Motorola ASTRO Digital Spectra, Detailed Service Manual, Page: 129 / 442

6881076C25-E September 5, 2008
Theory of Operation: Power Amplifiers 3-69
Final Stage
The final amplifier stage is the parallel combination of two 25-Watt input to 75-Watt output RF
transistors. The matching network from the collector of the driver device Q5851 to the bases of the
final devices Q5875 and Q5876 utilizes transmission lines as part of a combination matching
network and power splitter. The capacitors C5885, C5886, C5887, and C5888 are on the bottom
side of the PC board underneath the base leads of Q5875 and Q5876.
The DC bias path for the base of Q5875 is via L5877 and L5879. Q5876 has a similar network.
R5878 improves division of driver power between the final devices Q5875 and Q5876.
A feedback network consisting of C5890, R5879, and L5881 suppresses parasitic oscillations in
Q5875. Q5876 has a similar network.
The final stage output network serves the dual purpose of impedance matching and power
combining of the two final devices. C5891, C5892, C5893, and C5894 are on the bottom side of the
PC board underneath the collectors of the final devices. These capacitors are especially critical in
terms of their exact physical placement.
R5881 and R5882 help balance the load impedances presented to the collectors of the final devices.
Filtered A+ is routed to the final amplifier devices via the current sense resistor R5875, the ferrite
bead L5884, and the coil L5882. The final stage output network terminates at C5900 which is the
input to the antenna switch. The circuit impedance is 50 ohms at this point.
3.7.2.1.2 Antenna Switch and Harmonic Filter
Antenna Switch
The antenna switch utilizes PIN diodes to form a low loss, high isolation RF relay. During transmit,
PIN diodes CR5900, CR5902, CR5904, and CR5905,are forward biased during transmit via the K9.4
supply and resistors R5901, R5900, R5908, and R5909. In this state, a low loss path exists from the
final amplifier through PIN diode CR5900 and into the harmonic filter. PIN diodes CR5902, CR5904,
and CR5905 effectively shunt the path to the receiver front-end, which protects the preamp or mixer
device from excessive RF levels. A properly functioning switch will pass less than 10 mW of transmit
power to the receiver front-end.
During receive, all four PIN diodes remain unbiased. This opens a low loss path from the harmonic
filter to the receiver
Harmonic Filter
The harmonic filter is a 9-pole low-pass filter consisting of screened plate capacitors and air-wound
coils on a 0.035 inch thick ceramic substrate. The filter's primary function is to attenuate harmonic
energy generated by the amplifier stages. The filter also adds some selectivity for the receiver.
3.7.2.1.3 Power Control Circuitry
Command Board Circuitry
Inside U500, the Regulator Power Control IC ( Figure 3-25 on page 3-70 ) is an operational amplifier
that has four inverting inputs, and one non-inverting input (at pin 44) which is the reference input for
the entire power control loop of the power amplifier. The 3.2-V reference voltage at U500-44 is
produced by dividing SW +5-V with the voltage-divider circuit, R514 and R515.