I'm trying to understand the unusual current mirror circuit below. The current controlled by R2 is mirrored to R1. The strange feature of this circuit is that Q1's collector and base aren't connected directly as in a normal current mirror, but through two diode-connected transistors (Q3 and Q4).
What is the purpose of Q3 and Q4? I haven't seen this on current mirrors before. I looked at other current mirror circuits (Wilson current mirror, cascode current mirror) and this doesn't match. In LTspice, this mirror circuit functions, but I don't see any advantages to it. It seems to work just as well if I replace Q3 and Q4 with a wire.
This circuit is inside a chip I'm reverse engineering. The two-diode current mirror appears multiple times, so there must be some reason for it, but I can't figure it out. Any ideas? Does this circuit have a name?
(Ignore the part numbers in the schematic.)