I own a set of Soviet IN-14 Nixie tubes and I would like to use them to make a vintage clock. I found this example schematic of a "Nixie clock":
I don't understand what is the purpose of using a MOSFET in the second stage of this quasi-"Darlington pair" (as opposed to a second bipolar transistor in a normal Darlington pair) here:
Is there a particular name for this configuration?
As a secondary question, I also would like to know why a diode was used in place of a resistor in this configuration?
Best Answer
This is what's called a gate drive circuit, or gate driver. The BC558 aids in pulling charge out of the IRF840's gate, acting as a current amplifier to turn the FET off faster. The circuit designer apparently decided that the turn-on time wasn't as much of a concern, so a 1N4148 diode is used to pass the un-amplified current straight from the PIC's output.
The term gate driver is a generic term, referring to any circuit that aids a signal in charging and/or discharging the gate of a FET or IGBT. I don't think the particular instance in the circuit above has any more specific name.