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pwm need to increase frequency

I have built the PWM circuit located here,

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The triangle osc. frequency is about 150 hz. I want to increase it to about 15 khz. When I changed the capacitor, C1 and resistor, R1 the waveform got very distrorted. The risetime of the waveforms was slow. Are there adjustments that can be made to make the circuit work at 15 khz? Or is the LM 724 to slow for the application? Thanks, Mike

Reply to
amdx
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Yup, LM324 is a pitifully slow opamp. And on some of them, all sections screw up if one or more of them is driven open-loop, as two of yours are.

John

Reply to
John Larkin

Thanks John, Ok, so is there a direct replacement IC or do I need to start over. Mike

Reply to
amdx

I believe a TL 074 / TL 084 will drop in and be an OK starting point.

G=B2

Reply to
stratus46

I believe a TL 074 / TL 084 will drop in and be an OK starting point.

G²

In comparing the unity-gain bandwidth I see the LM324 is 1 and the TL084 is 3. I'm not sure that's enough, any changes I can make to the circuit to help? I need to order the part:-( Thanks, Mike

Reply to
amdx

084

The problem is in the output drivers in the LM324; they are very good at being power-stingy, but not suited (due to crossover distortion) for high-audio-frequency operation. The TL084 uses more quiescent current because it has higher output stage bias current, and lower output impedance. You can also add pullup resistors to an LM324, aim for about 1 mA of current and the outputs of the LM324 will stay in the low-output- impedance condition. That 1 mA of added current is triple the LM324's minimum power requirement, of course.

Reply to
whit3rd

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