# Electret microphone amplifier for computer using transistor

I'm trying to make a electret microphone amplifier using transistor to connect on my computer using a P2 connector, the problem is that using the amplifier the sound actually is lower than connecting the microphone without the amplifier. What I'm doing wrong?

The sound input is the electret microphone connected to 5V through a 10K resistor. The speaker is the P2 connector.

Schematic:

Q2 has to drive the speaker. Assume that speaker looks like ONE OHM. At the base of Q2, that ONE OHM becomes (maybe 100 Ohms ------ Q2 beta * Z(speaker)).

Now the base of Q2 is the AC load on collecter of Q1. The gain of Q1 is gm*Rload. Rload is 220K||10K||4.4K||100_Ohms, or approximately 100_Ohms.

What is the gm (transconductance) of Q1? Ie_ma/26. Ie = Ic = (+5v-Vce)/10K. Given the ratio of Rc to Rb (10K to 220K), 1:22 or a lot lower than BETA, I expect Vc of Q1 to be much lower than VDD/2. In fact, Vc may be only 1 volt, and that transistor is biased poorly for gain.

By operating the bipolar on the far left, we bring the very low Rout of the collector (normally predicted well as Vearly/Ic) into very inaccurate value that also is a serious load upon the gm behavior.

Right now with 4 volts across 10Kohm, you have Ic = 0.4mA, and gm = 0.4/26 or 0.016; the total gain is 0.016 * 100 = 1.6x.

Solution? Make that 220Kohm be 1MegaOhm; and convert the Q2 into a Darlington.

EDIT>>>> make that output cap Z(1KHz) be 1_ohm. To achieve that, use 160UF. For some base signal, use 1,000uF. EDIT>>>> corrected gm math error

The first and most obvious thing is that electret microphones need a voltage source to operate.

You need to put in a resistor from 5V to the point you have marked "sound.".

The recommended value can usually be found in the datasheet. If you don't have a datasheet, then a value of 1kOhm to 2kOhm should be close enough to get you started.

The rest of the circuit doesn't look really all that good, but should work.

I assume you have connected the cable for the PC in place of the speaker.

There is a way to power the amplifier directly from the microphone input on the PC.

PC sound cards always have a DC voltage present on the microphone input. This is because electret microphones require this voltage to be present.

You need a relatively small resistor (a few hundred Ohms) and a relatively large capacitor (10microfarads or so) to make a low pass filter connected to the PC. This gives you a (somewhat clean) DC that you use to power the amplifier. The output of the amplifier then goes directly back to the PC (amp out goes to before the low pass.)

For voice recordings, this can work well.

You need to make sure that the low pass for the DC is below the high pass in the amplifier (cutoff frequency wise, that is. They don't care much where they are physically located.)

A quick look around google didn't turn up an example, and since I'm posting from my phone I can't draw one for you.

It isn't complicated, but words make things sound complicated when diagram would show you how simple it really is.

• Last I checked electrec microphones have a built in bias and need no power, though some have a preamp built in that do require power. Apr 7, 2017 at 17:01
• @Trevor: The chance of having an electret mic with out the preamp is next to zero unless you go looking for one on purpose. We're talking about typical home consumer stuff.
– JRE
Apr 7, 2017 at 18:58
• yes I know, I'm just adding to your already great answer. Searches for schematics on the net tend to fall into the either or category, which can and I guess do, confuse people. Apr 7, 2017 at 19:01

My first guess would be that your 5V source is not suitable. Since you are talking about a computer, I'd guess that you powered this circuit from a USB port.

Don't do that, because the 5V on a USB port comes directly from a switching power supply, with little to no filtering. As such, your power supply is incredibly noisy, at least compared to the amplitude of a microphone signal. Since your amplifier has no power supply rejection to speak of, all this switching noise will go directly into your amplified audio signal.

My recommendation would be to power the circuit from a battery, which has virtually no noise, because the batteries don't contain switching power supply. Or from the microphone output's bias voltage, as JRE recommends.

• While that isn't bad advice, it doesn't explain the failure - the asker isn't reporting that they get noisy sound, they report that they get minimal volume. What you have here is more of an advisory comment than an answer to the actual problem posted. Sep 9, 2020 at 17:55
• @ChrisStratton Ah sorry, I misread the question. You are right, of course. Sep 9, 2020 at 18:01