prior Stages

The stages that come before the drivers must also be able to supply sufficient current forthe load imposed. The Class-A driver of a bipolar or MOSFET amp must be able to supplyenough current to satisfy the base current needs of bipolar drivers, or the gate capacitanceof MOSFETs.

Again, using the bipolar example from above, the maximum base current for the outputtransistors was 200mA. If the drivers have a minimum specified gain of 50, then their basecurrent will be ...

200 / 50 = 4mA.

Since the Class-A driver must operate in Class-A (what a surprise), it will need to operatewith a current of 1.5 to 5 times the expected maximum driver current, to ensure that itnever turns off. The same applies with a MOSFET amp that will expect (for example) amaximum gate capacitance charge (or discharge) current of 4mA at the highest amplitudesand frequencies.

This is not normally an issue with valve amps, as the early stages of the amp are notloaded with any significant impedance. No further determinations are needed (other thanthe normal loading effects of valve stages in general).

Input StagesThe input stages of all transistor amps must be able to supply the base current of theClass-A driver. This time, a margin of between 2 and 5 times the expected maximum basecurrent is needed. If the Class-A driver needs to supply a quiescent current of (say) 8mA,the maximum current will be 12mA (quiescent + driver base current. Assuming a gain of 50(again), this means that the input stage has to be able to supply 12 / 50 = 240uA, so itmust operate at a minimum current of 240uA * 2 = 480uA to preserve linearity.

Input CurrentThe input current of the first stage determines the input impedance of the amplifier. Usingthe above figures, with a collector current of 480uA, the base current will be 4.8uA for inputdevices with a gain of 100. If maximum power is developed with an input voltage of 1V,

then the impedance is 208k (R = V/I).

Since the stage must be biased, we apply the same rules as before - a margin of between2 and 5, so the maximum value of the bias resistors should be 208 / 2 = 104k. A lowervalue is preferred, and I suggest that a factor of 5 is more appropriate, giving 208 / 5 = 42k(47k can be used without a problem).

These are only guidelines (of course), and there are many cases where currents are greater (orsmaller) than suggested. The end result is in the sound of the amp, and the textbook approach isnot always going to give the expected result. Note that there are some essential simplifications inthe above - it is an overview, and is only intended to give you the basic idea.


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Types Of Amplifier Devices

For the purposes of this article, there are three different types of amplifying devices, and each willbe discussed in turn. Each has its strengths and weaknesses, but all have one common failing -they are not perfect.

A perfect amplifier or other device (known generally as "ideal") will perform its task within certain

set limits, without adding or subtracting anything from the original signal. No ideal amplifyingdevice exists, and as a result, no ideal amplifier exists, since all must be built with real-life (non-ideal) devices.

The amplifying devices currently available are:

Vacuum Tube (Valve)

Bipolar Junction Transistor (BJT)

Field Effect Transistor (FET)

There are also some derivatives of the above, such as Insulated Gate Bipolar Transistors (IGBT),

and Metal Oxide Semiconductor Field Effect Transistors (MOSFET). Of these, the MOSFET is apopular choice among many designers due to some desirable characteristics, and these will becovered in their own section.

All of these devices are reliant on other non-amplifying ("support") components, commonly knownas passive components. The passive devices are resistors, capacitors and inductors, and withoutthese, we would be unable to build amplifiers at all.

All the devices we use for amplification have a variable current output, and it is only the way thatthey are used that allows us to create a voltage amplifier. Valves and FETs are voltage controlleddevices, meaning that the output current is determined by a voltage, and no current is drawn from

the signal source (in theory). Bipolar transistors are current controlled, so the output current isdetermined by the input current. This means that no voltage is required from the signal source,only current. Again, this is in theory, and it is not realisable in practice.

Only by using the support components can we convert the current output of any of theseamplifying devices into a voltage. The most commonly used for this purpose is a resistor.

Common Limiting Ratings

All active devices have certain parameters in common (although they will have different namingconventions depending on the device). Essentially these are ...

Maximum Voltage - The maximum voltage that may be applied between the main

terminals of the device. This varies from perhaps as low as 25V (sometimes even less) forsmall signal transistors and FETS, and up to 1,200V or more for some valves and highvoltage transistors. MOSFET voltages are typically up to about 600 to 800V for switchingdevices for use in power supplies.

Maximum Current - The maximum current that the device may pass safely. Ranges froma few mA up to many amps. This will never be while the device also has the maximumvoltage across it, as this would result in power dissipation far in excess of ...

