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Old 26th May 2010, 02:52 AM   #1
pixpop is offline pixpop  United States
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Default Op-amp selectable gain +/- 1.0

I'm trying to find an op amp circuit I saw long ago. It had an analog input and a digital one. The output was the input times either +1.0 or -1.0, depending on the state of the digital input. From memory, there was only a single op-amp in it, but it's been many years now, and I could be wrong about that.

Anyone have any clues? Searching my usual places has yielded nothing. I may have seen it in my TAOE, but that's gone missing and I can't consult it at the moment.
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Old 26th May 2010, 03:32 AM   #2
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if you want the output to be buffered, the easiest way would be two inverting op-amp buffers (-1 gain), and a spdt relay to select one of the two outputs. You usually get better performance from an inverting buffer, so it is likely that performance is as good or better than the output of a non-inverting buffer. This uses two opamps and a relay though.
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Old 26th May 2010, 03:57 AM   #3
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Click the image to open in full size.

Connect inputs 1 and 2 together. Set R1 = Rf. Make R2 = 0 and Rg open. The circuit has an overall gain of +1. Make R2 open and Rg a short, then you'll have a gain of -1 overall. A single SPDT CMOS switch will perform that trick.
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Old 26th May 2010, 04:55 AM   #4
pixpop is offline pixpop  United States
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That's pretty cool, and looks something like what I remember. But I think it was somehow more clever, didn't use any switches or relays as far as I can remember. But this would do in a pinch.
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Old 26th May 2010, 05:04 AM   #5
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Have a look at Bob Cordell's design for a THD analyser on Cordell Audio: Home Page. In the state variable filter he uses a circuit very similar to this with a JFET in place of Rg. This gives continuously variable gain from inverting, through zero to non-inverting.
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Old 26th May 2010, 10:29 AM   #6
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I remember the same circuit and can never find it when I need it. I also remember that you still need a switch of some sort to ground or unground the non-inverting input. An FET or analog switch- I like the DG series. IMO, as long as one has an analog switch, it's still better to use a dual switch and fully switch the input rather than leaving one side connected as the trick did. Walt Jung's op amp book shows the better and more obvious way to do it.

CH
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Old 26th May 2010, 04:22 PM   #7
pixpop is offline pixpop  United States
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Yes, apparently it does have a switch. The circuit I remember is indeed in H&H, figure 4.14. Looks like 4.14b would work with a MOSFET as the switch.

Thanks for the responses.
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Old 26th May 2010, 04:41 PM   #8
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H&H fig4.14 is almost identical to E.A. Parr - How to use opamps. fig2.27
The difference is the addition of an extra resistor between +IN and the switch tapping/5k1. He also uses 4k7 instead of 5k1. Yes, the extra resistor reduces the output offset.
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