25/10-Watt Power Amplifier - Motorola ASTRO Digital Spectra Plus Detailed Service Manual

Theory of Operation: Power Amplifiers
Temperature Sensing
The temperature-sensing circuit of the PA works with the RPCIC to protect the PA devices from
excessively high temperatures. On the PA board, this circuit (formed by resistors R3916, R3841, and
thermistor RT3842), provides a temperature dependent voltage to the RPCIC via J1 pin 6. As the PA
temperature increases, the resistance of RT3842 decreases, causing the voltage at pin 6 to
increase. This voltage is routed to the RPCIC, U500 pin 13, which is the input to the thermistor
buffer. The buffer's output on pin 12 is connected to pin 2 via resistor R508. Note that pin 2 is the
control amp input and is a summing point for temperature, forward-power detect, and power set
signals. If the PA temperature becomes high enough so that the voltage at pin 7 exceeds 3.2 V, the
thermistor buffer starts supplying current to the node at pin 2. Due to the fixed output current of the
power-set buffer, the control loop can maintain 3.2 V at pin 2 only by reducing the forward-power
detect voltage and, therefore, reducing the PA output power. Since power output is reduced, the
generated heat is reduced to a safe level. If temperature decreases, the power output of the PA
gradually increases to its nominal value.
NOTE: Under severe environmental conditions, more than one circuit may be attempting to reduce
power output at the same time (i.e., during high VSWR conditions, the current limiter may
initially reduce power, but eventual heat buildup will cause further power reduction by the
thermal cut-back circuit).
The temperature sense circuitry can easily be tested by placing an ordinary leaded 4.7k ohm
resistor across RT3842. PA output power should drop significantly if this circuit is working
properly.

3.7.1.2 25/10-Watt Power Amplifier

Transmitter
The 25/10-Watt Spectra power amplifier is discussed in the following text.
Transmit Low Level Amplifier (LLA)
NOTE: The minimum input drive level to the PA into J3850 is 10 mW. Refer to the synthesizer section
if input drive is less than 10 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, 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. Proper operation of the LLA can be checked by monitoring
the voltage across the resistor R3804. The voltage should measure in the range of 0.1 V 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).
Driver Stage
The second stage of the PA, Q3804, is the driver. The purpose of this stage is to amplify the output
of the LLA to a level sufficient to drive the final device, Q3850. Input power to this stage is
approximately 100 mw; output power from this stage is 3.5 Watts.
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September 5, 2008