Electronic – Wilson current mirror


I built a super wilson current current mirror. The Iin is 4-20mA at 24Vdc. Iin to Iout accuracy was a really bad 5%. I put 100 ohm resisters on the source to ground, and that helped a little. How can i improve the accuracy between input and output? Below is my circuit

here is the simultor link for the circuit: sim link

EDIT: the 500 ohm is the load resistance of the circuit using the mirror. The 470 ohm is the max possible resistance of a PLC that reads the Iin current.

Screen shot:


Best Answer

I would use bipolars, not FETs. The FETs have to be particularly well matched for this to work, which is difficult to guarantee with discrete parts. Use two NPNs. With enough emitter resistance, you should be able to to better than 5%. Keep in mind you can't use 5% resistors if you want accuracy better than 5%. Try something like this:

If that doesn't do what you need, a opamp circuit should do it, but will be a bit more complex.