I have seen current mirrors being used as current source in differential stage of an amplifier design.
The reason given is that the current mirror has a very high output impedance and behaves like an active load. I am not sure what this means.
Why do we need to use a current mirror in differential stage of an op-amp? I understand that it is to provide the same current for collectors of both transistors in the differential stage. However, I do not understand why we do this in the first place, why does current mirror have a high output impedance and what does the current mirror being used as an "active load" means here?