1. MALVINO
Electronic
PRINCIPLES
SIXTH EDITION

2. Voltage Amplifiers
Chapter 10

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

4. 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

5. T model of the common-emitter amplifierrc
re’
This model best illustrates that RC RL
vout ic ie R1 R2
re’
rc = RC RL
vin ie
vout
A =
vin

6. 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’.

7. 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

8. To get more gain, a cascade amplifier can be used.
Stage 1 A1
Stage 2 A2 RL
vout
vin
zin(stage 2)
The 2nd stage loads the 1st stage:
rc1 = RC1 zin(stage 2)
The overall voltage gain: A = A1A2

9. ac equivalent circuit for the two-stage amplifierRg
zin(stage 2) RC RL
zin(stage 1) ic RC ic vg

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

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

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

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

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

15. Emitter ac feedback 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

16. Two-stage feedback is another possibility.RL
vout
vin rf
The feedback signal can be connected to
the emitter end of resistor re in stage 1.

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

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

19. Troubleshooting
Open coupling and/or bypass capacitors will not cause dc error.
Shorted coupling and/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.

20. 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 inject power supply ripple into the amplifier.