Complementary transistor pair with a bipolar transistor and a MOSFET

Question

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?

Answer 1

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.

Answer 2

• it is obviously an active buffered gate driver, but no name yet subtle characteristics.

The IRF has a Vt= 2 to 4 V thus with only 4.4=Vgs to 4.5V the RdsOn is not the same as rated @ Vgs=10V. Due to 5V-0.6 V nom gate drive on 1n4148 for high signal.

• the nominal curves show Id=1.1 A constant current with no inductor to Vds.

So less than ideal but well damped V=LdI/dt = 2fL I is the boost voltage. (450uH 1.1A x 2 x f) =V yet source is limited by 250mA.

• The emitter follower reduces the PIC output impedance /10 from 50 Ohms to 0.5 then 5 Ohms when saturated during Ie/Ib=10 to drive Ciss off, thus close to Gate input impedance of 2.8 ohm max.

• so it is simply slow turn on, fast turn off buffered non-inverting gate drive shunt switch output to gnd with approx a 1.1A current limit for nominal parts at 25’C.

• Possibly for PFM modulated boost voltage.(TBD determined by timer coding)

The LC resonant Load is 450uH 4.7uF or 3.3kHz so it operates in CCM at higher frequency and select this f determines the desired optimum Tube bias voltage.

p.s. I wonder why to music symbol across the Xtal to PIC.