1 and R6 C4 form a low-pass filter, and a single-power noninverting amplifier circuit can amplify signals with the same polarity without bias;

2. The only function of the second operational amplifier is probably shaping. Because the first-stage amplifier circuit has low-pass filtering, the original PWM rectangular wave is turned into an approximate sawtooth wave, so the open-loop amplifier circuit is used for once shaping and restoring the rectangular wave.

3. The triode at the back forms an emitter follower circuit to amplify the current.