Feedback Amplifiers

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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