Pa Stages; Directional Coupler; Antenna Switch - Motorola GM950E Service Manual

The collector current of Q8510 causes a voltage drop across the resistors R8623 and R8624.
Transistor Q8612 adjusts the voltage drop across R8621 through PA control line (PWR CNTL). The
current source Q8621 adjusts the collector current of Q8510 by modifying its base voltage until the
voltage drop across R8623 and R8624 plus VBE (0.6V) equals the voltage drop across R8621 plus
VBE (0.6V) of Q8611. If the voltage of PWR CNTL is raised, the base voltage of Q8612 will also rise
causing more current to flow to the collector of Q8612 and a higher voltage drop across R8621. This
in turn results in more current driven into the base of Q8510 by Q8621 so that the current of Q8510
is increased. The collector current settles when the voltage over the series configuration of R8623
and R8624 plus VBE of Q8621 equals the voltage over R8621 plus VBE (0.6V) of Q8611.
By controlling the output power of Q8510 and in turn the input power of the following stages the ALC
loop is able to regulate the output power of the transmitter. Q8611 is used for temperature
compensation of the PA output power.
9.2

PA Stages

The bipolar transistor Q8520 is driven by Q8510. To reduce the collector-emitter voltage and in turn
the power dissipation of Q8510 its collector current is drawn from the antenna switch circuit.
In transmit mode the base of Q8520 is slightly positive biased by a divided K9V1 signal. This bias
along with the RF signal from Q8510 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 two enhancement mode N-Channel MOS FET devices (Q8530, Q8531)
and requires for each device a positive gate bias and a quiescent current flow for proper operation.
The voltages of the lines BIAS VLTG and BIAS VLTG 2 are set in transmit mode by two Digital to
Analog (D/A) converters (U0731-4, U0731-11) and fed to the gates of Q8531 and Q8530 via two
resistive dividers. The bias voltages are tuned in the factory. If one or both transistor are replaced,
the bias voltages must be tuned with the Service Software (RSS). Care must be taken, not to
damage any device by exceeding the maximum allowed bias voltage. The collector currents are
drawn from the supply voltage A+ via L8531 and L8532.
The final stage uses the bipolar device Q8540 and operates off the A+ supply voltage. For class C
operation the base is DC grounded by two series inductors (L8533, L8534). A matching network
consisting of C8541-C8544 and two striplines transform the impedance to 50 Ohms and feed 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 Q8541. The coupled signal is rectified to an output power proportional
negative DC voltage by the diode D8553 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.
9.4

Antenna Switch

The antenna switch is switched synchronously with the K9V1 voltage and feeds either the antenna
signal coming through the harmonic filter to the receiver or the transmitter signal coming from the PA
to the antenna via the harmonic filter.
Theory of Operation
Transmitter Power Amplifier (PA) 5-25W
6.3-19