Question 1:What do R3, R4 and D1 do?
whether it is,
1. In the case of low voltage input the diode will not conduct, this part does not work;
2.When the diode is switched on at a high voltage input, the op amp input voltage is equal to the partial voltage of R3 and R4+Vf.
question 2:What is the magnification under the above premise？
