I'm having trouble designing a circuit using an Op-Amp (741) to generate pulses. The task given to me is:

Design a pulse generator with a duty cycle that is adjustable from 0 to 100% and whose frequency can be set between 500Hz and 1KHz. Only DC supply may be used.

I found the following diagram:

Now, I know you can set the frequency by adjusting R2, but I'm unsure how to adjust the duty cycle (make R a potentiometer?). Also, what kind of values can be expected for the resistors and capacitor?

Thank you

  • 1
    \$\begingroup\$ That circuit won't work. (1) It uses a dual-voltage DC supply. (2) There is no duty-cycle adjust. Since this is homework we will guide you when you show your work so far. You need to do some more research. \$\endgroup\$
    – Transistor
    Oct 15, 2019 at 19:51
  • \$\begingroup\$ That circuit is a square wave generator. varying R or R2 will adjust the frequency. As the cct stands there is no way of adjusting the mark to space ratio. \$\endgroup\$
    – user173271
    Oct 15, 2019 at 20:17
  • \$\begingroup\$ @Transistor Why won't the circuit work if the op-amp uses a dual voltage DC supply? \$\endgroup\$
    – Carl
    Jun 7, 2021 at 18:23
  • 1
    \$\begingroup\$ @Carl, the exercise is to generate pulses. That implies a signal switching between zero and Vcc. The diagram shows that the 741 will switch between +Vsat and -Vsat so I reckon that's a fail right there. Question says that "only DC supply may be used" so that also (vaguely) implies that a single-ended supply must be used. The lack of duty-cycle adjustment should be obvious. \$\endgroup\$
    – Transistor
    Jun 7, 2021 at 19:35

2 Answers 2



simulate this circuit – Schematic created using CircuitLab

This topology will work.

OP1 is a non-inverting schmitt trigger. OP2 is an integrator. OP3 is configured as a comparator.

The output of OP1 is a square wave which drives integrator OP2 the output of which is a triangle wave. OP3 compares the triangle wave to a settable threshold to produce the variable frequency pulse width modulated waveform.

Variable frequency PWM generator


THis will easily work with dual supply .

But use Pot for R to change frequency from 1k to 10K using fixed 1k R in series with 10k Pot and C=0.1uF

The Positive feedback is simply equal R values from output to Mid-supply (V+ +V-)/2 = 0V in this case.

This changes the threshold of the Relaxation Oscillation just like CMOS Schmitt triggers with 50% Hysteresis for the sawtooth input voltage on the Vin- RC side. to switch outputs.

The negative feedback R self biases the cap to cross the upper and lower Vin+ threshold switched bu the output.

Interactive Falstad Simulation here

enter image description here

Your design adjusts the duty cycle.

This design rises 50% duty cycle and adjusts frequency by RC=T rise time approx. 50% vs T@ 63%. R is shown as a fixed R but has a soft slider on right to adjust from slider option 1k to 10k. OK?

I also used a single supply, so I used Pull-up/down to get V+/2 with equal R equivalent values. I set Vout to only swing 2 to 10V from Vcc=12V due to some Saturation values of the driver . just approx. OK?

For a dual supply use equal R's and remove pullup R to V+.


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