# Small signal model and direction of current

1. ## Small signal model and direction of current

hello

I want to draw small signal model but I am confused with the direction of currents. Can anyone help. In the attached figure for small signal model the direction of current for M1 is shown downwards and for M2 is shown upwards although both are NMOS. Why M2 direction shown upwards?

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2. ## Re: Small signal model and direction of current

I think the current direction is swapped because M2 gate is controlled with M4 source actually, where the signal is the inverse of the M1 source, but in this simplified model it uses M1 source because it left M4.
Why M3,M4 transistors are missing from right figure? Are you sure you can simplify the left circuit without those? I am not.
Other things:
- on the right figure the capacitance values should be C1 and C2 I think, not C1/2 and C2/2, because on the left figure C1/2 and C2/2 are connected between differential signals, nodes.
- in NMOS small signal model regurarly the Vgs controls the Ids, now on the right figure in M1 model the Vsg controls Ids by the arrow. https://en.wikipedia.org/wiki/Hybrid...FET_parameters
- what is the purpose of this circuit?

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3. ## Re: Small signal model and direction of current

Originally Posted by frankrose
Why M3,M4 transistors are missing from right figure? Are you sure you can simplify the left circuit without those? I am not.
Yes the circuit can be drawn like this, considering single ended equivalent circuit.

Originally Posted by frankrose
on the right figure the capacitance values should be C1 and C2 I think, not C1/2 and C2/2, because on the left figure C1/2 and C2/2 are connected between differential signals, nodes.
Yes it should be C1 and C2. I think its a mistake.

Originally Posted by frankrose
in NMOS small signal model regurarly the Vgs controls the Ids, now on the right figure in M1 model the Vsg controls Ids by the arrow.
Yes Vsg is controlling Ids. This is I cant understand is it because the signal is inversed may be?. I am not sure thats why I have posted a thread here.

The circuit is of a biquad. lowpass filter

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