[Amps] Monitoring temperature in SS amp

[John Lyles]

I agree with Manfred's assessment of temperature sensors around RF amps. The BE FM broadcast transmitters always had an LM35 mounted on a small PCB above the cavity amplifier exhaust RF shielded port - since 1980 design time.  It ran to the central controller as a diagnostic (is it tuned up wrong and dissipating a lot?). No problems with RFI, with 100 MHz filtering on the board. 

However, my recent amplifier is 30-40 dB higher output power, pulsed, and with these levels, it doesn't take much leakage at all to have some finite but safe signal power. Thermocouples are worst in RF, as they tend to bias from the RF at the junction. The semiconductor sensors that came with DMM's like Flukes are notorious about detecting RF and biasing the reading. Nowadays I prefer the platinum RTD (PT100) as they have a nice low Z output, and are fairly easy to linearize and use with meters. They make some that mount with a screw. Like Manfred said, you can do brute force filtering around your sensor to prevent conducted RF from getting in and out via the leads. Mounting right at a transistor flange might not be possible due to the nearby high RF current in the circuit. 
73
John 
K5PRO