How close can Op Amp operate to open loop gain

If I needed a to gain up an really small signal, say 100uV peak, up to 1V peak. I'll need a gain of 10,000 to accomplish this. Could I practically push all of this gain onto one stage, using an op-amp like the LTC6406? It has an open loop gain of 90dB and I'm only looking for a gain of 80dB. I understand that bandwidth tends to drop off with gain but I'm operating at relatively low frequencies so this may not be an issue. I also understand that I'll need to limit my bandwidth with a filter for the noise to not absolutely kill me with such a large gain. The op-amp never mentions use above a gain of 100 but I'm wondering if that's because it would somehow become unstable or they just don't see it being very useful with a gain that high. Thanks.

HI,

I don't think it's a good idea.

What about offest voltage and current, will they be amplified also?

What signal frequency do you expect? My calculator doesn'understand "relatively low". ;-)


Klaus

A G=10000 amplifier must be implemented with lower DC gain for reasonable DC bias. The higher the gain and the lower the high-pass cut-off frequency, the longer the time for initial power-up. But not impossible though.

Having only 10 dB loop gain left (as in your example) means that amplifier non-linearity is only partly cancelled by feedback, also the gain can easily vary by several 10 percent if the open loop gain varies with temperature or supply voltage.

I think using a dual or quad opamp to spread out the gain between its stages will be appropriate.

What about offest voltage and current, will they be amplified also?

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Previously I've been using a DAC and a summing junction to cancel the offset, as long as that is introduced before this gain stage I don't think the increased gain will cause any issues.

What signal frequency do you expect?

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Certainly less than 100kHz, I say relatively since currently I'm operating around 20MHz. The application is an inductive sensor which gets a much larger response at higher frequencies. I'm trying to repurpose the same PCB for lower frequency measurements but to get to the signal levels I need I'll need a huge gain and without re-spinning the PCB I only have the one stage.

A G=10000 amplifier must be implemented with lower DC gain for reasonable DC bias. The higher the gain and the lower the high-pass cut-off frequency, the longer the time for initial power-up. But not impossible though.

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Is this the same issue Klaus mentioned about DC offset? Would my approach of using the DAC to cancel the offset address your concerns?

Having only 10 dB loop gain left (as in your example) means that amplifier non-linearity is only partly cancelled by feedback

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Is this because I'm operating near the "knee" of the open loop gain plot?

also the gain can easily vary by several 10 percent if the open loop gain varies with temperature or supply voltage.

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I don't understand why the feedback would not be as effective at controlling the closed loop gain in this region?

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I think using a dual or quad opamp to spread out the gain between its stages will be appropriate.

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I was typing that long response when you commented but I'm trying to re-purpose a PCB for this application and it only has the one stage :/ I suppose I could spider of some additional ICs as a test.

Is this the same issue Klaus mentioned about DC offset? Would my approach of using the DAC to cancel the offset address your concerns?

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Possibly not. Depends on the implementation.

I don't understand why the feedback would not be as effective at controlling the closed loop gain in this region?

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You can if you look at the expression for closed loop gain and how it depends on open loop gain and feedback factor.

I was typing that long response when you commented but I'm trying to re-purpose a PCB for this application and it only has the one stage :/ I suppose I could spider of some additional ICs as a test.

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If I was you, I would test the amplifier instead of writing long posts.

If I was you, I would test the amplifier instead of writing long posts.

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I'm going to test it when I have the opportunity to but personally I'm more interested in learning about the theory behind what I'm trying to do. So far I've been unable to track down the answers I'm looking for elsewhere and usually have good luck here. But thanks for the non snarky part of your response