Institute of Technology and Management Vadodara. Presentation By :- Malakar Devesh D. En No Electronic & Communication Eng. 2 nd year / 3 rd Sem ELECTRONIC DEVICES AND CIRCUITS : Krutika Newad En No Paras Bhrambhatt En No

Voltage Amplifiers

Voltage gain AAc output voltage divided by ac input voltage CCan be derived by using the  model of a transistor AAc collector resistance divided by the ac resistance of the emitter diode

A VDB common-emitter amplifier +V CC RERE R2R2 RCRC R1R1 RLRL v in v out

re’re’ R2R2 RCRC R1R1 RLRL v in  model of the common-emitter amplifier icic ibib v out z in(stage) = R 1 R 2  r e ’ This model best illustrates that

re’re’ R2R2 R1R1 v in T  model of the common-emitter amplifier icic RCRC RLRL v out ieie r c = R C R L A V = rcrc re’re’ This model best illustrates that v out v in A V = i c  i e

Calculating voltage gain  Solve the dc circuit to find the emitter current.  Use the emitter current to find r e ’.  Combine R C and R L to find r c.  Divide r c by r e ’.

Loading effect of input impedance  An ideal ac voltage source has zero source resistance  The input impedance of a stage includes biasing resistors and base input impedance  When the ac source is not stiff, the input voltage is less than the source voltage

Real signal sources are not ideal. vgvg RgRg z in(stage) v in When a source is not stiff, use: v in = z in(stage) + z in(stage) RgRg vgvg

Multistage amplifier TTwo or more amplifiers cascaded PProvides increased gain TTwo CE stages produce an amplified in-phase signal The overall voltage gain: A V = A V1 A V2 To get more gain, a cascade amplifier can be used. Stage 1 A V1 Stage 2 A V2 v in RLRL v out z in(stage 2)

vgvg RgRg z in(stage 1) RCRC icic RCRC icic RLRL Ac equivalent circuit for the two-stage amplifier z in(stage 2) The 2nd stage loads the 1 st stage: R c1 = R c1 ║ Z in (stage 2)

Swamped amplifier SSome of the emitter resistance is unbypassed to get negative feedback VVoltage gain is stabilized IInput impedance is increased LLarge-signal distortion is decreased

+V CC rErE R2R2 RCRC R1R1 RLRL v in v out RERE ac feedback resistor Swamped amplifier circuit

re’re’ R2R2 R1R1 v in T  model of the swamped common-emitter amplifier icic RCRC RLRL v out rere A V = rcrc r e ’ + r e z in(base) =  (r e ’ + r e ) Emitter feedback decreases the gain and increases the impedance.

V BE IEIE Q Large signals produce distortion since r e ’ is non-linear. Input signal r e ’ = v be ieie

re’re’ R2R2 R1R1 v in icic RCRC RLRL v out rere This resistor is linear and can swamp r e ’.

vbvb ieie Q v b versus i e in a swamped amplifier Input signal r e ’ + r e

Emitter ac feedback  A swamped amplifier is an example of single- stage feedback  Decreases voltage gain (but the gain is more stable)  Increases the base input impedance  Decreases large-signal distortion

Two-stage feedback Stage 1Stage 2 v in RLRL v out rfrf The feedback signal can be connected to the emitter end of resistor r e in stage 1.

+V CC rErE R2R2 RCRC R1R1 v in RERE To stage 2 Feedback from stage 2 Stage 1

Voltage gain with two-stage feedback: Stage 1Stage 2 v in RLRL v out rfrf A V = rfrf r e(stage 1) + 1

CC amplifier KKnown as an emitter follower TThe collector is at ac ground SStable voltage gain HHigh input impedance LLow distortion

+V CC RERE R2R2 R1R1 RLRL v in The common-collector or emitter follower amplifier v out ac ground

re’re’ R2R2 R1R1 v in T  model of the emitter follower amplifier v out = i e r e rere r e = R E R L v in = i e (r e + r e ’ ) rere r e + r e ’ A V =

rere R2R2 R1R1 v in  model of the emitter follower amplifier v out  (r e + r e ’ ) z in(stage) = R 1 R 2  (r e + r e ’ )

RCRC RLRL RCRC RLRL v th The output side of a common-emitter amplifier Applying Thevenin’s theorem: The output impedance is equal to R C.

re’re’ R2R2 R1R1 RGRG T  model of the emitter follower amplifier RERE RLRL A v th z out RLRL A Apply Thevenin’s theorem to point A:

Output impedance of the emitter follower amplifier re’ +re’ + R 1 R 2 R G  ( ) z out = R E The current gain of the amplifier steps down the impedance of the base circuit. Thus, the output impedance of this amplifier is small.

Common-collector buffer AA common-emitter (CE) stage with a low resistance load overloads with a small voltage gain AA common-collector (CC) stage placed between a CE stage and the load reduces overload and acts as a buffer

Darlington pair  Two transistors connected with the emitter of the first transistor connected to the base of the second transistor  The overall gain is equal to the product of the individual current gains

Darlington connection Q1Q1 Q2Q2  =     Darlington transistor Darlington transistors are manufactured with transistors in one case

Zener follower  A combination of a zener diode and an emitter follower  Provides a regulated output voltage with a large load current  The zener current is much smaller than the load current

Zener follower circuit +V CC RLRL RSRS V out VZVZ V out = V Z - V BE I B = I out  dc z out = r e ’ +  dc RZRZ

R1R1 Two-transistor voltage regulator VZVZ R2R2 Q1Q1 Q2Q2 R3R3 R4R4 RLRL +V in V out V out = R 3 + R 4 R4R4 (V Z + V BE )

Common-base amplifier  The base is at ac ground  The input drives the emitter and the output is taken from the collector  No current gain but has a high voltage gain  Low input impedance  High output impedance  High frequency applications

CB amplifier circuit

Comparison of Amplifier Configurations

Numerical Comparison of Amplifier configurations for the Same Transistor and DC Biasing

Thank You