# Finding Wilson Current Mirror Output impedance

I am trying to confirm my output resistance calculations with my simulation.

How can I measure the output resistance seen by Q4 in order to confirm my calculations?  How I measured $$\r_{o2}\$$ (image 1)

How I measured $$\r_{o4}\$$ (image 2)

My total output impedance as seen by Q4 can be calculated using the follow formula: $$\begin{split} g_{m4} &= 0.02267\mho\\ r_{o2} &= 42\mathrm{G}\Omega\\ r_{o4} &= 2.95\mathrm{G}\Omega \end{split} \Rightarrow \begin{split} R_o &= r_{o4}(1 + g_{m4} \cdot r_{o2})\\ &= 2.95\mathrm{G}\Omega(1 + 0.02267\mho \cdot 42\mathrm{G}\Omega) \end{split} \Rightarrow R_o = 2.7258\cdot e^{18}\Omega$$

• @BlairFonville No, they are different. This question asks for the total output impedance (ro) – Roo Apr 26 at 5:58

## 1 Answer

Some ideas:

I haven't used P-spice in a while. One way to solve this problem of measuring current in other spice programs is to insert a 0 ohm resistor and then that allows you to get the current measurement.

Another thing that might be messing with the calculation is the voltage source. If I remember right a voltage source in spice has infinite impedance, so that might be throwing the calculation off. A current source would probably be a better source to use in the calculation.