Exam problem

Jony130

Joined Feb 17, 2009
5,435
I think that means that Rg will be out of the hybrid equivalent model, right?
And as the source of the MOSFETs are supposed to be grounded according to the generic hybrid equivalent model, we will have a virtual ground at both MOSFETs sources, making Rs also out of the circuit, no?
Yes you can do that.
But why you simply not solve this circuit directly. Using whatever analysis methods are at your disposal to do this.
1.PNG
 

Thread Starter

PsySc0rpi0n

Joined Mar 4, 2014
1,684
Yes you can do that.
Are you referring to what I said about Rg to be out of the circuit as also Rs is out or were you referring to what I said about analysing the Diff amplifier separately from Q3 transistor? Is Rs shorted out or not? Do we have that virtual ground at the MOSFETs sources or not?
 

Jony130

Joined Feb 17, 2009
5,435
To be honest I do not like your new small signal diagram. Because know I do not see how MOS1 can have any effect on MOS2.
Try this
22.png

EDIT

I sent the wrong diagram
 
Last edited:

Jony130

Joined Feb 17, 2009
5,435
No ground at "sources" is needed.

One equation for Vs node

gm1*(Vin - Vs) + gm2*(0V - Vs) = 0 and because gm1 = gm2 we have:

gm*(Vin - Vs) + gm*(0V - Vs) = 0

And after we solve for Vs we have Vs = Vin/2.
 
In post #51, Rs=0; in post #52, Rs=∞. I don't think either of those is correct. Shouldn't Rs = 200 kΩ as it was in post #36 and post #41?

The gain expression I get includes Rs.
 

Thread Starter

PsySc0rpi0n

Joined Mar 4, 2014
1,684
Yesterday I spoke to my teacher and he told me we could split the circuit in 2.

@Jony130 why you said that the sources "don't need" to be connected to ground? I didn't understood that part!
 

Thread Starter

PsySc0rpi0n

Joined Mar 4, 2014
1,684
In post #51, Rs=0; in post #52, Rs=∞. I don't think either of those is correct. Shouldn't Rs = 200 kΩ as it was in post #36 and post #41?

The gain expression I get includes Rs.
I'm considering the symbolic expression. The parameters in the schematics I referenced are shown as R1, R2, gm, Rs, etc.
Yes, Rs is there, but I learned that if we're working with a Diff amplifier, the equivalent model says we have +vid/2- on one end of the hybrid equivalent and we have -vid/2+ on the other end making the 2 source currents to flow in the same direction, let's say, to the right of the circuit, leaving no current flowing to Rs, making there a virtual ground.

upload_2016-2-18_10-48-42.png
 
In post #51 and in your original post #1, you show a single-ended input. Now, in post #59 you have changed it to differential input.

Those two different methods of input drive give the same outputs only if Rs = ∞, but that's not the case in the circuit of post #1.

It's true that the outputs with Rs = 200k are nearly the same as if Rs = ∞, but you wouldn't know that unless you performed an analysis for both cases, would you?
 
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