Power AmpliÞer (PA) 5-25W.
Theory of Operation 4-19
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.
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 to allow 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 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 ßow for proper operation. The voltage of the line BIAS
VLTG is set in transmit mode by a Digital to Analog (D/A) converter (U0731-4) and fed to the gate of
Q5530 via a resistive divider. The bias voltage is tuned in the factory. If the transistor is replaced, the
bias voltage must be tuned with the Radio Service Software (RSS). 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+.
The Þnal 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 C5542-C5544 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 Q5536. The coupled signal is rectiÞed 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.
9.4 Antenna Switch
The antenna switch is switched synchronously with the K9V1 voltage and feeds either the antenna
signal coming through the harmonic Þlter to the receiver or the transmitter signal coming from the PA
to the antenna via the harmonic Þlter.
In transmit mode, this K9V1 voltage is high and biases Q5520 to allow a collector current to be
drawn. The collector current of Q5520 drawn from A+ ßows via L5542,L5541, directional coupler,
D5551, L5551, D5631, L5631, R5616, R5617 and L5611 and switches the PIN diodes D5551 and
D5631 to the low impedance state. D5551 leads the RF signal from the directional coupler to the
harmonic Þlter. The low impedance of D5631 is transformed to a high impedance at the input of the
harmonic Þlter by the resonant circuit formed by L5551,C5633 and the input capacitance of the
harmonic Þlter.
In receive mode the low K9V1 turns off the current through the PIN diodes and switches them to the
high impedance state. The antenna signal, coming through the harmonic Þlter, is channelled to the
receiver via L5551, C5634 and line PA RX.
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