6881076C25-D October 28, 2002
Theory of Operation: Power Amplifiers 3-81
3.7.3.1.4 Temperature Sensing
When the radio is keyed, K9.4-V is applied to pin 5 of the PA connector and on one side of thermistor
RT9560. As the temperature increases, the resistance of RT9560 decreases, creating more voltage
across R9561. This temperature voltage is routed via PA connector pin 7 back to U500, pin 13, which
is the input to a thermistor buffer. The thermistor buffer’s output on pin 12 is summed to U500, pin 2,
and passes through its scaling resistor, R508. When the temperature of the RT9560 causes its value
to change enough that the voltage exceeds 3.2 V, the thermister buffer starts supplying current to the
node at pin 2. Due to the fixed output of the D/A, the control loop can maintain 3.2 V at pin 2 only by
reducing power out and reducing the forward detected voltage. Since output is reduced, the
generated heat is held to a safe level. As temp decreases, the power output of the PA gradually
increases to its nominal value.
Q9515 and Q9510 switch A+ to one side of R9513. R9513 sums the A+ voltage into the same node
as TEMPSENSE. Together with temp-sense the circuitry protects the power amplifier from unsafe
operating conditions of high line and high temp.
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 inter-stage power limit
may initially reduce power, but eventual heat build-up will cause further power reduction by
the thermal cut-back circuit).
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