I'm studying semiconductor parameter analyzers. In particular, I have difficulties in fully understanding the circuit in the following slide, which represents an SMU operating in I-mode (i.e., it injects a current in the device under test and measures the voltage on it):
I start assuming ideal "error amplifier" block, thus the concept of virtual ground applies; as a consequence all the current which flows in R1 must flow in R2 as well. In this way, I easily find the output voltage of the "differential amplifier" block of the picture.
Question 1: why is it written that this voltage is also the voltage on Rr? The voltage on Rr should be de-amplified by the gain of the "differential amplifier" block
Question 2: It can be a stupid question, but this circuit has created in me a doubt: what about if I apply virtual-ground concept for the 2 inputs of the "differential amplifier" block? Rr would be short-circuited and the circuit would have no meaning, then the question: why am I allowed to use virtual-ground concept for the "error amplifier" block and not for the "differential amplifier" block?