# Finding the biasing resistor for MOSFET

Discussion in 'Homework Help' started by wind_blast942, Sep 12, 2010.

1. ### wind_blast942 Thread Starter New Member

Sep 7, 2010
22
0
Hi all

I have to find the Rs of the following P-MOSFET circuit such that my Id = 0.2mA. Vth and Kp are given.

I can assume that the transistor is operating in the saturation region. However i realised that there are 2 different methods in solving this and not sure which of it is correct.

Method 1:
Assume saturation, via Id = kp(Vgs - Vth)^2, find Vgs.
Setting Vds = Vgs - Vth (assumption for saturation), find Vds
Finally getting Rs = (Vdd - Vds - IdRd)/Id.

Method 2:
Realise at DC, Cin causes no current to flow through R2.
Assume saturation, find vgs using the formula in method 1.
Since there is no voltage drop across R2, Rs = (Vdd - Vgs)/Id.

Both methods sound reasonable to me, but since they both give different values, not sure which one is correct.

Thanks.

2. ### Ghar Active Member

Mar 8, 2010
655
73
Your assumption of Vds = Vgs - Vth is wrong, saturation is when Vds > Vgs - Vth

It's only equal at the exact border between saturation and triode.

You have everything else correct:

If there's no current in R2 then Vg = 0.
If Id = 0.2mA then you can find Vgs, and since you know Vg you can find Vs.

Both of these equations are correct:
Rs = (Vdd - Vds - IdRd)/Id.
Rs = (Vdd - Vgs)/Id.

3. ### wind_blast942 Thread Starter New Member

Sep 7, 2010
22
0
Thanks for the clarification. Just one more thing, when u mean triode are u referring to the linear region?

4. ### Ghar Active Member

Mar 8, 2010
655
73
Yes, it's just another name for it.
The names do get mixed up quite a lot... saturation is the linear amplification range, triode is the linear resistance region, in a BJT saturation is the resistance region while active is the amplification one... always need to stay on your toes as to what someone is talking about!