[SI-LIST] Re: Compensation scheme in two-stage opamp
- From: Raymond.Leung@xxxxxxxxxxx
- To: si-list@xxxxxxxxxxxxx
- Date: Thu, 20 Nov 2003 16:19:20 +1000
Yawei,
Likely this is the second time you have posted the same issue. I
will try to contribute my 2 cents.
If you have nmos input transistors in the first stage, then the load transistors
are pmos. If the decoupling cap is tied between the output of the 1st stage
and the vdd rail, then the PSRR of the vdd rail should be lesser than the
Miller configuration because the noise coupled to the drain/gate of the
diode-connected pmos will be in phase with the noise coupled to the
1st stage output and they will cancel each other. Of course if the
decoupling cap is connected to gnd the situation will be very different.
Since this topic is a bit off SI we can discuss it offline later.
Regards,
Raymond
"Guo Yawei" <ywguo527@xxxxxxxxxxx> on 20/11/2003 12:27:06
Please respond to ywguo527@xxxxxxxxxxx
To: si-list@xxxxxxxxxxxxx
cc: (bcc: Raymond Leung/sdc)
Subject: [SI-LIST] Compensation scheme in two-stage opamp
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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