LTSpice Transistor Only Analog Temperature Heater Control

[@vioariton8510]

What is the purpose of the pnp current mirror at the top?

[@manoA98A]

The differ ential Darlington compares the voltage of the adjustable reference divider with the divider formed from the thermistor and the other resistor. The Current mirror PNP at the top provides an active load to raise the gain (basically it's like an active resistor, you could put a resistor there at it would be the same) and the mirror npn provides the 1mA emitter current. It uses a beta helper to eliminate the effect of the base current. Ps: If you check most of opamp design like 741, you will notice that they always use the double current mirror configuration to provide both low side emitter current and high gain in the high side, an active load works better than a resistor. Current mirrors are widely used in Integrated Analog Circuits as a bias basically