A simple pulse width modulation circuit that directly modulates the self-excited oscillation circuit

Function of the circuit

The unstable multivibrator using an OP amplifier can obtain an oscillation output with a duty ratio of 1:1 with positive and negative symmetry. If the threshold voltage is changed externally, the pulse width can be modulated. This circuit is very simple. The use of a CR charge-discharge circuit will have an adverse effect on the linearity of large modulation inputs. The stability of the oscillation frequency depends on the time constant of the CR, so the stability will not be high. However, it can be used as a PWM power control circuit.

How the Circuit Works

In this circuit, if there is no input resistor R2, the oscillation frequency is determined by the period of the hysteresis voltage +VE, -VE, that is:

If the ratio of R3 and R4 is set so that the value in the brackets is 2.718, that is, IN2.718, then the calculation formula for the oscillation frequency becomes very simple: FO=1/2C1.R1. In this case, the duty cycle can be changed by using a positive and negative symmetrical external hysteresis modulation voltage to move the ±VTR balance through R2.