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Design >> Analog Design >> MOSFET OP parameters for a small-signal model
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Message started by IBurmistrov on Sep 7^th, 2019, 11:29pm

Title: MOSFET OP parameters for a small-signal model
Post by IBurmistrov on Sep 7^th, 2019, 11:29pm

Hi!

I'm working with the Specter circuit simulator.

I'm trying to get transfer function for the circuit on the first pic. At first step I make test bench for STB analysis and see some poles and zeros. Next I want to replace every element of the initial circuit with it small-signal model to get it's small-signal representation and I'm started from the PMOS.

This PMOS has a current biasing function so I replace it with resistor and capacitor as at second pic. Parameters I get from the OP analysis. Resistor have rout value and capacitor is sum of the cdg, cds and cdb. After simulation I get very different result - poles and zeroes are well shifted from it's original positions, gain and phase margin are increase. I think its predictable because rout is huge resistance so system gets more stable. Replace rout by ron also don't give desireble result in terms of gain, but what I should to do?

In saturation region small-signal resistance of the MOSFET described by it rout OP parameter. Small-signal model of the MOSFET if pretty straingforward in this case, and circuit simulator do the same things as me - replace MOSFETS with it small-signal models. So why I get such unpleasure results? I don't want to get 100% accuracy, but what I see makes me sad.

Title: Re: MOSFET OP parameters for a small-signal model
Post by IBurmistrov on Sep 8^th, 2019, 10:48pm

Ok, I found the cause of the problem.

The problem was that you can't replace only one element of a circuit with it's small-signal model, because this can change operating point. Either replace all elements at once or replace with saving an operating point.

I placed the current source in parallel with resistor and capacitor with current equal to id and now circuit works more or less like it should. The resistance is equal to rout like it should be in saturation region.