There are two important functions of RF amplifier:
1) To increase the strength of weak RF signal.
2) To reject image frequency signal. In FM broadcast the channel bandwidth is large as compared to AM broadcast.
Hence the RF amplifier must be design to handle large bandwidth.
The function of frequency mixer is to heterodyne signal frequency fs and local oscillator frequency fo. At the output, it produces the difference frequency known as intermediate frequency fi. The intermediate frequency used in FM receiver is higher than that in AM receiver. Its value is 12MHz (practical value of IF is 10.7MHz).
Since FM broadcast operates in VHF and UHF band, a separate local oscillator is used in FM receiver The local oscillator frequency fo is kept smaller than the signal frequency fs by an amount equal to the intermediate frequency fi (fi = fs-fo).
Two or more stages of IF amplifier are used to provide large gain to the receiver. This increases the sensitivity of a receiver. If amplifier should be designed to handle large bandwidth.
The function of amplitude limiter is to remove all amplitude variation of FM carrier voltage that may occur due to atmospheric disturbances. Use of amplitude limiter makes the system less noisy.
FM Discriminator or detector:-
It separates modulating signal from frequency modulated carrier signal. Thus it produces audio signal at its output.
Audio frequency voltage and power amplifier:-
Audio amplifier increases voltage and power level of audio signal to a suitable level.in FM broadcast, the maximum modulating frequency is 15 kHz. Hence the audio amplifier must have large bandwidth.