Microphone Amplifier

TM 11-5895-1141-34

referred to as P1 in this subparagraph, is actually P1201. The following circuits found in this module will be examined in

detail:

Microphone amplifier (U1)

Regulated amplifier (U2, U4, Q1)

Buffer amplifiers (Q2, Q3, Q4, U5-B, C, D)

Lamp supply bus regulator

(1) Microphone Amplifier. Refer to figure FO-8. Low level audio signals from the jack panel assembly are

supplied via pins 1 and 19 to microphone transformer T1001. Potentiometer R2 provides for adjustment of the level of the

signal entering amplifier stage U1 and can supply up to 20 db of signal attenuation. U1 is configured as a non-inverting

amplifier with a voltage gain of close to four as set by resistors R4 and R5. The dc output voltage of U1 is +12 v to permit

control element Q1 to operate properly.

(2) Regulated Amplifier. Refer to figure 2-6. U2, Q1 and U4 act as an automatic gain control or regulating loop

that serves to prevent undesirably large variations in audio output signal fed to the transmitter from the microphone

amplifier despite a wide variation in microphone audio input amplitude. R6 and field effect transistor Q1 form a dynamic

voltage attenuator. Q1 is operated as a variable shunt resistance, so that its drain to source resistance decreases as the

positive dc gate voltage increases. The purpose of U4 is to precisely half-wave rectify the audio signal received via C2,

filter the signal after rectification and deliver this positive dc voltage to the gate of Q1. Then, if the U2 output signal

attempts to increase because of an increase in microphone audio the drain to source resistance of Q1 is decreased,

increasing the attenuation of the R6, Q1 divider. The net result then, is the regulator circuit maintains the U2 audio output

fairly constant. Diode CR2 supplies the half-wave rectification so that only positive excursions appear at the U4 output.

Capacitor C3 and resistor R12 filter this rectified output providing smooth dc to the gate of Q1. Resistor R18 is a threshold

control which provides a maximum positive bias voltage of 7 vdc on the Q1 gate in the absence of signal. This voltage

determines the level of signal at which the regulator circuit becomes effective. The smaller the bias voltage the greater the

signal level is required before Q1 attenuator action is initiated, and the regulator circuit becomes activated. Note that the

filter capacitor C3 is allowed to charge much faster (through R13) than it can discharge (through R12). This prevents any

significant decay of voltage on the capacitor between spoken words which would have the effect of increasing the noise

level in the audio circuit since the gain of the regulator would increase between spoken words.

on those installations where the speech regulator action is not desired the jumper between E21 and E22 may be removed.

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