____________________________________________________________________________ Amplifiers
5-76. Figure 5-20 shows negative feedback in a CE transistor amplifier. The feedback
network of C2 and R2 couples part of the output signal of Q1 back to the input. Since the
output signal is 180 out of phase with the input signal, this causes negative feedback.
Figure 5-20. Negative Feedback in a Transistor Amplifier
5-77. Negative feedback is used to improve fidelity of an amplifier by limiting the input
signal. Negative feedback can also be used to increase the frequency response of an
amplifier. The gain of an amplifier decreases when the limit of its frequency response is
reached. When negative feedback is used, the feedback signal decreases as the output
signal decreases. At the limits of frequency response of the amplifier, the smaller feedback
signal means that the effective gain (gain with feedback) is increased. This will improve
the frequency response of the amplifier.
AUDIO AMPLIFIERS
5-78. An audio amplifier has been described as an amplifier with a frequency response
from 15 Hz to 20 KHz. The frequency response of an amplifier can be shown graphically
with a frequency-response curve. Figure 5-21 is the ideal frequency-response curve for an
audio amplifier. This curve is practically "flat" from 15 Hz to 20 KHz. This means that the
gain of the amplifier is equal between 15 Hz and 20 KHz. Above 20 KHz or below 15 Hz
the gain decreases or "drops off" quite rapidly. The frequency response of an amplifier is
determined by the components in the circuit.
5-79. The difference between an audio amplifier and other amplifiers is the frequency
response of the amplifier. There are certain techniques and components used to change and
extend the frequency response of an amplifier. The transistor itself will respond quite well
to the audio frequency range. No special components are needed to extend or modify the
frequency response.
5-80. You have seen the purpose of all the components in a transistor audio amplifier.
We will not look at schematic diagrams of several audio amplifiers and the functions of
each of the components will be discussed.
23 June 2005
TC 9-62
5-21
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