Question

Explain the operation of the PWM circuit with 741. Just typed, should not be written by hand.

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Answer 1

Pulse width modulation is technique of generating gate pulses for different switches to conduct for period for which pulse is defined . Depending on duration where pulse is on switch will conduct. To generate such pulses a carrier signal is compared with modulating signal . Carrier signal is generally sawtooth or triangular wave of high frequency and modulating signal can be dc or sinusoid signal of low frequency .

In this example carrier wave is sawtooth wave i e given to inverting terminal of op- amp and modulating signal is a dc signal whose amplitude is decided by potential divider circuit connected to supply. By using potential divider circuit variety of magnitude of dc signal can be obtained . This modulating signal is given at non inverting terminal. So when value of voltage at inverting terminal is less than non inverting terminal ( means sawtooth voltage is less than dc reference voltage) op-amp will give positive value i e on time of gate pulse and if voltage at inverting terminal will be more than non inverting terminal ( means sawtooth voltage is greater than dc reference voltage) output will be zero i e off time of gate pulse . Duty cycle can be varied easily by varying modulating dc signal ( V reference)with help of potential divider because carrier signal is fixed. Time period of gate pulse will be same as carrier signal time period .