Power Amplifiers; Vhf Band Power Amplifiers; High-Power Amplifier; Transmitter - Motorola ASTRO Digital Spectra Plus Service Manual

Theory of Operation: Power Amplifiers
3.7

Power Amplifiers

This section of the theory of operation provides a detailed circuit description of the power amolifiers.
When reading the Theory of Operation, refer to your appropriate schematic and component location
diagrams located in
detailed Theory of Operation will help isolate the problem to a particular component. However, first
use the ASTRO Digital Spectra and Digital Spectra Plus Mobile Radios Basic Service Manual to
troubleshoot the problem to a particular board.
3.7.1

VHF Band Power Amplifiers

3.7.1.1 High-Power Amplifier

3.7.1.1.1 Transmitter

The high-power ASTRO Spectra amplifier is discussed in the following text. A block diagram of the
circuit is shown on the foldout drawing.
Transmit Low Level Amplifier (LLA)
The LLA is the first stage of the PA and provides a gain that is a function of the control voltage. This
control voltage comes from the Regulator Power Control IC (RPCIC) on the command board. The
magnitude of the control voltage depends on PA output power, temperature, and final amplifier
current drain. See Section
explanation of the power control circuitry.
The LLA, Q3801, is unique in that its gain is controlled by varying the collector's current rather than
its voltage. Q3801 and associated circuitry (Q3806 Q3802, R3804, and R3818) are best described
as a voltage-controlled current source. This means that the collector current of Q3801 is controlled
by the magnitude of the control voltage.
Second Amplifier Stage
The second stage of the PA, Q3804, amplifies the output of the LLA to a level sufficient to drive the
third stage device, Q3805. Q3804 amplifies the LLA output from approximately 300 mW to 3.0 Watts.
Driver Stage (Q3805)
The third stage uses a 3.0-Watt input to 30-Watt output device. It is driven by the second stage
through a matching circuit that consists of C3824, L3808, C3819, and C3820. L3812 and L3809 give
the device a zero-Vdc base bias (required for Class-C operation). The network of L3811, L3810,
R3819, and C3821 provide A+ to the collector.
Final Stage (Q3870 AND Q3871)
The final amplifier stage is the parallel combination of two 15-Watt input to 75-Watt output RF
transistors. The matching network, from the collector of the driver device Q3805 to the bases of the
final devices Q3870 and Q3871, utilizes transmission lines as part of a combination matching
network and power splitter. The capacitors C3860, C3861, C3862, and C3863 are on the bottom
side of the PC board underneath the base leads of Q3870 and Q3871.
The DC bias path for the base of Q3870 is via L3930 and L3931. Q3871 has a similar network.
R4007, R4008, and R3859 improve division of driver power between the final devices Q3870 and
Q3871.
A feedback network consisting of C3870, R3870, and L3870 suppresses parasitic oscillations in
Q3870. Q3871 has a similar network.
6881076C25-D
"Chapter 7. Schematics, Component Location Diagrams and Parts
3.7.1.1.3, "Power Control Circuitry," on page 3-57
3-55
Lists". This
for a detailed
October 28, 2002