Back in the 1970's I was tasked with coming up with a thermometer that could
be read in the studio of an AM radio station. I bought a Heathkit
indoor-outdoor unit. It worked great at night but was all over the place in
the daytime when the AM transmitter was on the air. Turned out the sensor
was just a silicon diode forward biased. A small ceramic capacitor across
the diode sensor fixed the RF sensitivity.
Apparently, the forward biased silicon diode was temperature sensitive
enough that a small D.C. amplifier could drive a meter to read-out with
reasonable accuracy. Well, maybe not accurate by Time-nut standards but
close enough for its intended purpose.
A lot of better audio amplifiers use a silicon diode as a temperature sensor
in the output stages to sense the case temperature to control the biasing
and prevent thermal run-away.
Seems like there are IC's that contain two diodes, one as a sensor and one
as a heater. Part numbers escape me now.
Al, retired, mostly
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