Electronic PRINCIPLES MALVINO & BATES Electronic PRINCIPLES SEVENTH EDITION

Chapter 10 Voltage Amplifiers

Topics Covered in Chapter 10 Voltage gain The loading effect of input impedance Multistage amplifiers Swamped amplifier Two-stage feedback Troubleshooting

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

A VDB common-emitter amplifier +VCC RE R2 RC R1 RL vin vout

p model of the common-emitter amplifier This model best illustrates that zin(stage) = R1 R2 bre’ ib vin vout R1 R2 bre’ ic RC RL

T model of the common-emitter amplifier AV = rc re’ This model best illustrates that RC RL vout ic ic @ ie R1 R2 re’ rc = RC RL vin ie vout AV = vin

Calculating voltage gain Solve the dc circuit to find the emitter current. Use the emitter current to find re’. Combine RC and RL to find rc. Divide rc by re’.

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. Rg vin vg zin(stage) When a source is not stiff, use: vin = zin(stage) + zin(stage) Rg vg

Multistage amplifier Two or more amplifiers cascaded Provides increased gain Two CE stages produce an amplified in-phase signal

To get more gain, a cascade amplifier can be used. Stage 1 AV1 Stage 2 AV2 RL vout vin zin(stage 2) The overall voltage gain: AV = AV1AV2

Ac equivalent circuit for the two-stage amplifier Rg zin(stage 2) RC RL zin(stage 1) ic RC ic vg The 2nd stage loads the 1st stage: Rc1 = Rc1 ║ Zin (stage 2)

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

Swamped amplifier circuit +VCC Swamped amplifier circuit RC R1 vout RL vin rE R2 ac feedback resistor RE

T model of the swamped common-emitter amplifier Emitter feedback decreases the gain and increases the impedance. RC RL vout ic vin AV = rc re’ + re R1 R2 re’ zin(base) = b(re’ + re) re

Large signals produce distortion since re’ is non-linear. vbe ie IE Q VBE Input signal

This resistor is linear RC RL vout ic vin R1 R2 re’ This resistor is linear and can swamp re’. re

vb versus ie in a swamped amplifier re’ + re Q vb Input signal

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 1 Stage 2 RL vout vin rf The feedback signal can be connected to the emitter end of resistor re in stage 1.

+VCC Stage 1 RC R1 To stage 2 Feedback from stage 2 vin rE R2 RE

Voltage gain with two-stage feedback: RL vout vin rf AV = rf re(stage 1) + 1

Troubleshooting Open coupling or bypass capacitors will not cause dc error. Shorted coupling or bypass capacitors will cause dc error. An open coupling capacitor will cause a loss of signal. An open emitter bypass capacitor will cause the gain to decrease.

More troubleshooting The supply line is normally an ac ground. The supply line will not be an ac ground if a bypass or filter capacitor opens. An open bypass may allow other signals to affect the amplifier. An open filter capacitor will allow power supply ripple to get into the amplifier.