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Amplifying microphone signal using common emitter amplifier

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Advanced Member level 2
Apr 17, 2011
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I have an electret microphone that I connect to +ve supply using a 10k resistor (yes the value is arbitrary). The microphone itself has 2k resistance measured using an ohm meter. The voltage across the microphone is to be connected to a common emitter amplifier using a 1nF capacitor (again an arbitrary value) to filter out DC and obtain voltage gain for the AC signal.

Now the question; I have a few hundred mV of signal that develops across the microphone passes through the capacitor and want to amplify this before it can be input to an ADC. When we using an OpAmp, the calculations for gain are quite simple since the signal is thought of as a voltage as no current enters the (ideal) OpAmp anyway. But for the CE amplifier we talk of base current, look at the beta value and then choose a value of collector resistance to convert that to voltage.

I do not know the base current, I only know the range of voltages that shall be applied across the base, how then do I calculate what value resistor to use? Do you have any advice on how the values for the already arbitrary resistor and capacitor should be chosen?

A transistor has a wide range of current gain so you do not know how much base current it needs. Its base-emitter voltage is different for each transistor and changes when the temperature changes.
Therefore to make a common emitter transistor amplifier the emitter has a series resistor to ground cancelling most of the problems and the base gets a voltage divider that has 10 times as much current as the transistor's "typical" base current that is shown in its datasheet. The emitter resistor applies AC and DC negative feedback.

A capacitor parallel with the emitter resistor prevents AC negative feedback for high AC voltage gain.

The datasheet for some transistors shows its input impedance. For the 2N3904 its input impedance is 4k ohms when its collector current is 1mA. Then your 1nF input capacitor cuts frequencies below 40kHz so it is almost useless for audio. It should be 1uF to pass frequencies above 40Hz (don't you know the simple calculation?).

A single transistor with no AC negative feedback has high distortion at high levels. Usually two transistors are used for high outputs and low distortion. Here is an example:

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By the way, there is NO WAY you will get the very high output of a few hundred mV from an electret microphone. 10mV is typical when speaking at a normal conversation level 10cm away.


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