Thermal compensation

[lumanauw]

Threshold/Statis line of bipolar power amps are wonderful design(s) from NP. One (other) thing that makes it different than other desinger's drawing is how Threshold power amps compensate thermal change.
In the schematics, Threshold usually use Thermistor (1K typically) for thermal compensation. The place is in various places. Like in S300 it is on the CFP driver. On S300 optical, it is on the optocoupler. On S2-S3 it is on VBE multiplier's leg. There is also on 400A, 800A....

Others usually put the VBE multiplier in the heatsink. Some use bipolar, some use double bipolar, some use darlington VBE multiplier transistor attached to heatsink.

I assume in Threshold/Statis, the device who is attached in the heatsink is the Thermistor, while the VBE multiplier or Smart Bias is not attached to heatsink. Is this right?

What is the comparison between thermal comparison with VBE multiplier transistor attached to heatsink or Thermistor attached to heatsink? What are the pro's and con's for each method?

[Nelson Pass]

Originally posted by lumanauw
I assume in Threshold/Statis, the device who is attached in the heatsink is the Thermistor, while the VBE multiplier or Smart Bias is not attached to heatsink. Is this right?

What is the comparison between thermal comparison with VBE multiplier transistor attached to heatsink or Thermistor attached to heatsink? What are the pro's and con's for each method?

Both approaches work, and the choice of how to go about
compensation is often arbitrary, but is affected by the wattages
involved and the amount of sinking, ballast resistors, types of
devices, and the specific topology - in other words, everything.