Section 11: 2-4 VHF (136–174 MHz) 25–45W LDMOS Transmitter Power Amplifier (PA) 45 W
6881091C63-F
output causes the DC voltage from the PCIC to fall, and U3402-1 adjusts the bias voltage for a lower
drain current to lower the gain of the stage.
In receive mode the DC voltage from PCIC pin 23 (RX) turns on Q3442, which in turn switches off
the biasing voltage to U3401.
Switch S3440 is a pressure pad with a conductive strip, which connects two conductive areas on the
board when the radio’s cover is properly screwed to the chassis. When the cover is removed, S3440
opens and the resulting high voltage level at the inverting inputs of the current control op-amps
U3402-1 switches off the biasing of U3401. This prevents transmitter key-up while the devices do
not have proper thermal contact to the chassis.
3.2 Driver Stage
The next stage is an LDMOS device (Q3421) providing a gain of 13 dB. This device requires a
positive gate bias and a quiescent current flow for proper operation. The voltage of the line
MOSBIAS_1 is set during transmit mode by PCIC pin 24 and fed to the gate of Q3421 via the
resistive network R3417, R3415, and R3416. This bias voltage is tuned in the factory.
In receive mode, the MOSBIAS_1 supplied to Q3421 is grounded by the PCIC.
3.3 Final Stage
The final stage is an LDMOS MRF1550 device (Q3441). It provides a gain of 10 dB. This device also
requires a positive gate bias and a quiescent current flow for proper operation. The voltage of the
line MOSBIAS_2 is set during transmit mode by the ASFIC and fed to the gate of Q3441 via the
resistive network R3404, R3406, and R3431-2. This bias voltage is tuned in the factory. If the
transistor is replaced, the bias voltage must be tuned using Global Tuner Software. Care must be
taken not to damage the device by exceeding the maximum allowed bias voltage.
In receive mode, MOSBIAS_2 voltage is reduced to switch off Q3441.
The drain current of MRF1550 is drawn from the radio DC supply via filtering beads, bypass
capacitors and a current-sensing resistor (R3518). A differential DC amplifier U3503-1 compares the
voltage drop across R3518, which is proportional to the transmitter final stage DC current, with the
voltage across resistors R3511 and R3510, which is proportional to the current through transistor
Q1503. This transistor is controlled by the output of the differential amplifier, which varies the
transistor current until equilibrium of the two compared voltages is reached. The current through
Q1503 develops a voltage across R3517, which is exactly proportional to the DC current of the final
stage. This voltage is combined with power-sensing and temperature-sensing inputs and applied to
the RFIN port of the PCIC (pin 1).
3.4 Antenna Switch
The antenna switch consists of two PIN diodes, D3471 and D3472. In receive mode, both diodes
are off. Signals applied at the antenna jack J3401 are routed, via the coupler and harmonic filter,
through network L3511, C3451 and C3475, to the receiver input. In transmit mode, the keyed 9 volts
turns on Q3471 which enables current sink Q3472, set to 96 mA by R3473 and VR3471. This
completes a DC path from A+, through L3436, R3518, L3437, L3443, D3471, L3510, L3511, D3472,
L3471, and R3474 and the current sink, to ground. Both diodes are forward biased into conduction.
The transmitter RF from the PA is routed via D3471, C3561 and C3563 to the harmonic filter,
directional coupler and antenna jack. D3472 also conducts, shunting RF power and preventing it
from reaching the receiver port (RXIN). L3511 is selected to appear as a broadband quarter-wave
transmission line, making the short circuit presented by D3472 appear as an open circuit at the
junction of D3472 and the receiver path.
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