Text 9. Radio-Frequency Amplifiers

The functions of a radio-frequency amplifier are to increase the voltage ofthe radio-frequency (r. f.) signal and to secure the re­quired selectivity of the receiver.

The voltage applied to the input of a r. f. amplifier is from units to hundreds of microvolts depending on the sensitivity of the receiv­er. Before the signal reaches the detector it should be amplified a million times or more. Such voltage gain may be obtained only with the aid of several amplifier stages.

A r.f. amplifier stage contains a valve or a transistor and a load, which is a resonant circuit tuned to the frequency of the signal ap­plied to the input of the stage. This resonant circuit may be a signal tuned circuit or a band-pass filter.

R. f. amplifiers in which single-tuned circuits serve as a load are known as tuned amplifiers. In case r. f. amplifiers employ band-pass filters for load they are called band-pass or filter amplifiers.

Band-pass amplifiers have a nearly rectangular resonance curve. They are mostly fixed frequency amplifiers, i. e. their tuned circuits do not have to be retuned when the receiver is in operation. Band-pass amplifi­ers are widely used as i. f. amplifiers in superheterodyne receivers.

In a band-pass amplifiers the anode load is a band-pass filter which may have widely differing circuit configurations and may be con­nected to the anode of the amplifier valve in many ways.

Notes

r. f. (radio frequency)	частота
to secure [si'kjua]	забезпечувати
Band	частотна смуга
rectangular [rek'ueggjula]	прямокутний
heterodyne f'hetaradain]	гетеродинний
f. — intermediate frequency [inta'mirdjat]	— — проміжна частота
anode [a'noud]	— анод

Text 11. Detection

The process by which the signal being transmitted is reproduced from the radio-frequency currents present at the receiver is called detection, or sometimes demodulation. Where the intelligence is trans­mitted by varying the amplitude of the radiated wave, detection is accomplished by rectifying the radio-frequency currents.

The rectified current thus produced varies in accordance with the signal originally modulated on the wave radiated at the transmitter and so reproduces the desired signal.

Thus, when the modulated wave is rectified, the resulting current has an average value that varies in accordance with the amplitude of the original signal.

In the transmission of code signals by radio, the rectified current reproduces the dots and dashes of the telegraph code and could be used to operate a telegraph sounder. When it is desired to receive the telegraph signals directly on a telephone receiver, it is necessary to break up the dots and dashes at an audible rate in order to give a note

that can be heard, since otherwise the telephone receiver would give forth a succession of unintelligible clicks.

The detection of a frequency-modulated wave involves two steps.

First, the wave is transmitted through a circuit in which the rel­ative output obtained from the circuit depends upon the frequency.

The circuit output is then an amplitude-modulated wave since, as the frequency of the constant-amplitude input wave varies, the out­put will vary correspondingly.

The resulting amplitude-modulated wave is then rectified.

Notes

to detect [di'tekt]	випрамляти
to transmit [traenz'mit]	передавати
to rectify ['rektifai]	випрамляти (струм)
correspondingly ^kons'pondiijli]	відповідно
click	клацання
dot	крапка
dash [dse/]	тире
unintelligible [jAnin'telidjsbl]	незрозумілий