September 5, 2008 6881076C25-E
3-76 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 R5878, R5876,
R5877, and thermistor RT5875, provides a temperature-dependent voltage to the RPCIC via P0853,
pin 7. As the PA temperature increases, the resistance of RT5875 decreases, causing the voltage at
pin 7 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. Temperature cutback should occur at about
140°F (60°C).
The temperature sense circuitry can easily be tested by placing an ordinary leaded 6.8k ohm resistor
across RT5875. PA output power should drop significantly if this circuit is working properly.
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).
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