I've spent two

*long*days trying to work out what this circuit is doing:

mirror.png

I'm need to understand its operation so I can write a detailed description with equations, and be able to explain how to modify it to tailor its operation.

If this consisted of a straight forward current-mirror, I'd find this easier. However, with emitter resistors R3 and R5 I'm struggling to find/formulate the appropriate equations.

I have no idea what Q1 is really for. Calculating Ib of Q1 is easy of course, however Vce (or equivalent resistance) of Q1 is another matter... perhaps this is there for temperature stability?

So my fundamental questions are:

- What is the purpose of Q1, and how do you calculate the voltages and currents?
- What are the voltages/currents associated with Q3?
- What effect does changing R5 have on the output of Q3?
- Pulling Raise to Vs (0.95) effects Q2 how? How does the output of Q3 alter?

This circuit is implemented within an IC, so the transistors are matched and operate at the same temperature. I don't have values for hfe etc though, so I've been assuming sensible defaults.

I'd be really greatfull for any help you can provide!

Cheers,

Chris