Classical Pulse Width Modulation Circuit

This paper discusses the voltage waveforms of pins 4, 2 and 1 of integrated circuit operational amplifier LM339 and what functions are realized.

After power on, pin 7 of the IC generates a DC voltage of 8.25v from the resistance partial voltage. When it is just powered on, the potential of pin 6 is lower than that of pin 7, and pin 1 of the comparator (LM339) outputs a high potential. The positive feedback effect of R3 makes the comparator rapidly saturated. With the passage of time, the capacitor is gradually charged, and the potential of pin 6 increases gradually. When it is higher than the potential of pin 7 (8.25v), the comparator suddenly turns over and pin 1 outputs a low potential, Similarly, the effect of positive feedback makes the process stronger. At this time, the capacitor discharges to pin 1 of LM339 through R4 and diode D1. When the voltage on the capacitor is lower than the voltage of IC7 pin (it may not be 8.25v at this time, because the low potential of pin 1 will affect the voltage of pin 7), the circuit turns over again and repeats the previous process, so as to form a sawtooth voltage of 8000Hz at both ends of the capacitor. The sawtooth wave voltage is directly applied to the four pins of the comparator and compared with the control voltage. When the voltage at both ends of the capacitor is higher than the control voltage, the comparator outputs a low potential, and when it is lower than the control voltage, it outputs a high potential, which is equivalent to cutting off the upper half of the sawtooth. Therefore, the higher the control voltage, the less sawtooth is cut off, and the wider the output pulse width. Zener diode plays the role of clipping here to realize pulse shaping. This circuit design is very classic and is a very good pulse width modulation circuit.