Motorola PRO Series, Detailed Service Manual

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Section 9: 2-4 UHF Band 1 (403–470 MHz) 25–40 W Bipolar Transmitter Power Amplifier (PA) 40W
6881091C63-F
the PCIC pin4(U4501). The control voltage simultaneously varies the bias of two FET stages within
U4401. This biasing point determines the overall gain of U4401 and therefore its output drive level to
Q4421, which in turn controls the output power of the PA.
In receive mode the voltage control line is at ground level and turns off Q4473 which in turn switches
off the biasing voltage to U4401.
3.2 Pre-Driver Stage
The next stage is a 13dB gain LDMOS device (Q4421) which requires a positive gate bias and a
quiescent current flow for proper operation. The voltage of the line PCIC_MOSBIAS_1 is set in
transmit mode by PCIC pin 24 and fed to the gate of Q4421 via the resistive network R4407, R4408,
R4416 and R4415. The bias voltage is tuned in the factory.
3.3 Driver Stage
The following stage is an enhancement-mode N-Channel MOSFET device (Q4431) providing a gain
of 10dB. This device also requires a positive gate bias and a quiescent current flow for proper
operation. The voltage of the line Bias_2_UHF_PA_1 is set in transmit mode by the ASFIC and fed
to the gate of Q4431 via the resistive network R4630, R4631, and R4632. This bias voltage is also
tuned in the factory. If the transistor is replaced, the bias voltage must be tuned using the Customer
Programming Software (CPS). Care must be taken not to damage the device by exceeding the
maximum allowed bias voltage. The device’s drain current is drawn directly from the radio’s DC
supply voltage input, A+, via L4421.
3.4 Final Stage
The final stage uses the bipolar device Q4441. The device’s collector current is also drawn from the
radio’s DC supply voltage input. To maintain class C operation, the base is DC-grounded by a series
inductor (L4441) and a bead (L4440). A matching network consisting of C5541 – C5544 and two
striplines transform the impedance to 50 Ohms and feeds the directional coupler.
3.5 Bi-directional Coupler
The bi-directional coupler is a microstrip printed circuit, which couples a small amount of the forward
and reverse power of the RF power from Q4441. The coupled signal is rectified to an output power
proportional DC voltage by the diodes D4451 and D4452 and sent to the RFIN of the PCIC. The
PCIC controls the gain of stage U4401 as necessary to hold this voltage constant. This ensures the
forward power out of the radio is held to a constant value.
3.6 Antenna Switch
The antenna switch utilizes the existing DC feed (A+) to the last stage device (Q4441). Basic
operation is to have both PIN diodes D4471 and D4472 turns on during key-up by forward biasing
them. It is achieve by pulling down the voltage at the cathode end of D4472 to around 11.8V (0.7V
drop across each diode). The current through the diodes needs to be set around 80mA to fully open
the transmit path through resistor R4496. Q4472 is a current source controlled by Q4471 and is
eventually connected to pin ANO of PCIC. VR4471 ensures the voltage at the resistor R4511 never
exceeds 5.6V.