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I've been trying to make a Oscillator circuit with a Op Amp and an crystal (7.68 MHz) Similar to the one in the schematic below, but for a 7.68 MHz crystal. op-amp oscillator

Is this possible? (it says low freq only). If so, what modifications do I need?

Ideally I would just make a Pierce Oscillator circuit. However I do not have any NOT gate logic chips on hand, but I do have Op Amps

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    \$\begingroup\$ If you're limiting the part selection, let us know what parts you do have on hand. Thanks! \$\endgroup\$ – W5VO Aug 12 '12 at 3:39
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    \$\begingroup\$ The LM111 is not an op-amp, but a comparator. \$\endgroup\$ – Renan Aug 12 '12 at 15:50
  • \$\begingroup\$ I have the crystal, various Capacitors, Resistors and a couple of Op Amps. But no digital Logic chips. "LM111 is not an op-amp", is it even possible I make an oscillator then from just from a crystal, capacitors, resistors, and some op amps? If you need more details on the parts, i can list them completely \$\endgroup\$ – RMDS Aug 13 '12 at 15:44
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As the saying goes: If all you have is a hammer, then everything looks like a nail.

You wouldn't use a Mini Cooper instead of a truck to move your bedroom furniture, so why use an op-amp instead of some not-gates?

Get some not-gates. They are cheap and appropriate for your task. Using random op-amps that you happen to have lying around will only result in much frustration.

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  • \$\begingroup\$ I tried to build an oscillator using the SN74AHCT14N Schmidt trigger NOT gate a 13.56MHz crystal, and a couple of 20pF capacitors. At least in my case I found that the NOT gate produced a very distorted waveform. I had much better luck using the KSP10BU NPN transistor. That design produced a sinewave with about 1-2% THD. \$\endgroup\$ – user4574 Aug 2 '16 at 2:29
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According to TI,

Op amp oscillators are restricted to the lower end of the frequency spectrum because op amps do not have the required bandwidth to achieve low phase shift at high frequencies. The new current feedback op amps are very hard to use in oscillator circuits because they are sensitive to feedback capacitance. Voltage feedback op amps are limited to a few hundred kHz because they accumulate too much phase shift.

The point being that even if you have an op-amp with 10 MHz or even 100 MHz GBW, because of its multi-stage design, it will very likely have too much phase shift at high frequencies to be usable in an oscillator circuit. Which isn't to say there isn't somewhere out there some device and circuit that could achieve an 8 MHz oscillator, but the odds of it working with any op-amp you pull out of a drawer are pretty low.

In any case, many oscillator designs are available using a single transistor as the gain element. There's no need to use an op-amp when a single transistor will do.

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