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I have inherited a circuit from a colleague using a MAX4063 pre-amplifier and an electret microphone. It's almost identical to figure 4 from the preamp datasheet:

enter image description here

But where that circuit has 2× 1 kΩ resistors in series, I have just one. I'm not happy with the overall volume and noisiness of the audio, and I'm starting to wonder whether the 1k resistors are too small, as they're smaller than the 2.2k output impedance of the microphone.

How, in general, should I select the bias resistor for an electret in this situation? What's the relevant theory?

Edit: While all the answers so far have been useful, I'm still interested in having an analytic understanding of why the bias resistor should have a particular value; I've seen other electrect circuits on the internet with values up to 10k, which must make some difference to something?

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I'm starting to wonder whether the 1k resistors are too small, as they're smaller than the 2.2k output impedance of the microphone.

Those are the output impedance of the microphone. If you look at the mic capsule's datasheet you'll see an equivalent circuit:

capsule equivalent circuit

I don't know why manufacturers always show the FET as a triangle. This is how it's actually configured:

same thing but with normal FET symbol

So this is really a common source amplifier:

common source amplifier

The output impedance of a common source amplifier is just \$R_\text{D}\$, the drain resistor, so when the datasheet says "output impedance (Zout) 2.2 KΩ", they really mean "output impedance of our example circuit".

With \$R_\text{S}\ = 0\$, the voltage gain of the common source amplifier is proportional to \$R_\text{D}\$, since the FET acts like a current source, so the resulting voltage is determined by V = I(FET) * Rd.

What resistor should you choose? It depends. Generally you want high gain in the first stage so you can lower the gain of subsequent stages, which lowers noise. The distortion also decreases as gain increases. You can't increase \$R_\text{D}\$ forever, though, there's a point at which current is too low and distortion increases and gain drops suddenly. Also, if your microphone is expected to pick up high SPLs, you shouldn't increase the gain too much or it will clip.

I don't know how to optimize the gain based on the parameters in the datasheet, but I'd like to know. For mass production, the gm of the FETs will vary from unit to unit (and possibly the FET type will be changed from one capsule to the next even though they have the same part number), so optimizing for maximum gain for a specific FET is probably a bad idea.

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Note that in this case you actually have 2 kΩ bias, not 1 kΩ. That is because the bias resistor is split on both sides of the mic, probably hoping for most of the noise pickup to be common mode, which the amp looks like it is designed to reject.

The outer (argh, use component designators already!) 1 kΩ resistors are for filtering the power supply, not adding bias resistance. They are effectively shorted AC-wise by the two caps to ground. It is important to treat each side of the mic similarly so that ambient noise will be coupled about equally to both leads.

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It's a 2Vdc microphone (according to the spec sheet) so if you supply a bias much smaller it probably will stop working and if your bias is too high it may also reduce its sensitivity as the in-built JFET starts to run into potantial problems. This is my intuition.

Also, I suspect that the case of the one in the drawing may also be connected to one lead and this lead should be grounded rather than feed into a diff input. It doesn't look like pin 10 needs any dc on it so I'd leave it open circuit.

This might reduce the noise you are getting. You should try and connect the mic to ground and feed it 2.0V (or whatever?) via a larger resistor and check with a meter that it's still about a couple of volts or if the power supply is say 5V try a 3k3 and check that the mic has 1.5V to 2.5V across it.

Again more intuition than hard fact.

Pin 14 and pin 1 have a resistor that looks like it defines the gain/amplification - if it's anything like an instrumentation amp you could lower this and see if you get more decibels.

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The input noise voltages on that datasheet are rather high - 70nV/rtHz at a gain of 20, which the datasheet p.8 shows corresponds to Rgain=11.1k between pins 1 and 14.

The noise falls to about 12nV/rtHz at a gain of 200 (Rgain = 1.1k between pins 1 and 14). 12nV/rtHz is still noisy but probably what you could expect for such a low power device. So try this first; it sounds as if the extra gain would be useful too.

If that's not enough and you can afford the power, you could achieve much lower noise with a classic audio opamp (the ancient NE5534A at 3.5 nV/rtHz or down to 1nV/rtHz with others like the AD797.

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  • \$\begingroup\$ Thanks for pointing that out, it seems counterintuitive to get lower noise with higher gain. \$\endgroup\$ – pjc50 Jul 23 '13 at 9:26
  • \$\begingroup\$ It's actually quite normal in mic amps; high gain is usually achieved with low emitter resistance and hence low noise resistance. The trick is how to keep the noise low and reduce gain (increasing headroom) but that's another story. \$\endgroup\$ – Brian Drummond Jul 23 '13 at 20:05
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If you take the supply voltage VCC and subtract the design voltage of the electret insert microphone then divide by the electret's current draw you get the bias resistor in k ohms . It looks something like this 5 volt - 2 volt -:- .5 = 6k resistance . Work it out for your device data . The four 1 k resistors in series add up to 4 k total across mic .

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