amplifiermicrophone
Can you explain about microphone preamplifier in the next circuit?
http://circuitndiagram.blogspot.ru/2013/04/fm-transmitter.html
Is this mic preamplifier of class C? If it is then how can it works? Will I hear the sound?
I just can't understand why we can use class C amplifier instead A?
The noise is very unlikely to be coming from any of the components in the circuit. The biggest culprit is likely to be the power supply. Add a 22uF and 0.1 uF (helps with high frequencies) capacitor and a 100 ohm resistor that feed the electret part of the circuit and a 22uF across the 5V supply.
And why the signal from microphone is so weak?
The microphone picks up air vibrations and the energy hitting it is proportional to the surface area of the microphone's moving element - consider a loud speaker a couple of metres away pumping out one watt - how much power/energy does the microphone diaphragm receive - Imagine how many microphones you can fit side by side at a couple of metres from this speaker in all directions.
How many would that be? Let me guess at ten thousand and, each one receives one ten-thousandth of one watt - that's a tenth of a milliwatt each. 600 ohm is normal for microphones so the signal amplitude (with a bit of handwaving and magic) is \$\sqrt{0.1 mW\times 600}\$ = 245 mV
Just how big did you expect a microphone signal to be?
The circuit is wrong as pointed out by others - it doesn't work.
Best Answer
No, the microphone pre-amplifier circuit is not biased class C.
The transistor is certainly biased on and, assuming the microphone signal is small enough, the transistor will always be conducting so this is a class A voltage amplifier.
Why do think it is a class C amplifier?
There isn't much to explain about the operation.
A bias voltage is applied via the resistors \$R_2\$ and \$R_3\$. The operating point is stabilized by degenerative feedback, i.e., if the collector current increases, the collector voltage decreases which decreases the forward bias decreasing the collector current.
This type of bias is described at Wikipedia article section "Collector feedback bias"