Current feedback - It’s not that difficult at all!
A current feedback has an input that consists of a unity gain buffer and two current mirrors in the configuration you see here:
The buffer marked Buf1 has a quiescent current running in through +V terminal and out -V terminal. This current is mirrored so the same current runs from Q3 collector to Q4 collector.
When +IN is higher than -IN, a current will flow out of the -IN terminal (this is the error current, hence the name current feedback). This current causes the current in terminal +V to be higher than the current in terminal -V. This in turn causes the upper current mirror to source more current than the lower one sinks. This difference in current is flowing to capacitor Cdom and so the voltage on Cdom rises and the output voltage follows and rises too. And so +IN > -IN = Vout rises, and vise versa. And so when you connect the output to the negative input you get a circuit that functions very much like a conventional opperational amplifier.
This also explains why the feedback resistor value from out to -IN affects the bandwith of the amplifier. A lower resistor value will cause a higher feedback current to flow at the same error voltage and so the Cdom charges faster and voltage rises faster.
