1. Solid-State Devices & Circuits
ECE 342
Solid-State Devices & Circuits
14. CB and CG Amplifiers
Jose E. Schutt-Aine
Electrical & Computer Engineering
University of Illinois

2. Common Gate Amplifier
Substrate is not connected to the source must account for body effect
Drain signal current becomes
And since
sedr42021_0627a.jpg
Body effect is fully accounted for by using

3. Common Gate Amplifier

4. Common Gate Amplifier

5. Common Gate Amplifier
Taking ro into account adds a component (RL/Ao) to the input resistance.
The open-circuit voltage gain is:
The voltage gain of the loaded CG amplifier is:

6. CG Output Resistance

7. CG Amplifier as Current Buffer
sedr42021_0630.jpg
Gis is the short-circuit current gain

8. High-Frequency Response of CG
- Include CL to represent capacitance of load
- Cgd is grounded
- No Miller effect
sedr42021_0631a.jpg

9. High-Frequency Response of CG
2 poles:
fP2 is usually lower than fp1
fP2 can be dominant
Both fP1 and fP2 are usually much higher than fP in CS case

10. CB Amplifier
sedr42021_0633a.jpg

11. High-Frequency Analysis of CB Amplifier
The amplifier’s upper cutoff frequency will be the lower of these two poles.
From current gain analysis

12. MOS Cascode Amplifier
Common source amplifier, followed by common gate stage – G2 is an incremental ground

13. MOS Cascode Amplifier CS cacaded with CG  Cascode
Very popular configuration
Often considered as a single stage amplifier
Combine high input impedance and large transconductance in CS with current buffering and superior high frequency response of CG
Can be used to achieve equal gain but wider bandwidth than CS
Can be used to achieve higher gain with same GBW as CS

14. MOS Cascode Incremental Model

15. MOS Cascode Analysis
KCL at vs2

16. MOS Cascode Analysis
Two cases

17. MOS Cascode Analysis
CASE 1
The voltage gain becomes

18. MOS Cascode Analysis
CASE 2
The voltage gain becomes

19. MOS Cascode at High Frequency
The upper corner frequency of the cascode can be approximated as:

20. MOS Cascode at High Frequency
Capacitance Cgs1 sees a resistance Rsig
Capacitance Cgd1 sees a resistance Rgd1
Capacitance (Cdb1+Cgs2) sees resistance Rd1
Capacitance (CL+Cgd2) sees resistance (RL||Rout)

21. MOS Cascode at High Frequency
If Rsig is large, to extend the bandwidth we must lower RL. This lowers Rd1 and makes the Miller effect insignificant
If Rsig is small, there is no Miller effect. A large value of RL will give high gain

22. Effect of Cascoding
(when Rsig=0)
sedr42021_0639abc.jpg

23. Cascode Example

24. Cascode Example The internal conductance of the current source is:
The cascode circuit has a dc drain current of 50 mA for all transistors supplied by current mirror M3. Parameters are gm1=181 mA/V, gm2=195 mA/V, gds1= 5.87 mA/V, gmb2=57.1 mA/V, gds2= mA, gds3= 3.76 mA/V, Cdb2 = 9.8 fF, Cgd2 = 1.5 fF, Cdb3=40.9 fF, Cgd3= 4.5 fF. Find midband gain and approximate upper corner frequency
The internal conductance of the current source is:
Therefore, we use Case 2 to compute the gain

25. Cascode Example Gain can be approximated by
Upper corner frequency is approximated by

26. BJT Cascode Amplifier
Common emitter amplifier, followed by common base stage – Base of Q2 is an incremental ground

27. BJT Cascode Incremental Model

28. BJT Cascode Analysis
Ignoring rx2

29. BJT Cascode Analysis

30. BJT Cascode Analysis
If Rs << rx1+rp1, the voltage gain can be approximated by

31. BJT Cascode at High Frequency
Define rcs as the internal impedance of the current source. R includes generator resistance and base resistance rx1 base of Q1

32. BJT Cascode at High Frequency
There is no Miller multiplication from the input of Q2 (emitter) to the output (collector).

33. BJT Cascode at High Frequency

34. BJT Cascode at High Frequency

35. Cascode Amplifier – High Frequency
High-frequency incremental model

36. Cascode Amplifier – High Frequency
Applying Kirchoff’s current law to each node:
Find solution using a computer

37. Cascode Amplifier – High Frequency
As an example use:
gm=0.4 mhos b=100
rp=250 ohms rx=20 ohms
Cp=100 pF Cm=5 pf
GL=5 mmhos GS’=4.5 mmhos
ZEROS
POLES
sa=8.0 ns sd= nsec-1
sb= j se=-0.644
sc=-2.02 – j sf=-4.05
sg=-16.45

38. Cascode Amplifier – High Frequency
If one pole is at a much lower frequency than the zeros and the other poles, (dominant pole) we can approximate w3dB
For the same gain, a single stage amplifier would yield:
Second stage in cascode increases bandwidth

39. CE Cascade Amplifier Exact analysis too tedious  use computer
CE cascade has low upper-cutoff frequency

40. Cascode Amplifier First stage is CE and second stage is CB
If Rs << rp1, the voltage gain can be approximated by

41. Cascode Amplifier – High Frequency
High-frequency incremental model

42. Cascode Amplifier – High Frequency
Applying Kirchoff’s current law to each node:
Find solution using a computer

43. Cascode Amplifier – High Frequency
As an example use:
gm=0.4 mhos b=100
rp=250 ohms rx=20 ohms
Cp=100 pF Cm=5 pf
GL=5 mmhos GS’=4.5 mmhos
ZEROS (nsec-1)
POLES (nsec-1)
sa= sd=
sb= j se=-0.644
sc=-2.02 – j sf=-4.05
sg=-16.45

44. Cascode Amplifier – High Frequency
If one pole is at a much lower frequency than the zeros and the other poles, (dominant pole) we can approximate w3dB
For the same gain, a single stage amplifier would yield:
Second stage in cascode increases bandwidth