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Need explanation for problem with oscillator when swapping opamp

Hi,

I try to build this simple square wave oscillator which is supposed to use opamp TL062;

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According to formulas this circuit should swing at about 55 kHz and give several volts AC out. Now I didn't have this opamp at hand so I tried LM324 which also is supposed to be able to operate with a low voltage single supply. But when measuring with my Fluke 79 I get swing at about 19 kHz at 0,85 Volt AC. Can you please explain to me why these figures does not match with the calculated ones? Thank you for your help!

ottar

Reply to
ottarh
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ottarh a =E9crit :

Probably because the TL062 is a FET opamp while the LM324 is a bipolar opamp.

Interestingly your oscillator circuit is in the LM324 datasheet, the only difference is the value of R3 (100k vs 8k2). I couldn't find a LM324 so I've setup your oscillator with a TL084, which is a close relative of the TL062. I measured with an oscilloscope a frequency of almost 55kHz and a squarish output of 2.9Vtt superimposed on 1.45V. My multimeter said 1.2VAC and 54.7kHz.

Thanks, Jenalee K.

Reply to
Jenalee K.

Sounds like a slew rate limitation, but if I'm reading my data sheet correctly (never a sure thing) it should be plenty fast. Either the thing isn't happy at low supply levels or you're loading it down somehow. If you can, take a look at it with an oscilloscope. If you can't experiment with the feedback resistor or cap to see if it'll work more cleanly at way lower frequencies (like 10x lower).

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Reply to
Tim Wescott

If you look at the 'large signal output swing', versus frequency graph, for the 324, with a 5v supply, it only shows it managing about 15KHz, for the full swing. Your example, sounds to be manging slightly better than this. The same graph for the TL062, manages about 400KHz. I'm afraid at this low voltage, with an output that swings this far, the frequency performance of the LM324, is just not good enough.

Best Wishes

Reply to
Roger Hamlett

I think it is slew rate. An LM324 data sheet shows a voltage follower pulse response that would imply 0.5 V/us, while the TL062 claims 3.5 V/us, or 7 times faster. The 324 is actually happier on a low supply, since it goes closer to the rails, but the voltage swing should cancel out with this circuit.

-- john

Reply to
John O'Flaherty

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