RF AMPLIFIERS A8A4Q1, ASA5Q1/Q2, A8A6Q1/Q2

TO 31R2-2GRC171-2

TM 11-5820-815-14

NAVELEX 0967-LP-544-5010

rf amplifiers A8A5Q1 or A8A5Q2. In a like manner,

differential input voltage to develop the base drive signal

isolation resistors A8R2 and A8R6 dissipate reflected

to A8Q1. Potentiometer A8A1R11 is adjusted to set the

power from rf amplifiers A8A6Q1 or A8A6Q2.

collector voltage of A8Q1 to about 6 V dc.

4-266.

RF AMPLIFIERS A8A4Q1, ASA5Q1/Q2,

4-271.

When transmit audio is applied, capacitor

A8A6Q1/Q2. The rf amplifier (A8A4Q1 of figure FO-29)

A8A1C3 couples the audio signal to the noninverting

of rf driver A8A4 and the rf amplifier pair (A8A5Q1/Q2

input of A8A1U1. The audio signal, when amplified by

and A8A6Q1/Q2 of figure FO-30) of rf amplifiers A8A5

A8A1U1, causes the collector voltage of A8Q1 to vary at

and A8A6 are all identical. Therefore, only rf amplifier

the audio rate which, in turn, causes the collector voltage

ASA5Q1 of figure FO-30 will be discussed in detail.

of A8A2Q2 and A8A2Q3 to vary at the audio rate. By

Transistors Q1 and Q2 or rf amplifiers A8A5 and A8A6

varying the collector voltage, the gain of A8A2Q2 and

are a matched quad so that the gain through each of the

A8A2Q3 is modulated at the audio rate which causes the

four parallel paths is identical.

transmit rf signal to be amplitude modulated.

4-267. Refer to figure FO-30. The output of hybrid HY1

4-272. Ac gain of the modulator is controlled by negative

is coupled to the base of rf amplifier transistor Q1

feedback of the modulation signal applied to the inverting

through the impedance matching network composed of

input of ABA1U1. The ac component of the modulation

inductors L1 and L11 and capacitors C1, C2, C3, and

feedback signal (P1-5) is applied to the inverting input of

C4. Transistor Q1 is biased by a reference voltage

A8A1U1 through voltage divider A8A1R7-R15, resistor

developed across diode CR1 of bias network CR1-R2-

A8A1R9, and capacitor A8A1C9

C1 and applied to the base of Q1 through resistor R1.

Capacitor A8A1C7 provides an ac ground for resistor

The amplified output of Q1 is coupled to hybrid HY2

A8A1R10. The ac component of the pa forward power

through the impedance matching network composed of

signal detected at reflectometer A8A7 and amplified by

inductors L12, L4, L3, and L5 and capacitors C5 through

A8A1U3A is fed back as envelope feedback to the

C9. Capacitors C12, C13, and C10 and inductor L2

inverting input of A8A1U1 through voltage divider

decouple the rf amplifier from the +26-V dc line. The

A8A1R19-R20, (A8A1R20 grounded through A8A1U2D),

amplifier power gain is approximately 8 dB.

resistor A8A1R18, and capacitor A8A1C9. When a vswr,

pa/antenna power, or temperature fault is detected, the

4-268. REFLECTOMETER A8A7. Refer to figure FO-

envelope feedback control switch, A8A1U2F and

31. The reflectometer senses forward and reflected

A8A1U2D, opens the ground circuit to resistor A8A1R20

power at the output of the power amplifier module. The

to increase the level of the envelope feedback signal.

forward power sensor consists of resistor R2, capacitors

The ac gain of the modulator circuit is such that

C1 and C3, and detector diode CR1. The reflected

approximately 0.85 volt of transmit audio modulates the

power sensor consists of resistor R1, capacitors C2 and

transmit rf signal to 90 percent. Envelope feedback

C4, and detector diode CR2. Both sensors develop

improves the audio quality (reduces distortion) of the

approximately 1 volt for 20 watts (0.5 volt at 5 watts) of

transmitted audio signal.

forward or reflected power.

4-273. ALC Circuit. Refer to figure FO-26. Reference

4-269. ALC/MODULATOR A8A1. ALC/modulator A8A1

designators apply to A8A1. The ALC circuit consists of

contains the modulator circuit, ALC circuit, antenna vswr

the following circuits of ALC/modulator A8A1: pa forward

monitor, pa/antenna power monitor, and the temperature

power amplifier U3A, buffer U7A, ALC differential

monitor.

amplifier U6B, ALC differential amplifier U4A, ALC output

amplifier Q3-Q4, pa power output reference source U6A,

4-270. Modulator Circuit. Refer to figures 4-9, FO-25,

ALC precharge amplifier U4B-Q5, and time constant

FO-26, and FO-27. With no transmit audio applied,

control Q1-Q6-Q7. In the primary control loop, the ALC

modulation amplifier A8A1U1 sets the quiescent

circuit receives the antenna forward power signal (P1-1)

modulation input voltage applied to rf preamplifier/

from the rf filter module and provides ALC voltage (P1-

modulator A8A2 at about 6 V dc. In A8A2, this voltage is

11) to the pin diode ALC attenuator of rf predriver/ALC

applied as collector voltage to the high-level modulated rf

attenuator A8A3 to control power output at the antenna.

amplifiers, A8A2Q2 and A8A2Q3. To set the quiescent

In the secondary control loop, the ALC circuit receives

modulation input level, modulation amplifier ASA1U1

the pa forward power signal (P1-6) from reflectometer

compares a reference voltage to a modulation feedback

A8A7 and provides ALC voltage (P1-11) to the pin diode

voltage (P1-5) to control base drive to modulation

ALC attenuator (A8A2) to control power output from the

transistor A8Q1.

The reference voltage developed

rf power amplifier module. The secondary control loop

across resistor A8A1R6 of voltage divider A8A1R5-R6 is

takes over control if, for some reason, the rf filter module

applied to the noninverting input of ASA1U1. The

fails to provide power to the antenna or has a high

feedback voltage developed at the arm of potentiometer

insertion loss. The ALC circuit functions as follows.

A8A1R11 of voltage divider A8A1R8-R11-R12 is applied

through resistor A8A1R10 to the inverting input of

A8A1U1.

A8A1U1

amplifies

the

4-46