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folded cascode & improve PSR

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ansu_s

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Hi,

I have folded cascode amplifier but it not gives good PSR. With AC source on Vdd, look at differential output (in monte carlo) and PSR rises to -20dB at 10MHz, then drops. At low freq I understand - noise rejected by opamp gain. At high freq, PSR improves because AC supply noise shorts through output cap. But at medium freq, what sets PSR?

Please see attached diagram. VbFCPmos and Vbias set by ideal VDC source. VbFCTopPmos and Vcmc set by current mirror. So noise on PM3/4 source changes drain current? Or doesn't change drain current as VbFCTopPmos itself set by mirror (which also moves with supply)? How do PM3A/4A affect PSR? I need -50dB at 10Mhz, how I improve medium freq PSR?

Ansu
 

In your schematic you will be coupling in your AC signal into your input pair through the body connections. Try first connecting the pmos locally in there own wells. I know the layout area will increase but atleast you will not have body effects.

Also you shouldn't be using Vdc sources in the design since this is not how it will be built in real life. Everything should be based off of currents and diode connected mosfets! This could be also part of your problem as well!

JGK
 

Thank you, I not notice the PMOS body problem. Maybe replace with NMOS input stage and try again. Thanks!
 

Being a differential opamp body connection should not cause any issues. It is a common mode signal as far as the inputs are concerned.

The PSRR is bad because of the output stage. One way to improve it would be to add a high bandwidth gate boost stage on the cascode current sources
 

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