Source follower using an NMOS transistor as current source.

[Screen1988]

Hello!
I am new here. 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.

 

[crutschow]

Circuit (b) would not be used in the real world since the current is dependent upon the gain and threshold voltage of M2 which varies greatly between two transistors of the same type and would also vary with temperature. To minimize those effects you would need to add a resistor in series with the source of M2 to ground to add some negative feedback. Typically you want the voltage drop across the resistor at the desired I1 current to be significantly larger than the normal tolerance variation of the Vgs threshold voltage of M2.

The value of I1 depends upon the minimum load resistance and maximum signal voltage at Vout.