Distortion and Stability in output
It is well known that the
output of an amplifier does not have a voltage waveform exactly a replica of
the input signal. Because of the presence of inductance and capacitance in the
tubes and circuits, the various frequency components of the signal are affected
by different amplifications and phase shifts. Also because of nonlinear action
of the tube itself, the output voltage is not exactly prop optional to the
input voltage. Again, if one or more of the bias voltages E0, Eb0, etc., varies,
the
operating point shifts and the resulting changes in , r, and Urn
affect the
amplification. The latter effect is sometimes useful as a volume control, or it
may be the basis for a method of changing the waveform of the output in an
easily controlled manner. The latter process, called modulation, sometimes
takes place when not wanted, as, f or example, when a poorly filtered
plate-supply voltage is used. Whether this occurs in an a-f amplifier or in an
r-f amplifier, the result may be noticeable in a loudspeaker as hum. The change
in gain with change in operating point may be very objectionable for another
reason:
If a battery-operated amplifier has a certain normal gain, this gain
may increase upon renewal or recharge of the batteries. This may not seem
serious at first thought, but suppose we consider a long-distance telephone
system. The energy given by the microphone alone may be sufficient for
satisfactory local operation, but it normally needs building up by repeater
(amplifier) units two or more times every 100 miles of line. Since these
repeaters are distant from one another, they operate independently and a rise
in gain for one is not automatically compensated for by a lower gain in the
next. Hence a distant repeater may find itself with a signal far too weak or
maybe too strong. What is needed for each repeater is a stabilizer which will
make the amplification independent of small changes in operating biases.
Interference in Feedback Amplifier Output
In addition to the distortion
and modulation products discussed in the preceding article, the output of an
amplifier contains interference and noise components which have frequencies
that are unrelated to the input signal. Plate-supply hum in a-f amplifiers may
be considered as interference. This hum may be removed by more comp lete
filtering, or it may be eliminated by the feedback processes discussed in this
chapter. Other interference sources will now be considered.
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