# High frequency MOSFET amplifier , CS with Rs

Discussion in 'Homework Help' started by greenland, May 22, 2018.

1. ### greenland Thread Starter New Member

Jan 28, 2018
7
0

a) AM=? (when Cgs and Cgd are 0) --> AM = -gmRL / (1+gmRs)
b) ωH=? (using OCTC method)
Rgs =? Rgd=?
I'm stacked on this point.
In the case of CS MOSFET without Rs:
Rgs=Rsig
Rgd= Rsig(1+gmRL)+RL

What will change in CS MOSFET with Rs?

I'd really appreciate any help. Thanks in advance.

2. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,638
1,300
You need to solve this circuit

If you whant to find Rgs seen by Cgs capacitor.

Rgs = Vx/Ix

Also notice that now Vgs = Vx and Is = Id - Ix.

Can you solve it ?

As for Rgd resistance seen by Cgd capacitor, you need to solve this circuit:

And once again

Rgd = Vx/Ix

Last edited: May 23, 2018
greenland likes this.
3. ### greenland Thread Starter New Member

Jan 28, 2018
7
0

VG = Vgs + gmVgsRs
VG = IxRsig
Vgs = IxRsig / (1+gmRs)

Vx= VG -VD=IxRsig - (-IDRD) = IxRsig + (Ix + gmVgs)RD = Ix(Rsig + RD) + gmVgsRD
Vx / Ix = Rsig + RD + gmVgsRD / Ix

=> Rgd = Rsig + R
D + gmRsigRD / (1 + gmRs) = Rsig + RD ( 1 + gmRsig / 1+gmRs )

Thanks a lot. I find it really helpful.