The two op-amp in the upper left corner are responsible for generating a triangle waveform. It is then fed in the thirs op-amp with all the diodes, it then generates a pseudo-sine wave (a simple way to obtain something very close to a sine wave with the exponential curves of the diodes. An other triangle wave generator with two op-amps is in the lower left corner, but generating a much higher frequency, which is going to be the carrier wave. A cap is used to keep the AC wave only, since the triangle wave is always shifted up or down a bit. Finaly both the sine wave ang the triangle carrier are fed to a simple comparator to optain the PWM wave.
