Feedback circuit analysis

[gggould]

Hi all,

I have been confused by how to identify series / shunt / mixing / sampling in real circuit.
It is clear to me in testbook two port network example, but hard to figure out when it comes to real circuit.
Can someone explains to me a little bit?

Also whats Rin/Rout/Av in the following ckt?


Thanks a lot~~

 

[gggould]

Can someone give me a hint, like what is Rin and Rout?
Here is the pic:
h**p://imgur.com/esM1ven

 

[monsoon]

Loop gain is 1+gm3*R. Input impedance is (1/gm2) /(1+gm3*R). Output impedance is R/(1+gm3*R). The circuit is a current controlled voltage source. Voltage is sensed at the output and current is fedback at the input. You can also find the impedances by usual methods of KVL/KCL. But while finding output impedance do not ground the input, as the input is a current source.

 

[gggould]

Thanks for the comment~~~ But why is the current fedback at input not voltage?? The feedback portion looks like a source follower isn't it?

 

[monsoon]

If you consider the feedback as a voltage signal (i.e output of the source follower), then the input must also be a voltage. To implement the feedback, the input voltage and the feedback voltage must be added. Two voltages cannot be added by connecting them together (as in this circuit), instead the currents add. That is why if you drive this circuit by an ideal voltage source, feedback will not play any role and output will simply be -gm3*R*vin.