A radio receiver always has a radio frequency section, which is tunable circuit connected to the antenna terminals. It is there to select the wanted frequency and reject some of the unwanted frequencies. However such a receiver need not have a radio frequency amplifier following this tuned circuit. If there is an amplifier its output is fed to the mixer at whose input another tunable circuit is present.
In many instances, however the tuned circuit connected to the antenna is the actual input circuit of the mixer. The receiver is then said to have no radio frequency amplifier.
The receiver having the radio frequency stage is undoubtedly superior in performance to the receiver without one, all else being equal. On the other hand there are some instances in which a radio amplifier is uneconomical like where its inclusion would increase the cost of the receiver while increasing performance.
Radio frequencies amplifiers have many advantages like better sensitivity, improved image frequency rejection, improved signal to noise radio, improved rejection of adjacent unwanted signals, better coupling of the receiver to the antenna , prevention of spurious frequencies from entering the mixer and heterodyning there to produce an interfering frequency equal to the intermediate frequency from the desired signal and prevention of re radiation of local oscillator through the antenna of the receiver.
The single tuned, transformer-coupled amplifier is most commonly employed for radio frequency amplification. The sensitivity of a radio receiver is its ability to amplify weak signals. It is often defined in the terms of the voltage that must be applied to the receiver input terminals to give a standard output power, measured at the output terminals. For amplitude modulation broadcast receivers several of the relevant quantities have been standardized.
The selectivity of the receiver is its ability to reject unwanted signals. Selectivity is measured at the end of the sensitivity test with conditions same as for sensitivity, except that now the frequency of the generator is varied to the either side of the frequency to which the receiver is tuned.
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