Radio-Frequency Power Amplifier (Rf Pa) And Output Network (On); Gain Stages; Power Control; Circuit Protection - Motorola ASTRO XTL 5000 Basic Service Manual

Basic Theory of Operation: Radio-Frequency Power Amplifier (RF PA) and Output Network (ON)
3.9 Radio-Frequency Power Amplifier (RF PA) and Output Network
(ON)
The RF PA is a three-stage power amplifier consisting of discrete LDMOS transistors:
• Controlled stage
• Driver stage
• Final stage
The RF PA is followed by the ON section consisting of discrete circuitry with the following functions:
• Antenna switch
• Harmonic Filter
• Power Detector
3.9.1

Gain Stages

The controlled stage consists of a two-stage, integrated amplifier with external matching which
amplifies the input signal from the VCO buffer and provides drive to the driver stage. Power is
controlled via gate bias to both internal stages and drain bias is supplied via K9.1V. The drive stage
has a fixed gate bias and drain bias is supplied by the A+ (battery) voltage. The driver stage drives
the final stage consisting of two transistors operating in parallel. Both devices have separate, fixed
gate biases and their drain biases are supplied by the A+ voltage. The output of the final stage feeds
the antenna switch which routes the RF PA to the harmonic filter/power detector/antenna and
isolates the RX front-end in transmit mode. Antenna switch routes antenna/power detector/harmonic
filter to RX and isolates TX in RX mode. Mode is determined via K9.1V. The harmonic filter is a low-
pass filter that attenuates harmonics generated by the RF PA in transmit mode and provides
additional receive selectivity in receive mode.
3.9.2

Power Control

Power is regulated by an automatic-level control (ALC) circuit. The transmitter ALC consists of a
distributed power detector with a detection diode, buffer/amplifier, digital-to-analog converter (DAC),
and loop integrator. During transmission, the RF PA gain and output power is adjusted by a control
voltage. The power detector senses incident power transferred to the antenna via a directional
coupler whose signal is converted to a DC voltage by the detection diode. This DC voltage is
buffered/amplified and then added to the DAC voltage which is then compared to a fixed voltage
reference. The carrier power level is set tby adjusting the DAC voltage while monitoring the output
power, which is saved tin radio memory.
3.9.3

Circuit Protection

RF PA final-stage drain current, RF PA final-stage temeperature, RF PA control voltage, and battery
voltage are sensed by the power-control circuitry. If a fault condition is detected, the control voltage
is reduced, which cuts back the output power to a level that is safe for the particular conditions.
3.9.4

DC Interconnect

The DC connector at the edge of the board carries the A+ supply for the entire board. This supply is
routed directly to the controller and transmitter circuitry for both direct supply and regulating
additional supplies. The radio chassis is grounded through the PCB screws and also via direct
contact to the board. The control head receives the A+ supply through the 50-pin flex connector.
6881096C73-O
3-9
June 11, 2003