Lecture 6 Active Load Amplifier Multistage Amplifier Circuit.
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Transcript of Lecture 6 Active Load Amplifier Multistage Amplifier Circuit.
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