This circuit was found in National Semiconductor documentation from the 90s. The circuit uses two sensors, one of which is a reference thermocouple for cold junction.
A narrow range of frequencies around 1 kHz is rejected by the high Q filter shown in the figure,...
This simple converter does not use a transformer and the transistor can be the BD135 for currents up...
This is a circuit from the time when some still called the transistor a crystal triode. It...