This is the circuit in question. The operation is thus: The opamp drives the emitters of Q1 and Q2. The bases of these devices are grounded, so the output of the opamp is sometimes negative. The opamp works to establish Vref at its inverting input. Thus, the voltage on Rin will be VCC - Vref, and the current I_in will be (VCC-VREF) / Rin
Since the opamp is controlling the V_be of both transistors, they should both sink approximately the same current; of course the Early Effect must be considered, which would cause mismatch if R_out was different from R_in.
I have prototyped the circuit on a breadboard using an LM741 opamp and an LS313 Matched Pair audio BJT transistors. The datasheet isnt that great but they are similar in performance to the SSM-2210 matched pair, which has a much better datasheet.
For certain combinations of Rin and Rout, the matching error becomes huge. But there is almost no discernable trend!
For all of the graphs, the vertical axis is matching proportion, calculated as ABS[(Iout - Iin)/Iin]
It doesnt just happen at low current, or high resistor mismatch, or anything that I can tell. does anyone have any ideas where the mismatch is coming from??
Note - Amp Out is the voltage at the output of the amplifier, or V_E of the BJTs
Note 2 - Vin and Vout are the voltages ACROSS the resistors, not referenced from ground.