# Small Problem Microphone Amplifier Design

#### jegues

Joined Sep 13, 2010
733
Hello everyone.

See figure attached for design requirements and components provided.

Before just recently, my amplifier was meeting all the requirements and everything was fine.

It seems that as though just recently there are a couple of things I had overlooked in the design of my amplifier.

Firstly, on the second page of the data sheet for the microphone (See figure attached, squared in red) there is a resistor that must be hooked to the battery in order to power the circuit for the microphone. This was not included in the circuit I've been working with thus far.

Secondly I did not account for the output impedance of the microphone. (See figure attached, squared in red)

I decided to add these things into my circuit to see how it was going to effect it's preformance.

Before doing so, I had some confusion about whether or not the output impedance of the microphone is in series, or parallel with the small ac voltage obtain from the microphone circuit. For this reason, I divided things into 2 cases, one where it was in series and one where it was on parallel. (Case 1 = Parallel, Case 2 = Series)

As you can see in Case 1, my circuit holds its gain and other various characteristics and works fine.

In Case 2, my circuit loses it's gain, as the voltage at the input is much larger than before. (The order of mV not uV)

First question that needs to be adressed is which case is indeed correct based on the datasheet from the microphone? Case 1 or Case 2?

The second question relates to the answer of the first question.

If it is Case 1: is this output impeadance going to ruin my input impedance that I've established previously? If so, how do I go about fixing that?

If it is Case 2: How do I go about correcting the issues I'm having with my gain? Is there any effect on my input impedance? And if so, how do I correct that?

Thanks again!

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#### Audioguru

Joined Dec 20, 2007
11,248
Something is wrong with your simulated 'scope in case #2.
The schematic shows a channel A input of 1mV peak but the scope lists much more input but does not show the trace.

The electret mic needs a DC current of about 500uA so change the value of R7 to 10k.
The mic has a series 500 ohms output impedance. The input impedance of the amplifier is 52k ohms so it is not loading down the mic.

The circuit works much better if R6 is reduced to 680 ohms and if R1 is reduced to 56k ohms. The voltage gain is 15.