Lect. 24: CG and Cascode (Razavi 7.3 and 9.1)

Common-Gate Increase in Vin  Decrease in VGS

 Decrease in ID  Increase in Vout

ID: - gmVin = 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

!  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)=100A/V , rO=10/Ibias () for both 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