# feedback circuit analysis

Discussion in 'Electronics Homework Help' started by only6512, Feb 15, 2014.

1. ### only6512

18
0
Feb 15, 2014
Hello, everyone
I have problem about the ratio of the Vout/I(in)
as shown in the attachments

the method i use:
let the gate voltage of M2 be vx
Vout/I(in)=Vout/vx*(vx/I in)

I don't know how to figure out (vx/I in)
does the small signal model include R(D1)?
and the current through RD1 is?
can any one help me?

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Last edited: Feb 15, 2014
2. ### Harald KappModeratorModerator

11,655
2,696
Nov 17, 2011
Welcome to the forum.
I'm sure we can guide you along your work.

I assume that you are looking for the small signal solution, right? (By the way it's customary to use small letters to denote the small signal sloution, capital letters for the DC operating point).

I'd go about solving the circuit as follows:

- find the operating point (Iin=0A, note this is only the small signal current)
- replace the transistors by their small signal models (see e.g. here). The parameters are defined by above operating point. The small signal model of the transistor does, of course, not include the external resistors. The small signal model of the circuit does include them.
- solve for Vout=f(Iin) and calculate Vout/Vin.

3. ### duke37

5,364
772
Jan 9, 2011
A very strange circuit. Are you sure that the two RF resistors are connected to ground?

4. ### only6512

18
0
Feb 15, 2014
That's why i am confused
the small signal model includes RD1
but the textbook's answer to vout/ I in doesn't(shown in attachment 2)
is it wrong?

5. ### only6512

18
0
Feb 15, 2014
yes.actually this circuit is used to calculate the closed loop gain with finite impedance
of a feedback system
the two "RF" are respectively "sense duplicate "and "return duplicate"