RF AMPLIFIERS A8A5 AND A8A6

TO 31R2-2GRC171-2

TM 11-5820-815-14

NAVELEX 0967-LP-544-5010

produce a reference voltage across diode CR4 that

cut to the center of the uhf operating frequency. Refer to

biases Q1 to about 20 milliamperes. Thermistor RT12

A8HY1 and A8HY2 of figure FO-25. When terminals E2,

provides temperature compensation for the stage 4

E3, and E4 of A8HY1 are terminated into 50 ohms

linear rf amplifier.

(characteristic impedance of the quarter wavelength

lines), the input rf signal applied to terminal E1 divides

4-261. The amplified output of Q1 is coupled to the

equally and appears at output terminals E3 and E4, 3 dB

stage 5 linear rf amplifier, Q2, through the impedance

down (half power) and 90-degree phase related. No

matching network composed of capacitors C17, C18,

signal appears at terminal E2. The isolation between

and C19, inductor L5, and resistors R18 and R19.

output terminals is approximately 25 dB. When two rf

Transistor Q2 is biased by base current supplied from

signals are simultaneously applied to terminals E3 and

bias network R13-R10-RT14-CR5 and applied to the

E4, the signals add together and appear at either

base of Q2 through inductor L7 and impedance network

terminal E1 or terminal E2 depending upon the 90-

Z2 (ferrite beads). Resistor R10 is test selected to

degree phase relationship of the input signals. In the

produce a reference voltage across diode CR5 that

case of A8HY2 of figure FO-25, the rf signals add at

biases Q2 to about 20 milliamperes. Thermistor RT14

terminal E1 and cancel at terminal E2. If the loading at

provides temperature compensation for the stage 5

terminals E3 and E4 of A8HY1 becomes greater than 50

linear rf amplifier. The amplified output of Q2 is coupled

ohms, the reflected power that results from the mismatch

to the output of A3 through the impedance matching

adds together and appears at terminal E2. This reduces

network composed of inductors L8, L9, and L10 and

the power output at terminals E3 and E4 and increases

capacitors C20, C21, C22, C25, and C26.

the power at terminal E2. The hybrid coupler used in the

pin diode ALC attenuator of figure FO-28 uses this

4-262. When the transmitter is unkeyed, the voltage

principle to attenuate the input rf signal.

applied to the +5-V dc switched input is reduced to 0 V

dc and the applied ALC voltage is increased to a high

4-259. Refer to figure FO-28. ALC voltage applied

positive value. This removes the bias voltage from Q1

through rf chokes L1 and L2 to the anode of pin diodes

and Q2 and causes maximum signal attenuation by the

CR1 and CR2 causes the resistance to ground at

pin diode ALC attenuator. The result of this circuit action

terminals E3 and E4 of HY1 to vary as a function of the

is to provide maximum attenuation in the rf signal path

ALC voltage. When the ALC voltage is a high positive

through A8A3.

value, the resistance of CR1 and CR2 is low causing the

resistance to ground at terminals E3 and E4 to be

4-263. RF DRIVER A8A4. Refer to figure FO-29. The rf

approximately 50 ohms (R2 in parallel with R3 and R4 in

input signal is coupled to the broadband rf power

parallel with R5). For this condition, the pin diode ALC

amplifier, Q1, through the gain slope pad composed of

attenuator provides maximum signal attenuation since

inductors L1 and L2, resistors R1, R2, and R3, and

almost all the rf input signal applied to terminal E1

capacitor C1. The gain slope pad compensates for loss

appears across resistors R2, R3, R4, and R5 and very

in power amplifier gain as the frequency of the applied rf

little appears at terminal E2. The rf signal that does

input signal increases. The pad loss is about -5 dB at

appear at terminal E2 is shunted to ground through pin

225 MHz, -2 dB at 300 MHz, and -0.5 dB at 399 MHz. Rf

diode CR3 and capacitor C4. Pin diode CR3 offers

power amplifier Q1 amplifies the rf signal to

minimum resistance because of ALC voltage applied

approximately 12-watt carrier power. Refer to paragraph

through HY1 from terminal E4 to terminal E2. As the

4-266 for a description of the rf amplifier.

ALC voltage decreases toward zero and becomes

negative, the resistance of CR1, CR2, and CR3

4-264. RF AMPLIFIERS A8A5 AND A8A6. Refer to

increases causing the resistance to ground at terminals

figure FO-30. The rf input signal is applied to terminal

E2, E3, and E4 to increase. As the resistance increases

E2 of hybrid HY1. HY1 splits the signal into two separate

from 50 ohms, the increase in reflected power resulting

signals that are 3 dB down from the input signal and 90-

from the mismatch appears at terminal E2. Thus, as the

degree phase related and applies them to two broadband

ALC voltage decreases, the attenuation of the pin diode

rf amplifiers, Q1 and Q2. The amplified outputs of Q1

ALC attenuator decreases causing the signal at terminal

and Q2 are applied to terminals E3 and E4, respectively,

E2 to increase. This signal is applied to the base of

of hybrid HY2 where they are shifted back into phase

transistor Q1 of the stage 4 linear rf amplifier through the

and coupled to the rf output at terminal E2. Refer to

impedance matching network composed of capaci-tors

paragraph 4-266 for a description of the rf amplifiers.

C5, C6, C7, C8, and C9; inductors L3 and L4; and

resistor R7.

4-265. Refer to figures FO-25 and FO-30. Isolation

resistors A8R3 and A8R5 connected to rf amplifier A8A5

4-260. Transistor Q1 is biased by base current supplied

dissipate any reflected power that may result from

from bias network R11-R9-RT12-CR4 and applied to the

impedance unbalance at the input or output of

base of Q1 through resistor R8 and impedance network

Z1 (ferrite beads). Resistor R9 is test selected to

4-45