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A problem about Class-AB output stage

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Ziqi

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1.png

Hi, friends

This Class-AB output stage circuit worked in open-loop condition to drive a 50ohm load. But the fn noise contributed by the MOS in red circle is too strong, nearly 50% contribution. The output integrated noise below 1MHz is about 0.17mV. the bias current in output branch is about 2.5mA.

I think the most noisy MOS is just for biasing, but I dont know how to reduce its noise.

Thanks

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2.png

This another type circuit is tried, the low-frequency noise performance is even worse. the MOSs in red circles are most noisy(fliker noise).
Could you help me how to deal with the LF noise?

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3.png

This output noise spectrum like this. my interested band is from 1MHz to 500MHz. but the fn noise dominant.
 

There are many ways to reduce the current mirror noise contribution, e.g. degeneration resistors or larger bypass capcitors.
 

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