Opto in amplifier screws up output

Fibo · 2015-12-17T19:28:46+00:00

Hello, I'm trying to make a voltage controlled amplifier. I only need a gain of about 11 max. So I thought I would use an opto as a variable resistor on an inverting amplifier. I went with the H11F3M I built up the Automatic Gain Control circuit in the datasheet. I used a 50k resistor as my feedback resistor. When I fired it up, my output looked kinda distorted. I swapped out the opto with a regular resistor and it cleaned up. Any idea why my output looks like that? Can I not use the opto this way? I tried a bunch of different opamps, but the scope shot is from an LF411 with +/- 5V rails. Scope shots are on the link below. Much thanks!

Dec 17, 2015 21 Replies

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Fibo 10 years ago

ahhh, thanks, that makes sense

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Tim Williams 10 years ago

Also, operation would be more obvious if you used a *much* smaller transistor, or a much lower value resistor that's actually loading the transistor to a modest fraction of its rating. Like, a few ohms I would guess?

And FWIW, R2 is superfluous because SPICE voltage sources are ideal*.

*LTSpice hides ESR and ESL parameters inside components, so beware if they've been set nonzero by default.

Tim

Seven Transistor Labs Electrical Engineering Consultation Website: http://seventransistorlabs.com

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