Audio Frequency Amplifier AM -3506/PRC-47 (A8A1)

TM 11- 5820- 509-35

resistance through the diode legs of CR17 so that audio

2-9. Audio Frequency Amplifier AM -3506/PRC-47

peaks from the microphone input are limited and the

(A8A1) (fig. 2-5 through 2-7)

amplified output from Q1 remains essentially constant

for wide variations in microphone input level. When the

a. The audio from the microphone is applied to

incoming audio is a low-level signal, the forward bias

microphone amplifier Q1 and to the shunt compressor

applied to compressor CR17 is small and the individual

circuit (CR17). After amplification, part of the output

diodes are operated in their high forward-resistance

signal from Q1 is routed to audio amplifier Q3 and the

region. As the audio signal increases, the bias voltage

remainder is amplified in compressor amplifier Q2. The

from compressor rectifier CR1 through CR4 also

amplified output of Q2 is then coupled by transformer

increases and the diodes of CR17 operate in a lower

T1 to compressor rectifier CR1 through CR4, filtered by

forward-resistance region and load the input circuit of

capacitors C3 and C4, and applied as an un. grounded

microphone amplifier Q1. The operating bias for the

forward bias voltage across compressor CR17. This

carbon microphone element in the handset is obtained

bridge rectifier assembly is connected so that two

from the 20-volt dc power connection. Resistor R3 and

variable-resistance paths are available to the incoming

capacitor C] provide decoupling and control the amount

audio signal. The amount of forward bias applied by

of bias applied to this microphone circuit.

compressor rectifier CR1 through CR4 determines the

path

Figure 2-5 Compressor/Amplifier Circuit Schematic Diagram.

connected to the cw key input (P1-18) and to the audio

b. When the radio set Is operated in either the cw

oscillator circuit. When this ground is connected to the

or fsk mode, or when OPR/TUNE switch S102 is placed

cold end of its tank circuit (L2-

in the TUNE position, a ground is

2-14