Transmitter Power Control; Control Line Functions; Power Control Operation; Rf Power Ampliþers - Motorola GM300 Service Manual

Theory of Operation
RF Power Amplifiers

Transmitter Power Control

Control Line Functions

As discussed earlier, all DC operating voltage for the
radio originates from the RF power ampliÞer via J7-5
(UNSW B+). In addition, J7 receives DC control volt-
ages from, and supplied DC control voltages to, the RF
power ampliÞer circuitry.
In the transmit mode, Q455 conducts, supplying 9.6T
(keyed 9.6V DC) via J7-1 to the antenna switch and pro-
viding bias (and collector voltage, in UHF models) for
the Þrst transmitter stage.
Controlled B+ is supplied via J7-2 to the collector of the
Þrst (VHF) or second (UHF) transmitter stage. As this
voltage varies over the range of 3 to 12.5V DC, trans-
mitter output power is varied over the range of 10 to 25
watts.
A small-value (.01 to 0.2 ohm) resistor is placed in
series with the DC supply to the RF Þnal ampliÞer, and
the voltage across this resistor is measured via the cur-
rent sense high (J7-3) and low (J7-4) lines.
The temperature of the Þnal RF ampliÞer is monitored
by a thermistor mounted next to it. The thermistor is
part of a voltage divider network whose output volt-
age is routed, via J7-6, to an A/D input of the micro-
computer, U802-49.

Power Control Operation

Power level is programmed electronically during radio
tune-up at the factory. The software varies the DC out-
put voltages of D/A converters U801-2 and U801-4
(two D/AÕs are summed for increased resolution) and
applied to the (+) input of opamp U451A. As D/A volt-
age increases, U451A-1 output voltage increases,
which causes greater conduction in Q453 and Q452,
thus increasing the control voltage and RF power out-
put. The values of R810, R811, R462 and R463 reduce
the D/A output voltage range (0 to 9V DC) to the
required 4V to 6V DC range at U451A-3.
If the DC current of the Þnal RF ampliÞer increases
excessively due to, for example, an antenna system
fault, the increased voltage drop across the series resis-
tor will be sensed by U451B, causing the current
through Q454 to increase until the two inputs of U451B
are again equal (the collector resistor of Q454 is on the
RF power ampliÞer board since its value is optimized
for different power ampliÞers). The increase in Q454
current raises its emitter voltage which is applied to the
(-) input of U451A. This is in a direction to reduce the
output voltage of U451A-1, reducing conduction in
Q453 and Q452, lowering the controlled B+ voltage to
counteract the excessive current condition.
Excessive RF Þnal ampliÞer temperature causes an
increase in the THERMAL DC voltage at U802-49. The
2-10
microcomputer reduces power via the D/A converter
to achieve a safe equilibrium.
Under any circumstance, controlled B+ is prevented
from exceeding a pre-programmed DC voltage. The
voltage is divided by R471 and R472 and monitored by
microcomputer A/D input U802-45 every 17 msec. If
voltage exceeds the maximum allowable, the DAC is
decremented one step, and this process is repeated as
needed.
RF Power Amplifiers
This manual covers several different power ampliÞer
kits. Each is optimized for the speciÞc frequency and
power output range of the radio model in which it is
used. All are similar in that they can be divided into
three basic circuits:
¥ Power ampliÞer
¥ Antenna switch
¥ Harmonic Þlter
1-10 Watt VHF Power Amplifier
The 1-10 watt VHF power ampliÞer is designed to
cover the range of 146-174 MHz. It consists of three
stages. The Þrst stage, Q2210, operates in Class A with
its operating voltage supplied by the 9.6T source. It has
a gain of 9 dB and delivers 160 mW of output power.
The second stage, Q2220, is operated in Class C and
also has 9 dB of gain. The collector voltage is supplied
from controlled B+. The output level of this stage is
varied by changes in the controlled B+ voltage. This
stage delivers up to 1.3 watts output.
The third stage, Q2230, is the Class C RF Þnal power
ampliÞer. It can output in excess of 14 watts. Collector
current is monitored by the power control circuit by
measuring the voltage drop across a 0.2 ohm resistor,
R2274, placed in series with the collector DC supply
lead.
The antenna switch consists of two pin diodes, CR2250
and CR2251. L2252 is parallel-resonant with the off
capacitance of CR2250, increasing its attenuation when
off. C2253 is a DC block.
In the receive mode, both diodes are off. Signals
applied at the antenna jack J1 are routed, via the har-
monic Þlter, through network L2251, C2251 and C2252
to the receiver input. In the transmit mode, 9.6T is
present and both diodes are forward-biased into con-
duction. The transmitter RF from Q2230 is routed
through CR2250, and via the harmonic Þlter to the
antenna jack. CR2251 conducts, shunting RF power
and preventing it from reaching the receiver. L2251 is
selected to appear as a 1/4 wave at VHF, so that the low
68 No.
Name of Manual
March, 1997