# Microphone preamplifier

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?

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"

You do realize that this is a wireless FM microphone circuit. T1 is the microphone preamplifier but T2 is an oscillator and T3 is a class C power amplifier driving the antenna. Since the modulation is FM, which means all of the information is contained in the zero crossings of the modulated carrier, the power amplifier feeding the antenna can operate in Class C without distorting the modulation. If AM modulation had been used, than the power amplifier would have to be a linear type, class A for example.