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Design >> Analog Design >> Source follower using an NMOS transistor as current source.
https://designers-guide.org/forum/YaBB.pl?num=1371596992

Message started by baab on Jun 18^th, 2013, 4:09pm

Title: Source follower using an NMOS transistor as current source.
Post by baab on Jun 18^th, 2013, 4:09pm

Hello!
Today, I got stuck with this circuit: Source follower using an NMOS transistor as current source.

I want to know what calculation you would do to use the circuit in real world.
What value of I1 do I need?
I think I can choose any value, no problem!
What is the value of Vb?
From the saturation mode, I can figure out Vb.
How do you choose Vin to make M2 operates in saturation mode?
(M1 is always in saturation mode)
I want to know how you would choose Vin to operate.
For me the first thing is to analyse large signal model then choose an operating point.

Can you comment on my method?
Thanks.

Title: Re: Source follower using an NMOS transistor as current source.
Post by saket on Jun 19^th, 2013, 11:10am

Vin needs to be at least V_{DSAT, M2} + V_{GS, M1} higher than ground level.

The situation is a little complicated if there is back-gate biasing on M1. That is, its source and substrate are at different potentials. This will make the threshold voltage vary with the output.

You will have to make sure that Vin-(V_{DSAT, M2}+V_GS,M1) is is always greater than zero across corners.