Motorola GM1200E

Transmitter Power Amplifier (PA) 5-25W
3.1-20 Introduction/Theory of Operation
The collector current of Q5510 causes a voltage drop across the resistors R5623 and R5624.
Transistor Q5612 adjusts the voltage drop across R5621 controlled through the PA control line
(PWR CNTL). The current source Q5621 adjusts the collector current of Q5510 by modifying its
base voltage via (R5502, L5501) until the voltage drop across R5623 and R5624 plus V
BE
(0.6V)
equals the voltage drop across R5621 plus V
BE
(0.6V) of Q5611. If the voltage of PWR CNTL is
raised, the base voltage of Q5612 will also rise causing more current to flow to the collector of
Q5612 and a higher voltage drop across R5621. This in turn results in more current driven into the
base of Q5510 by Q5621 so that the collector current of Q5510 is increased. The collector current
settles when the voltage over the series configuration of R5623 and R5624 plus V
BE
(0.6V) of
Q5621 equals the voltage over R5621 plus V
BE
(0.6V) of Q5611.
By controlling the output power of Q5510 and in turn the input power of the following stages the ALC
loop is able to regulate the output power of the transmitter. Q5611 is used for temperature
compensation of the PA output power.
In receive mode the PA control line (PWR CNTL) is at ground level and switches off the collector
current of Q5612 which in turn switches off the current source transistor Q5621 and the RF
transistor Q5510.
9.2 PA Stages
The bipolar transistor Q5520 is driven by Q5510. To reduce the collector - emitter voltage and in turn
the power dissipation of Q5510 its collector current is drawn from the antenna switch circuit.
In transmit mode the base of Q5520 is slightly positive biased by a divided K9V1 signal. This bias
along with the RF signal from Q5510 allows a collector current to be drawn from the antenna switch
circuit and in turn switches the antenna switch to transmit, while in receive mode the low K9V1
signal with no RF signal present cuts off the collector current and in turn switches the antenna
switch to receive.
The following stage uses an enhancement mode N-Channel MOS FET device (Q5530) and requires
a positive gate bias and a quiescent current flow for proper operation. The voltage of the line BIAS
VLTG is set in transmit mode by a Digital to Analogue (D/A) converter (U0731-4) and fed to the gate
of Q5530 via the resistive network R5521, R5522 and R5523. The bias voltage is tuned in the
factory. If the transistor is replaced, the bias voltage must be tuned with the Dealer Programming
Software (DPS). Care must be taken, not to damage the device by exceeding the maximum allowed
bias voltage. The collector current is drawn from the supply voltage A+ via L5532.
The final stage uses the bipolar device Q5536 and operates off the A+ supply voltage. For class C
operation the base is DC grounded by two series inductors (L5533, L5534). A matching network
consisting of C5541-C5544 and two striplines transforms the impedance to 50 Ohms and feeds the
directional coupler.
9.3 Directional Coupler
The directional coupler is a microstrip printed circuit which couples a small amount of the forward
power off the RF power from Q5536. The coupled signal is rectified to an output power proportional
negative DC voltage by the diode D5553 and sent to the power control circuit in the controller
section via the line PWR DETECT for output power control. The power control circuit holds this
voltage constant, thus ensuring the forward power out of the radio to be held to a constant value.