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": Nixie clock schematic

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: quasi-Darlington pair with a MOSFET

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?

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  • \$\begingroup\$ @Hearth: This is the answer. \$\endgroup\$ – Janka Jul 7 '19 at 13:54
  • \$\begingroup\$ @Janka Thanks. Wanted at least one other person to confirm before I made it an actual answer, as I wasn't completely certain that was the whole story. \$\endgroup\$ – Hearth Jul 7 '19 at 13:58
  • \$\begingroup\$ It is the simple answer without a name \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jul 7 '19 at 14:19

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.

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  • 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.

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  • \$\begingroup\$ That's a music player and amplifier IC. I wonder why they did not put a music player into the PIC software. It's simple. \$\endgroup\$ – Janka Jul 7 '19 at 15:20

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