I am being asked to find the output resistance of the current mirror below.
the correct answer is supposed to be R_out = 103k ohms.
but when am simulating the circuit, I get R_out = 3.82k ohms.
Can somebody please help me and tell what wrong is going on????
Btw, R_s in the circuit diagram is 3k ohms.
Here is the problem statement along with the simulation results.
Thank you for your help.
Best Answer
The one way of finding the\$R_{OUT}\$ resistance in a proper way is shown here:
And what you should know that the LTspice first computes the dc operating point and then based on this result (gm, ro) the AC analysis is performed.
All this means that if you include \$\lambda\$ in your MOSFET model. You no longer can assume that \$I_{D1} = I_{D2}\$ and \$r_{o1} = r_{o2}\$
Because the lambda will also affect the DC-operating point also.
And the equation for \$R_{OUT}\$ is \$R_{OUT} = ro2*(1+gmR_S)+R_S \$
I/O Resistance of common source MOSFET with source degeneration