40-Watt Power Amplifier; Transmitter - Motorola ASTRO Digital Spectra Service Manual

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3.7.2.2 40-Watt Power Amplifier

3.7.2.2.1 Transmitter

The 40-Watt ASTRO Spectra power amplifier is discussed in the following text.
Transmit Low Level Amplifier (LLA)
NOTE: The minimum input drive level to the PA into P5850 is 30 mW. Refer to the synthesizer section
if input drive is less than 30 mW.
The Low Level Amplifier, the first stage of the PA, provides a gain that is a function of a 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.
The LLA, Q5801, is unique in that its gain is controlled by varying the collector's current rather than
its voltage. Q5801 and associated circuitry (Q5806, Q5800, R5805, and R5818) are best described
as a voltage-controlled current source. This means that the collector current of Q5801 is controlled
by the magnitude of the control voltage. Proper operation of the LLA can be checked by monitoring
the voltage across the resistor R5805 The voltage should measure in the range of 0.1 to 1.0 V,
depending on the value of control voltage. A 0.1-V reading corresponds to a low control voltage
(1 to 5 V) and a 1.0-V reading corresponds to a high control voltage (up to control voltage limit).
Predriver Stage
The second stage of the PA, Q5803, is the predriver which amplifies the output of the LLA to a level
sufficient to operate the driver device, Q5850. This stage amplifies the LLA output from,
approximately, 250 mW in to 2.0 Watts out.
Driver Stage
The driver is a six-leaded 2.5- to 16-Watt device. It is driven by the predriver device through a
matching circuit that consists of C5851, C5852, C5850, C5858, and L5850. L5851 and L5852 give
the driver a zero-DC bias (required for the driver's Class-C operation). L5852, a ferrite bead, helps
lower the driver base Q and prevent unwanted oscillations. The network of L5853, L5854, C5856,
C5857, and R5850 provide A+ to the collector. L5853 and L5854 provide the DC path and block RF
from coming up the DC line. R5850 resistively loads down the collector at low frequencies,
preventing unwanted oscillations. C5856 and C5857 are bypass capacitors.
Final Stage
The final device is a six-leaded 15- to 50-Watt device and is driven by the driver through a quasi-low
pass matching circuit that consists of C5853, C5854, C5855, C5875, C5876, and associated
transmission lines. Base network, L5875, L5876, L5883, C5891, R5881, and R5882, provide the
zero-DC bias required by the final device's Class-C operation. L5875, L5876, and L5883 provide the
DC path from base to ground. C5891, in parallel with L5875, presents a high impedance at UHF
frequencies, thus minimizing RF losses in the base network. R5881, R5882, and L5883 resistively
load down the base at low frequencies, thus preventing unwanted oscillations. The collector DC
network consists of L5878, L5879 R5879, R5880, R5883, R5884, R5875, C5881, C5883 C5884,
C5885, C5886, C5893, and CR5875. This network provides the A+ voltage to the final while blocking
RF from getting up the DC line. L5878 and L5879 provide the DC path and block RF. R5879, R5880,
R5883, and R5884 resistively load down the final's collector at low frequencies and prevent
unwanted oscillations. C5881, C5883, C5884, C3885, C5886 and C5893 are all bypass capacitors
ranging from very low frequencies up to UHF frequencies. R5875 is the current-sense resistor.
CR5875 protects against reverse polarity. Finally, the power goes through a low-pass matching
network (C5877, C5878, C5887, C5892, C5880, and associated transmission lines) to the rest of the
output network (Directional Coupler, Antenna Switch, and Harmonic Filter).
July 1, 2002
Theory of Operation: Power Amplifiers
68P81076C25-C