1. Common Base and Common Collector Amplifiers

2. Reminder: BJT Small Signal ModelC ic ib r
ib ie E

3. Definition of input and output voltage and current directions
iin
iout
Amplifier
vin
vout

4. Common Base Amplifiers
Uses the same stabilised bias scheme as CE configuration.

5. Common Base Amplifiers
Uses the same stabilised bias scheme as CE configuration.
The base is grounded for a.c. signals using a large capacitor.
Signal input at the emitter (between the emitter and the common base)
Signal output at the collector (between the collector and the common base)

6. Common Base Amplifiers
+VCC
GND RE R1 RC RL
vout R2 CB
vin

7. Common Base Amplifiers
We will ignore the a.c. coupled load RL
vin
+VCC
GND RE
vout R1 RC R2 CB

8. CB Amplifier, a.c. equivalent.
iin E C
ic= iout ie
ib
vout
vin RC RE r ib B

9. Common Base Amplifier

10. Common Base Amplifier

11. Common Base Amplifier
Input impedance to transistor

12. Common Base Amplifier iin E C ic= iout ie ib vout vin RC vin RE r ib

13. Common Base Amplifier
Input impedance to stage (whole amplifier)

14. Common Base Amplifier
iin E C
ic= iout ie
ib
vout
vin RC RE r ib B

15. Common Collector Amplifier
Signal input between base and common collector.
Signal output between emitter and common collector.

16. Common Collector Amplifier
VCC R1 R2
vin RE
vout

17. CC Amplifier, a.c. equivalent circuitvs RS
ib r B C ib io RE
RB = R1//R2
vin
vout

18. Common Collector Amplifier

19. Common Collector Amplifier
Transistor input impedance =

20. Common Collector Amplifier

21. Common Collector Amplifier

22. Common Collector Amplifier
To find the output impedance we note that:
Hence

23. Common Collector Amplifier
The voltage gain is slightly less than unity.
The CC circuit is sometimes known as an emitter follower since the output voltage at the emitter follows the input voltage at the base.

24. Common Collector Amplifier
The prime function of this circuit is to connect a high resistance source to a low resistance load (“a buffer amplifier”).
Since voltage gain is approximately one this is achievable as the current gain is high.