Power Amplifier; Gain Stages; Power Control; Circuit Protection - Motorola ASTRO Digital Spectra Plus Detailed Service Manual

General Overview: Power Amplifier
2.5

Power Amplifier

The power amplifier (PA) is a multi-stage, discrete-transistor RF amplifier consisting of the following:
• Low-level power controlling stage
• Drivers
• Final amplifier
• Directional coupler
• Antenna switch
• Harmonic filter
2.5.1

Gain Stages

The first stage buffers the RF signal, filters harmonics, and acts as a variable amplifier. All of the
amplifying stages are matched using transmission lines, capacitors, and inductors and are supplied
with DC from either A+, keyed 9.4-V, or 9.6-V sources. Following the last gain stage, PIN diodes
switch the signal flow either from the antenna to the receiver, or from the last gain stage to the
antenna.
2.5.2

Power Control

A directional coupler and detector network controls power. It senses the forward power from the last
gain stage and feeds the detected voltage back to the command board control circuitry where it is
compared to a reference voltage set during power-set procedures. The DC feed voltage is corrected
and supplied to the "controlled" stage of the power amplifier. Circuitry on the power amplifier board
controls the gain of the first stage and is proportional to the DC control voltage.
2.5.3

Circuit Protection

Current and temperature sensing circuitry on the power amplifier board feed sensed voltages to the
command board for comparison. If the command board suspects a fault condition, it overrides the
power control function and cuts the power back to a level that is safe for the conditions. In addition,
some high-power amplifier boards include circuitry that monitors the power supply line. If the battery
voltage exceeds or drops below a pre-determined level, the power output of the amplifier is adjusted
to ensure proper operation of the transmitter.
2.5.4

DC Interconnect

The ribbon cable connector carries sensed voltages for power and protection to the command board.
It also carries A+ feed to the command board for distribution throughout the internal transceiver
housing and carries control voltage from the command board to the power amplifier board.
The rear battery connector carries A+ from the battery to the power amplifier board. The red lead
goes directly to the A+ terminal on the PA board. The black lead from the battery connector ties to
the chassis, and connection to the power amplifier board is made through the board mounting
screws.
A+ ground connection for the internal transceiver housing is through the RF coax ground connectors
and through the mechanical connection of the power amplifier heatsink to the rest of the radio.
During test conditions in which the power amplifier assembly (board and heatsink) is physically
disconnected from the rest of the radio, it is acceptable to rely on the coax cable connections to carry
ground to the internal housing.
6881076C25-E
2-5
September 5, 2008