[SI-LIST] Compensation scheme in two-stage opamp
- From: "Guo Yawei" <ywguo527@xxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Thu, 20 Nov 2003 10:27:06 +0800
Hello,
This is a question about frequency compensation scheme in two-stage opamp.
The most direct and simple scheme in my opinion is Miller compensation.
However, I have some confusions about the capacitors that I can use.
First, if I use Poly to Poly cap, it is easy to emplement. The parasitic
capacitor is small and has insignificant influence on the frequency
response of the opamp.
Second, to reduce cost I need to avoid poly to poly cap, the sanwich
capacitor implemented with Metal1-Metal2-Metal3 is an alternative. Though
its parasitic cap is small, it may occupy much larger area than poly to
poly cap.
Third, if I use MOS transistor as Miller capacitor, in which the bottom
plate is tied to the output of the opamp. I want to know the nonideal
influence of the parasitic cap on the frequency response of the opamp.
Last, if I give up Miller compensation, just put a MOS transistor on the
output of the first stage as compensation capacitor, in which the source
and drain of the transistor are tied to ground. But it may deteriorate
negaive PSRR. Though the opamp using Miller compensation has poor PSRR,
this sompensation scheme seems to have worse negative PSRR than does Miller
compensation because it has a much larger cap which connect between the
output of the first stage and ground.
Who can evaluate the above compensation schemes, or advise any better
compensation scheme?
Thanks.
Yawei Guo
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