Lecture 6

Active Load AmplifierMultistage Amplifier Circuit

Differential to Single-Ended Conversion

Reason for converting the signal from differential to single ended—to drive an offchip load.

A Simple Circuit for Differential to Single-Ended Conversion

Problem: the drain signal current of Q1 is wasted!

If Q4 is perfectly matched to Q3

I(Q3)=I(Q4)=I/2Vo=VDD-VSG3

If Q4 is NOT perfectly matched to Q3

• If I(Q3) is I/2+ and I(Q2) is I/2• will flow into the high output resistances of

Q2 and Q4.• The DC output voltage will deviate significantly

from VDD-VSG3

• Need a common mode Feedback circuit

Small Signal Operation

See next slide

Virtual Ground

Common Drain Gain: (Drain Resistance)/(1/gm+Source Resistance)

Low Res intothe drain of Q3

High Res intothe drain of Q4

Not a virtual ground (Strictly Speaking)

Analysis Technique

Short circuit transconductance

Output resistance

Short Circuit Transcondutance

Virtual Ground Justification:1. The drain of Q4 is grounded!2. The resistance into Q3 is low! (e.g. 1/gm3)3. Therefore, we have an approximate virtual ground.

Approximations:1. ro1>>1/gm3

2. ro3>>1/gm3

Implications of the short at the output:3. ro4 is shorted out.4. ro2 is shorted out.

Output Resistance

A 2-Stage CMOS Op-Amp

A Bias Circuit that Stabilizes gm

Materials Not Covered in Class

CMRR Circuit