Hi..im new here.hope for good companion..i have this circui and i need to find the voltage gain.well i dont succeed drawing the ac equivilent system.i added the dc analyze for better view .thanks

 

I'll give you a tip.
At the beginning try to draw the small-signal diagram in exactly the same way as you have drawn the DC equivalent circuit.

 

Do you mean thevenin equilivent? How i do that with 3 resistors in series

 

No, simply drawn the amplifier as shown in the first figure and then replace the transistors with small-signal equivalent circuits.

 

Having a little bit of a problem..can you help with sketch

 

Look here



As you can see I remove the transistor symbols from the schematic. Now all you need to do is to replace the transistors by using their small-signal equivalent circuit (pi -model).

Can yo do it ?

 

Wow clearly open my eyes..ill give it a try

 

Well.i jeed to find the voltage gain..but how il find vin in term of vgs..

 

Very Good, but you forgot about C2 capacitor. C2 will act as a "short circuit" hence you need to add this "short circuit" into your schematic. And after that, you should see how simple the circuit becomes.

 

Wait a second..c2 will act as a short circuit and it 'shut down' the bjt amplifier with re resistor.it suppose to vanish?

 

What will gone ?and what about it input of the bjt

 

C2 will "short" BJT completely. So you can simplify the circuit and connect a MOSFET source terminal to GND.

 

You helped a lot.. thankyou man

 

Is that correct?
What is vin?

 

Is that correct?

Yes.

What is vin?

I do not understand the question

 

Yes.


I do not understand the question

I need to find the input voltage as a term of vgs and probably gm*vgs ..

 

But simply look at the diagram and you should see that Vin = Vgs. So, where is the problem?

 

Voltage gain is made of transistor.vo/vin..can you find it ?

 

I do not need to find it I already know the answer. You must show us your best attempt.

 

First of all i think that r3 is shorted too.what is left r1 r2 in parallel.now in order do find voltage gain in term of the drain resistor voltage plus transconductance.now if vout=gm*vgs*rd
Yhen vin is I*Rin but in order to vanish the vgs term in the numerator i need that vgs will conclude in the vin term.so how i add the Rin to the buisness.understand?