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I've built an Electret Microphone preamp based on this application note from the OPA1692 datasheet (page 27):

enter image description here

The circuit seems to work fantastic, but I've realized that I'd ideally need a bit more gain than the proposed 24.8 (~28 dB). Ideally I'd like to aim at something around 40 dB gain.

The problem I'm facing now is that I'm not quite certain what gain the OPA1692 can sustain. In the data sheet it says that it has a GBW of 5.1 MHz. 40 dB would equal to a gain of 100, which would narrow my bandwidth down to 50 kHz. I obviously only need 40 dB for 20 kHz, but I've heard things about not going too close to the amplifiers limits.

So how far is too far? How much headroom do I have to consider?

Oh and one more thing: The 2.2nF capacitor in the circuit limits the gain of the amplifier to a corner frequency of ~20 kHz. I've realized too late that I should have better selected a corner frequency higher than that so that my passband (20 kHz) actually has a reasonably flat response (do they really not consider this when designing these circuits or am I missing something?). I could simply reduce the capacitor size to fix that, but since I'd be already close to my amplifiers limits, would the extra amplification outside the audio band cause any problems?

PS: Sadly I've already built this circuit on a PCB. Changing components would be a trivial thing, but before I redo the board I'd rather just live with not enough gain.

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  • \$\begingroup\$ What's the other half of the 1692 doing? Two stages with 20dB each would give you more than enough GBW and margin \$\endgroup\$
    – Neil_UK
    Jun 15, 2021 at 17:36
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    \$\begingroup\$ For a gain of 40 dB I would use two stages in series, with each having 20 dB gain. That should give you better performance than a single 40 dB stage. And less issues (oscillations, bandwidth limitations) as well. \$\endgroup\$ Jun 15, 2021 at 17:36
  • \$\begingroup\$ @Neil_UK Unfortunately the other half is already used as rail splitter, so at least in this design (which I already have a PCB for) I can't do that. As I've said. if there is no way to "cheat", I'd rather just prefer to not have enough gain. Though, how much gain would be the upper limit? Trying different gains seems tricky since I wouldn't know how to properly measure the performance/distortion. \$\endgroup\$
    – theIpatix
    Jun 15, 2021 at 17:50

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The closed loop gain of an amplifier rolls off at high frequencies due to its GBW limitation. You have calculated correctly that with a GBW of 5.1MHz and a gain of 100 (40dB) the amplifier's bandwidth would be limited to about 50kHz.

The bandwidth of an amplifier follows the roll-off response of a single pole RC low pass filter and so at 50kHz (the cut-off frequency) the amplifier gain will be down 3dB (0.707) of its midband gain. Beyond this frequency of 50kHz the gain rolls off at -6dB/octave - the gain halves for every doubling of frequency.

If you were to leave out completely the parallel feedback capacitor (2.2nF) and rely on the natural rolloff of the amplifier then, because the gain is down to 0.707 at 50kHz, I estimate that the gain will be down to approximately 0.9 of its mid-band gain at 20kHz - I would say a fairly reasonable response.

The following resistor values give a mid-band gain of approx 100 (40dB).

Amplifier

The 2.2uF cap (in conjunction with the resistors) in the original circuit gave a lower cut-off frequency of about 30Hz - The gain was down to 0.707 (-3dB) of mid-band gain at 30Hz. I wasn't sure how to calculate this and so I simulated it in Circuit Wizard. To achieve a similar lower cut-off freq with the new resistor values (actually 20Hz) you need to replace this cap with a 10uF cap. This cap should be non-polarised despite being shown as polarised in my circuit diagram.

You'll need to test the circuit once modified but hopefully running the circuit without a parallel feedback cap (2.2nF) won't give any stability issues. Increasing the closed loop gain reduces the feedback fraction (beta) which has a reducing effect on the loop gain (B.Aol). This in turn increases phase margin which will hopefully result in an overall stability increasing effect on the amplifier.

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