How to verify MOSFET resistance implemented as a nulling resistor in an OpAmp

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rmanalo

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Hello everyone,

I'd like to verify the MOSFET's resistance (Rz) in an opamp. showin in the figure.



Usually I see this in an operating point simulation where I simply measure the Vds/Id for a MOSFET in linear region. But in this case it's blocked by capacitor CC.
 

One simple way would be to get the terminal voltages for that PMOS resistor from the operating point. Then, place it in a separate testbench, just that transistor, connect the same voltages to the gate, source, bulk and give a tiny increment on the drain - for example 1mV higher than the source voltage. Measure the resistance. 1mV/Id
 

Hi,
In DC analysis you can plot with calculator the ON resistance easily, just tick the "save operating points" at the analysis window, and use this expression: OP("/Mz", "ron")
 
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