Lect. 24: CG and Cascode Amplifiers (Razavi 7.3 and 9.1)
Common-Gate Amplifier Increase in Vin Decrease in VGS
Decrease in ID Increase in Vout
ID: - gmVin = g V R Vout: - IDRD m in D
Gain: gmRD
(Without rO consideration)
With small-signal circuit model
Rin: 1/gm
Rout: RD
Electronic Circuits 1 (13/2) Prof. Woo-Young Choi Lect. 24: CG and Cascode Amplifiers
Common-Gate Amplifier with RS With small-signal model:
(Ignore rO) Voltage Gain: gmRD / (1 + gmRS)
VX = 1/gm / (1/gm + Rs)Vin
= 1 / (1+ gmRs)Vin
Vout= gmRDVX
Input loading due to small Rin
Rin: 1/gm
Rout = RD
Electronic Circuits 1 (13/2) Prof. Woo-Young Choi Lect. 24: CG and Cascode Amplifiers
Common-Gate Amplifier with RS and rO
It can be shown
Rin = (1 + RD/rO )/ (gm + 1/rO)
If rO > RD, Rin 1/ gm
Rout = [(1+gmrO) + rO] Rs || RD
gm rO Rs || RD
Voltage Gain = (gm + 1/rO) Rout
gm Rout What is this good for?
Electronic Circuits 1 (13/2) Prof. Woo-Young Choi Lect. 24: CG and Cascode Amplifiers
Current amplifier
Consider an equivalent circuit for a current amplifier
Ais: Short-circuit current gain
Small Ri and large Ro desired
Electronic Circuits 1 (13/2) Prof. Woo-Young Choi Lect. 24: CG and Cascode Amplifiers
CG has small Rin and large Rout
Ais 1
CG: Current buffer
Electronic Circuits 1 (13/2) Prof. Woo-Young Choi Lect. 24: CG and Cascode Amplifiers
Small signal model with current buffer for CG
Rin :
Rout :gm2 rO2 Rs||RD gm2 rO2 Rs gm2 rO2 rO1
Voltage Gain: - gm1 gm2 rO1 rO2
If Q and Q are identical under same bias CS + CG 1 2 2 R ~ Ar Cascode Amplifier Agvo~( mr o ) out v2 o
Electronic Circuits 1 (13/2) Prof. Woo-Young Choi Lect. 24: CG and Cascode Amplifiers
CS vs Cascode Amplifier
Rout: ro (gmro)ro
2 Avo: - gmro -(gmro)
Can be faster for the same voltage gain
Electronic Circuits 1 (13/2) Prof. Woo-Young Choi Lect. 24: CG and Cascode Amplifiers
CS CG
Cgd between input and output No direct capacitor between input and output
Miller effect! No Miller effect!
Cascode: Smaller gain in CS stage is sufficient for achieving the target gain For the same overall gain, cascode is faster than CS!
Electronic Circuits 1 (13/2) Prof. Woo-Young Choi Lect. 24: CG and Cascode Amplifiers
CS vs Cascode Amplifier
R : r (g r )r out o m o o Disadvantages:
2 Avo: - gmro -(gmro) - Smaller voltage headroom
Can be faster for the same voltage gain
Electronic Circuits 1 (13/2) Prof. Woo-Young Choi Lect. 24: CG and Cascode Amplifiers
Folded Cascode Double Cascode
With BJT, CE + CB Cascode
Electronic Circuits 1 (13/2) Prof. Woo-Young Choi Lect. 24: CG and Cascode Amplifiers
Homework:
Determine W/L and Vb2 so that Rout=200k in the following cascode circuit. 2 Assume Vt= 0.4V, kn’(=nCox)=100A/V , rO=10/Ibias () for both transistors Use Ibias = 0.5mA, (W/L)1= (W/L)2 and the approximate expression for cascode Rout.
Electronic Circuits 1 (13/2) Prof. Woo-Young Choi