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@ unity gain you must have a certain margin in phase in order to maintain stability.
I have heard many people say 45 deg is the magic number, but I usually aim for about 60 deg.
Lets take 60 deg as an example.
You need to have a margin of 60 deg or more from the nearest multiple of 180 deg @ unity gain freq.
You should be at about -120 deg. Also, depending on the application, a phase reversal might be bad as well.
If you tell me what topology of OTA you are implementing, maybe I can tell you how to improve phase margin.
i feel there is something wrong with simulation setup..Usually we try to measure phasemargin for -ve f/b loop...I assume you are feeding ac signal in the negative feedback loop..so at dc itself output is inverted i.e 180' phaseshift,but you are saying PM=-180' which means there is around 360' phaseshift by the loop..in this case loop becomes positive and system becomes unstable...
Oh... in that case it`s fine.
The amplifier will be stable, with a phase margin of 72 deg @ unity gain.
The problem is that a big phase margin usually means less bandwidth,
so just check that the bandwidth is also within spec.
a phase margin of 72 deg @ unity gain is large ? thanks anyway.
another problem is a PSRR of 33 db , is too small? when I apply my designed OPA with PSRR of 33db to a practical circuit, the output voltage is flutating.
From the low PSRR I'm guessing you are designing a Miller compensated two stage opamp?
If not, tell me what topology you are using, since dealing with PSRR depends on the topology.
From the low PSRR I'm guessing you are designing a Miller compensated two stage opamp?
If not, tell me what topology you are using, since dealing with PSRR depends on the topology.
I was talking about the inside of the amplifier. Can you post a transistor schematic?
PSRR problems can come from many places. Two stage miller compensated opamps have PSRR problems due to the coupling of the compensation capacitor,
for example.
If I see your schematic, maybe I can see where the PSRR prob is coming from...
By the way, are talking about simulation or real, on silicon measurment?
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