Maximum Power Dissipation - The maximum power that the device may dissipate (in

mW or W), under any condition of voltage and current. (Called plate dissipation for valves).


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Heater Voltage/Current - (Valves). The operating voltage and / or current for the filament(directly heated cathodes) or heater (for indirectly heated cathodes). This should always bewithin 10% of the quoted value, or cathode life will be severely shortened.

Maximum Junction Temperature - (Semiconductors) The maximum temperature that thesemiconductor die will tolerate without failing. At this temperature, most semiconductorswill be unable to perform any work, as this would raise the temperature above themaximum permissible.

Temperature Derating - (Semiconductors). Above a specified temperature, the allowablepower rating of semiconductor devices must be reduced to remain below the maximumallowable junction temperature. The power is normally derated above 25 C.

Thermal Resistance - (Semiconductors). The thermal resistance between junction andcase (high power) or junction and air (low power). Measured in Degrees C/W, This allows asuitable heatsink to be determined.

This is by no means all of the ratings, there are many more, and vary from device to device.Some MOSFETs for example will have Peak Current ratings, which will be many times thecontinuous rating, but only for very limited time. Bipolar transistors have a Safe Operating Area(SOA) graph, which indicates that in some circumstances you must not operate the device

anywhere near it's maximum power dissipation, or it will fail due to a phenomenon called secondbreakdown (described later).

With most semiconductors, in many cases it will not be possible to operate them at anywherenear the maximum power dissipation, because thermal resistance is such that the heat simplycannot be removed from the junction and into the heatsink fast enough. In these cases, it mightbe necessary to use multiple devices to achieve the performance that can (theoretically) beobtained from a single component. This is very common in audio amplifiers.

Essential Electronics Formulae

There are some things that you just can't get away from, and maths is one of them. (Sorry.) I willonly include the essentials here, but will describe any others that are needed as we go. I am notabout to give a lesson in algebra, but the best reason for ever doing the subject is to learn how totranspose electronics formulae ! Transposition is up to you (unless I am forced to do it for acalculation here or there).

Ohm's LawThe first of these is Ohm's Law, which states that a voltage of 1V across a resistance of 1 Ohmwill cause a current of 1 Amp to flow. The formula is ...

R = V / I (where R = resistance in Ohms, V = Voltage in Volts, and I = current in Amps)

Like all such formulae, this can be transposed (oops, I said I wasn't going to do this, didn't I),

V = R * I (* means multiplied by), andI = V / R

ReactanceThen there is the impedance (reactance) of a capacitor, which varies inversely with frequency (asfrequency is increased, the reactance falls and vice versa).

Xc = 1 / (2 * * f * C)


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where Xc is capacitive reactance in Ohms, (pi) is 3.14159, f is frequency in Hz, and C iscapacitance in Farads.

Inductive reactance, being the reactance of an inductor. This is proportional to frequency.

Xl = 2 * * f * L

where Xl is inductive reactance in Ohms, and L is inductance in Henrys (others as above).

FrequencyThere are many different calculations for this, depending on the combination of components. The-3dB frequency for resistance and capacitance (the most common in amplifier design) isdetermined by

fo = 1 / (2 * * R * C) where fo is the -3dB frequency

When resistance and inductance are combined, the formula is

fo = R / (2 * * L)

PowerPower is a measure of work, which can be either physical work (moving a speaker cone) or (muchmore commonly in audio), thermal work - heat. Power in any form where voltage, current andresistance are present can be calculated by a number of means:

P = V * IP = V / RP = I * R

where P is power in watts, V is voltage in Volts, and I is current in Amps.

Decibels (dB)It has been known for a very long time that human ears cannot resolve very small differences insound pressure. Originally, it was determined that the smallest variation that is audible is 1dB - 1decibel, or 1/10 of 1 Bel. It seems fairly commonly accepted that the actual limit is about 0.5dB,but it is not uncommon to hear that some people can (or genuinely believe they can) resolvemuch smaller variations. I shall not be distracted by this!

dB = 20 * log (V1 / V2)dB = 20 * log (I1 / I2)

dB = 10 * log (P1 / P2)

As can be seen, dB calculations for voltage and current use 20 times the log (base 10) of thelarger unit divided by the smaller unit. With power, a multiplication of 10 is used. Either way, adrop of 3dB represents half the power and vice versa.

There are many others, but these will be sufficient for now. I do not intend this to be a completeelectronics course, so I will give you that which is needed to understand the remainder of thearticle - for the rest, there are lots of excellent books on electronics, and these will have everyformula you ever wanted.

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Amplifier Basics How Amps Work