1. Common-Base Amplifier

2. Common-Base Amplifier

3. Response of CB Stage to small Input Change

4. Input Impedance

5. Application of CB Stages

6. Response to a Source of Finite Source Resistance

7. Output Impedance

8. Input Impedance

9. General CB (VA=infinity, RB>0)
Base resistance (RB) decreases voltage gain.

11. CB Stage with Biasing

12. Design Goal
Goal: i2>>i1

13. Bypass Capacitor
(Base resistance decreases gain!)

14. Design Guideline Given: Rin, AV Select RE>>(1/gm). gm →IC.
RE=10Rin
Av=gmRC →RC
Vb=IERE+VBE
Let I(R1)=40IB →(R1+R2)
R2 is known from Vb and R1.
1/(C1ω)=(1/gm)/20
1/(beta+1)/(C1 ω)=(1/gm)/20

15. Common Base Amplifier

16. ADS Simulation ADS Analysis Vs,pp(mV) 4 Vin,pp(mV) 2 1.90 Vout,pp(mV)17
N/A
Vout,pp/Vin,pp
8.5 11
Vout,pp/Vs,pp
4.25
5.23

17. Common Base and Common Collector Amplifiers

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

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

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

21. 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)

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

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

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

25. Common Base Amplifier

26. Common Base Amplifier

27. Common Base Amplifier
Input impedance to transistor

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

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

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

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

32. Common Collector Amplifier
VCC R1 R2
vin RE
vout

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

34. Common Collector Amplifier

35. Common Collector Amplifier
Transistor input impedance =

36. Common Collector Amplifier

37. Common Collector Amplifier

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

39. 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.

40. 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.