LESSON 4
AUDIO CIRCUITS AND REPRODUCERS
SCOPE ....................................Noise discrimination; AF amplification;
coupling;
distortion;
degeneration;
reproducers;
transistorized
AF
amplifiers.
CREDIT HOURS .............................1
TEXT ASSIGNMENT ..........................TM
11-665,
para
115,
144-148,
151;
Attached Memorandum, para 4-1.
MATERIALS REQUIRED .......................None
SUGGESTIONS ..............................Read assignment in TM 11-665
before
reading the attached memorandum.
LESSON OBJECTIVES
When you have completed this lesson, you will be able to:
1. Explain the operation of audio amplifiers.
3. Explain the operation and application of the various methods of coupling between
audio stages.
4. Explain how AF current variations are converted into sound waves.
5. Explain the operation of transistorized audio circuits.
ATTACHED MEMORANDUM
4-1.
TRANSISTORIZED AF AMPLIFIERS
a. Preamplifiers.
In transistor receivers, audio preamplifier stages are used
whenever the detector output is insufficient to drive the audio output stage.
Figure 4-1 illustrates such a preamplifier stage, using a medium-power PNP
transistor in a class A amplifier circuit.
(1) Biasing. The emitter-base forward bias is obtained from a +12-volt source
connected to the R1-R2-R3 voltage-divider network.
(2) Signal path.
The detector output is transferred into the secondary of
input transformer T1 which carries both the input signal and the bias
voltage to the base. The C1 is a bypass capacitor to prevent
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