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I'm designing a triangle wave generator to power a detector. The triangle wave is coming out at the right frequency (0.4 Hz), but I'm using an LM324 and the detector that I'm driving needs 100 mA to run. I tried connecting the triangle wave output to a 1k resistor, and then into the base of a transistor but the output from the emitter was only the positive half of my triangular waveform, and I need the detector to oscillate with the positive and negative peaks. I know that I could order an op amp ic that drives a higher current, but if possible I'd like to use parts that I have on hand.

I have a spice schematic attached for reference. Spice doesn't have the LM324 so I just used the LT1014 to draw up the schematic. I also attached the schematic with the added transistor.

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Thanks in advance for any ideas!

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  • \$\begingroup\$ Use another op-amp which adds the needed offset voltage to your signal. \$\endgroup\$ – Janka Sep 25 '17 at 22:49
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Since you are already using an LM324 as a oscillator at 400 milliHz (0.4 Hz). Just use one of the addition 2 opamps in package as a buffer amplifier with a class B output stage.

schematic

Select Q1 and Q2 based on load requirements.

There will be small amount of crossover distortion as the load current requirements change sign. If that's an issue bias the output for class-AB.

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  • \$\begingroup\$ Nice idea circuit. \$\endgroup\$ – Michael Karas Sep 26 '17 at 4:58
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I wanted to enhance the understanding of the buffer idea that @sstobbe posted in another answer. Here is a combined circuit of the OP with the buffer added. Note that I had to add some rise time to the supplies and delay one some from the other to coax the simulation to start the triangle generator going.

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The circuit waveforms below show the triangle wave, the base drive waveform and the net output waveform. The output chart shows the voltage and load current through the 15 ohm resistor selected to achieve ~100mA max current output.

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Note that with a higher bandwidth opamp there will be less output cross over distortion. Also note how the LT-SPICE made the two output waveforms in the bottom plot panel be the same lines. They scaled the current axis on the right so it just so happens that the two plots end up right on top of each other.

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  • \$\begingroup\$ @sstobbe, and Michael Karas, This works perfectly, thanks! Do you know why it doesn't oscillate perfectly around 0V? I've tried to find a way to change that, but I haven't been able to come up with anything so far. \$\endgroup\$ – Bex.1233 Sep 28 '17 at 16:30
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You want to look into the Fast 150mA Power Buffer from Linear Technology. It will easily drive + / - 10V into a 75 ohm load. Their part number is LT1010.

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