Basic MOS amp question: Finding dc output voltage

[EEPC]

Hi, I have a basic MOS common-drain amplifier as described below:

- Drain connected to 5 V DC
- Gate connected to an AC source on top of a 3 V DC source
- Source connected to a 200 uA current source with a resistor R connected in parallel. The parallel connection goes to ground, and the output voltage (Vo) is taken across resistor R.

If R approaches infinity, I heard someone explain that Vo > 3 - Vt - (Vgs - Vt)

where Vt = thermal voltage.

I'm not understanding how to get to this expression, because I had simply thought: Vo = 3 - Vgs, since it's just the voltage applied at the gate minus gate-to-source voltage. I'm not sure where the Vt comes from in the expression above.

Thanks for clearing this up for me!

 

[niladri.s.debnath]

Hi eepc
Well I first of all would like to tell u that Vt is actually threshold voltage of the mos not thermal voltage and this topology is also called source follower whose output dc voltage will be equal to 3 - Vth because a source follower o/p follows the i/p voltage and to turn the mos on the condition is that Vgs>Vth so Vs < Vg - Vth or u can say that Vs can be at the max equal to Vg - Vth
I hope U will find this post helpful.

 

[goldsmith]

Hi
I confused! without your accurate schematic why i can help you?