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Allen and Holberg - CMOS Analog Circuit Design

Problems with the two-stage OTA:

•Insufficient gain

•Poor stability for large load capacitance

•Poor PSRR

These problems can be addressed using various cascode structures.

We will consider several approaches:

•Cascoding the first stage

•Cascoding the second stage

•Folded cascode

Allen and Holberg - CMOS Analog Circuit Design

First Stage Cascode

VBIAS

VSS

ro1 ≈ (gmc2rdsc2)rds4 || (gmc1rdsc1)rds2

Gain ≈ gm2ro1

•Requires voltage translation to drive next stage

•Requires additional biasing for cascode devices

•Common-mode problem at drains of M1 and M2

Allen and Holberg - CMOS Analog Circuit Design

First Stage Cascode - Continued

Common-mode improvement:

VBIAS

VSS

Common-mode circuitry (M9) maintains Vds of M1 and M2

AV = gm1ro1

ro1 ≈ (gmc2rdsc2)rds4 || (gmc1rdsc1)rds2

p1 ≈ -1 CLro1

Output range of this amplifier is poor when used by itself. It needs an output stage to be practical.

Allen and Holberg - CMOS Analog Circuit Design

First Stage Cascode - Continued

Implementation of ICM

I5+ICM

VSS

Allen and Holberg - CMOS Analog Circuit Design

Level Translator for First Stage Cascode

Vo

VSS

M2

M7

VBIAS

M5

VSS

Allen and Holberg - CMOS Analog Circuit Design

Improved PSRR For Two-Stage OTA

Use cascode to reject Cc feedforward

VDD

VSS

+PSRR is reduced by M9

positive input common mode range is restricted

Allen and Holberg - CMOS Analog Circuit Design

Complete Two Stage Cascode