Op amp PWM circuit (HELP please !)

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jc256

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Hi there I have been working the following circuit:

Built in LTSpiceIV.

I am trying to input a square waveform, integrating this to become a triangular waveform, then through a summing amplifier to be able to vary the offset with a dc bias, then this triangular waveform is compared to a reference voltage which sets the output duty cycle of the square wave produced.

My problem is that my first voltage divider circuit does not provide the voltage I require, could anyone help with this ?

James
 

I'm not sure what you mean with "first voltage divider circuit". If it's node bias, it will be very low (mV range) because it's shorted by R3.

The LT1017 circuit seems to refer to the "two wire comparator" example in the datasheet, but it does it in an erroneous way because it exceeds the LT1017 common mode range. Neither I do see a reasonable pupose to use this circuit here. No need for two-wire supply.

If you want others to check your LTSpice design, you'll post the *.asc file.
 


Apologies, new to this forum. I will remember that though, thanks.

Yeah the node "bias" is the one i meant. yes it is very low and you are correct in the mv range, I have the 100 ohm resistor as it was to set the gain of my summing amplifier, Is there anyway I can stop this resistor (R3) from shorting ?

Why is there no need for two wire supply ?

Thanks.
 

The LM741 opamp has a very old design and has horrible high frequency response and slew rate.
The extremely low value for some of the output and feedback resistors (R4 and R2) overloads the opamps.
The value of R5 is so low that opamp U1 is not an integrator.
 

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