Ask the following circuits, especially the function of R4 and R7, and why R3 and R5 are divided, is it because the operational amplifier+and-are all broken?

Let's talk about why R3 and R5 are divided. It's very simple that R3 and R5 are series circuits, and they are divided in series. So it has nothing to do with what you said because the operational amplifier+and-are all imaginary. Let's talk about the function of R4 again. We can use the limit method. Assuming that R4 is short-circuited, the voltages of the G and S poles of the transistor BBS84 (transistor polarity: P) are the same, which means that Vgs=, so BBS84 will never turn on. So R4 is the bias resistance between the 4G pole and the S pole of BBS 88. The same R7 is BBS138? Bias resistance and protection resistance of S-pole (the internal resistance of transistor is very small after conduction). Attach a simple voltage diagram of each node. The above analysis is for reference only.