Чтение общенаучной литературы. Кытманова О.А. - 9 стр.

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10
current flowing in the same direction. Once this field has vanished and the current
has ceased, the capacitance is found to be in a charged condition, the upper plate
being negative and the lower plate positive.
Having reversed the charge on the capacitance, the above process will repeat
itself, this time the electron current surging clockwise around the circuit. Thus the
current rushes first in one direction, then in the other, oscillating back and forth in
an electrical way just as any spring pulled to one side and released vibrates in a
mechanical way.
When a straight spring is pulled to one side and released, the kinetic energy it
gains upon straightening keeps it moving and it bends to the other side. Just as the
vibration amplitude of the spring slowly decreases because of friction, so also does
the current in the electrical circuit decrease due to electrical resistance. A graph
showing how current slowly dies out in an electric circuit is given in Fig. 16.
These are called damped vibrations, or damped oscillations. If the resistance of
the circuit is high, the damping is high and the current quickly dies out after but
few oscillations. The resistance being low, however, the damping is small, the
amplitude decreases slowly, and there are many oscillations.
Radio-Frequency Amplifiers
The functions of a radio-frequency amplifier are to increase the voltage of the
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 single 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 amplifier 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.

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