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MALVINO & BATES Electronic PRINCIPLES SEVENTH EDITION Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter Voltage Amplifiers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 10 Topics Covered in Chapter 10 • • • • • • Voltage gain The loading effect of input impedance Multistage amplifiers Swamped amplifier Two-stage feedback Troubleshooting Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. A VDB common-emitter amplifier +VCC R1 RC RL vout vin R2 RE Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. p model of the common-emitter amplifier This model best illustrates that zin(stage) = R1 R2 bre’ ib vin R1 R2 bre’ ic RC Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. RL vout T model of the common-emitter amplifier rc This model best illustrates that AV = r ’ e ic vin R1 R2 re’ RC RL vout ic @ ie ie rc = RC RL vout AV = v in Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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’. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Real signal sources are not ideal. Rg zin(stage) vin vg When a source is not stiff, use: zin(stage) vin = Rg + zin(stage) vg Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Multistage amplifier • Two or more amplifiers cascaded • Provides increased gain • Two CE stages produce an amplified inphase signal Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. To get more gain, a cascade amplifier can be used. Stage 1 AV1 vin Stage 2 AV2 zin(stage 2) The overall voltage gain: AV = AV1AV2 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. RL vout Ac equivalent circuit for the two-stage amplifier Rg zin(stage 2) zin(stage 1) ic RC RC ic RL vg The 2nd stage loads the 1st stage: Rc1 = Rc1 ║ Zin (stage 2) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Swamped amplifier circuit +VCC R1 RC RL vout vin R2 rE ac feedback resistor RE Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. T model of the swamped common-emitter amplifier Emitter feedback decreases the gain and increases the impedance. vin R1 R2 ic re’ re RC RL vout rc AV = r ’ + r e e zin(base) = b(re’ + re) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Large signals produce distortion since re’ is non-linear. re’ = vbe IE ie Q VBE Input signal Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ic vin R1 R2 RC RL vout re’ re This resistor is linear and can swamp re’. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. vb versus ie in a swamped amplifier ie Q re’ + re vb Input signal Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Two-stage feedback Stage 1 vin Stage 2 RL vout rf The feedback signal can be connected to the emitter end of resistor re in stage 1. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. +VCC Stage 1 R1 RC To stage 2 Feedback from stage 2 vin R2 rE RE Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Voltage gain with two-stage feedback: Stage 1 Stage 2 RL vout vin rf AV = rf re(stage 1) +1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 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. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.