opamp switchable gain

Clive Arthur · 2017-10-17T16:25:19+00:00

I have a standard inverting op-amp with a 1k input resistor. The feedback comprises one fixed resistor with three more parallel resistors which can be connected or disconnected by analog switches, giving 8 possible gain settings. I want a minimum gain of 1 and a maximum of 20. (ok, -1 and -20, it's inverting) Clearly, the fixed resistor should be 20k to give a gain of 20 with all three switches open. Likewise all four in parallel (all three switches closed) should equal 1k. I'd like to space the gain settings evenly, so the eight gain settings would ideally be 1, 1.53, 2.35, 3.61, 5.54, 8.5, 13.0, 20. What's the best way to work out resistor values to get as close as I can to this ideal? The circuit exists and can't be changed. Cheers -- Clive

Oct 17, 2017 41 Replies

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Jasen Betts 8 years ago

The switches are SPST and are soldered directly to the PCB?

This email has not been checked by half-arsed antivirus software

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oldschool 8 years ago

That explains the obvious virus between the letters P and S in the word SPST. :)

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