Power Controlled Stage; Pre Driver Stage; Pre-Driver Stage; Driver Stage - Motorola GM328 Service Manual

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3.1

Power Controlled Stage

The first stage (U5401) amplifies the RF signal from the VCO (TXINJ) and controls the output power
of the PA. The output power of the transistor U5401 is controlled by a voltage control line feed from
the PCIC (U5501, pin 4). The control voltage simultaneously varies the bias of two FET stages within
U5401. This biasing point determines the overall gain of U5401 and therefore its output drive level to
Q5421, which in turn controls the output power of the PA.
In receive mode, the voltage control line is at ground level and turns off Q5473, which in turn
switches off the biasing voltage to U5401.
3.2

Pre-Driver Stage

The next stage is a 13dB gain LDMOS device (Q5421) 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 Q5421 via the resistive network R5407,
R5408, R5416, and R5415. The bias voltage is factory tuned.
3.3

Driver Stage

This stage is an enhancement-mode N-Channel MOSFET device (Q5431) 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
Q5431 via resistive network R5630, R5631, and R5632. This bias voltage is also factory tuned.
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 L5421.
3.4

Final Stage

The final stage uses bipolar device Q5441 whose collector current is also drawn from the radio's dc
supply voltage input. To maintain class C operation, the base is dc-grounded by series inductor
L5441 and bead L5440. A matching network consisting of C5541-C5544 and two striplines
transform the impedance to 50 ohms and also feed 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 Q5441. The coupled signal is rectified to an output power
proportional dc voltage by diodes D5451 and D5452 before being sent to the RFIN input of the
PCIC. The PCIC controls the gain of stage U5401 as necessary to hold this voltage constant. This
ensures the forward power out of the radio is held to a constant value.
Transmitter Power Amplifier (PA) 40W