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Diode as Clamper

• A clamping circuit is used to place either the positiveor negative peak of a signal at a desired level.

• The dc component is simply added or subtractedto/from the input signal. The clamper is also referredto as an IC restorer and ac signal level shifter.

• A clamp circuit adds the positive or negative dccomponent to the input signal so as to push it eitheron the positive side (positive clamper) or negativeside (negative clamper).

• For a clamping circuit at least three components — adiode, a capacitor and a resistor are required.Sometimes an independent dc supply is alsorequired to cause an additional shift

• The shape of the waveform will be the same, butits level is shifted either upward or downward.

• The values of the resistor R and capacitor Caffect the waveform.

• The values for the resistor R and capacitor Cshould be determined from the time constantequation of the circuit, t = RC. The values mustbe large enough to make sure that the voltageacross the capacitor C does not changesignificantly during the time interval the diode isnon-conducting. In a good clamper circuit, thecircuit time constant t = RC should be at leastten times the time period of the input signalvoltage

RC charging circuit

Vc=Vs (1-e-t/RC)

Negative clamper

• Discuss the physical structure and operation of the bipolar junction transistor.

• Understand the dc analysis and design techniques of bipolar transistor circuits.

• Investigate various dc biasing schemes of bipolar transistor circuits, including integrated circuit biasing.

BJT: Bipolar Junction Transistor

• Two individual signal diodes back-to-back,give two PN-junctions connected together inseries that share a common p or n terminal.The fusion of these two diodes produces athree layer, two junction, three terminaldevice Bipolar Junction Transistor, or BJT.

• Revolutionized electronics industry from1950-1990

• First BJT was invented in 1947• Responsible for computer age as well as

modern era communication.• Transistors are three terminal active

devices made from different semiconductormaterials.

• Base is very narrow– Emitter ~ 1019 /cm3

– Base ~ 1017 /cm3

– Collector ~ 1015 /cm3

• Electrically unsymmetrical• Four biasing conditions

– Depending on forward or reverse bias

Transistor Resistance Values

Between Transistor Terminals PNP NPN

Collector Emitter RHIGH RHIGH

Collector Base RLOW RHIGH

Emitter Collector RHIGH RHIGH

Emitter Base RLOW RHIGH

Base Collector RHIGH RLOW

Base Emitter RHIGH RLOW

Cross Section of Integrated Circuit npn Transistor

Complex structure

npn BJT in Forward-Active

Understanding the current flow in npn transistor

TBE

TBE

Vv

SV

v

Sc eIeIi ≈

−= 1

Emitter current

Depends on cross-section (10-12 -10-15 A)

TBE

TBE

Vv

EV

v

EE eIeIi 00 1 ≈

−=

• Due to large concentration gradient,electrons injected from emitter diffuse acrossthe base in to B-C space region where EFsweeps them into collector, forming collectorcurrent which depends on the B-E voltage

Collector current

Collector

Base

Emitter

Movement of electrons and holes in npn transistor

• ic <iE ; ic = α iE , where α is called as common base current gain and is less than unity.

• Base Current:– Two components

• Flow of holes• Recombination with majority carriers

(recombination current)

TBE

Vv

B ei ∝

Electrons and Holes in pnp BJT

Circuit Symbols and Current Conventions

Common emitter current gain• Defined as ratio of collector and base current

• Key parameter– Assumed as constant for any given transistor

• 50<β<300• Highly dependent on fabrication

β=B

c

ii

Current Relationships

ααβ

αβ

β

−=

=+=

=+=

1

)1(

EC

BE

BC

BCE

iiii

iiiii

ββα+

=1

• Three possible ways to connect it within anelectronic circuit with one terminal beingcommon to both the input and output.

• Each method of connection respondingdifferently to its input signal within a circuit asthe static characteristics of the transistor varywith each circuit arrangement.– Common Base Configuration – has Voltage Gain

but no Current Gain.

– Common Emitter Configuration – has both Currentand Voltage Gain.

– Common Collector Configuration – has CurrentGain but no Voltage Gain.

Common-Emitter Configurations

Common-Base Configuration

• Current source provide emitter current

Modes of Operation

• Forward-Active– B-E junction is forward biased– B-C junction is reverse biased

• Saturation– B-E and B-C junctions are forward biased

• Cut-Off– B-E and B-C junctions are reverse biased

• Inverse-Active (or Reverse-Active)– B-E junction is reverse biased– B-C junction is forward biased

Current-Voltage Characteristics of a Common-Base Circuit

• B-C/C-B junction forward biased, transistor no longer in forward active mode• B-C/C-B, reverse biased, ic ~ iE• common-base: ideal constant current-source

Cut-off

Saturation

Current-Voltage Characteristics of a Common-Emitter Circuit

VCE ≥ VBE (on)

Finite slope to the curves-due to base-width modulation

- observed by J.M. Early- called as Early effect

Common-base Common-emitter

Early Voltage/Finite Output Resistance

+=

A

CEVv

sc VveIi T

BE

1.

1

constvCE

c

oBE

vi

r=

∂∂

=

C

Ao I

Vr ≅

DC analysis of transistor circuits

• Use of piecewise linear model of pn junction• Assume transistor in forward active mode• Common emitter configuration• VCE>VBE (on)

DC Equivalent Circuit for npn Common Emitter

Example 1.

DC Equivalent Circuit for pnp Common Emitter

Load LineHelps to visualize the characteristics of a transistor circuit

Saturation mode

VBB IB Q-point

As base current continue toincrease, a point is reachedwhere the collector current ICcan no longer increase.

- Transistor is biased insaturation mode

IC/IB < β

Example 2:

circuit Circuit showing values with an assumption of forward active mode

Circuit showing values with an assumption of saturation mode

Problem-Solving Technique: Bipolar DC Analysis

1. Assume that the transistor is biased in forward active modea. VBE = VBE(on), IB > 0, & IC = βIB

2. Analyze ‘linear’ circuit.3. Evaluate the resulting state of transistor.

a. If VCE > VCE(sat), assumption is correctb. If IB < 0, transistor likely in cutoffc. If VCE < 0, transistor likely in saturation

4. If initial assumption is incorrect, make new assumption and return to Step 2.

Voltage Transfer Characteristic for npn Circuit

Voltage Transfer Characteristic for pnp Circuit

BJT Biasing

• Single base resistor biasing• Voltage divider biasing

– Biasing stability

Single Base Resistor Biasing

Common Emitter with Voltage Divider Biasing and Emitter Resistor

• RB is replaced by R1 and R2.

• Emitter resistor is added. AC signal is coupled through Cc.

• Analyzed by Thevenin equivalent circuit for the base circuit.

CCTH VRRRV )/([ 212 +=

21 || RRRTH = (1)

Use of KVL in B-E loop

( ) EEQBETHBQTH RIonVRIV ++=

If the transistor is biased in forward-active mode

( ) BQEQ II β+= 1

(2)

(3)

From (2), the base current can be calculated. Hence, the collector current can be found

The design requirement for bias stability is

The collector current is therefore,

( ) ETH RR β+<< 1

( )( )( ) E

BETHCQ R

onVVIβ

β+−

≅1

( )( )E

BETHCQ R

onVVI −≅

β >> 1, β/(1+β) ~ 1

Example: