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I will try to summarize my problem the best I can. I have a circuit I use to generate a voltage pulse. This circuit consists on a voltage-controlled current source with a 54.9ohm shunt resistor, a PNP placed on the high-side controlled by an NPN transistor, and a couple of electrodes. The load is connected between these two electrodes.

I know how these two parts work isolatedly, however, there's one thing I don't understand when I simulate the circuit and analyze the current waveforms.

The current source is always adjusted with 0.6, so that the current through the shunt resistor is roughly 10mA.

The voltage pulse through the load is therefore generated when the NPN is “enabled”.

The PNP will start flowing current and will go from active to saturation…

When observing the Q1 currents and Assuming Ie = Ib + Ic (approx.). How could it be that my base current is negative? Is there something very obvious that I'm missing?

LTSpice simulations. current waveforms.

Circuit schematics

LTSpice simulations

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  • \$\begingroup\$ C604/C3 shouldn't be there - it will cause instability. R601 and R601/R54 are much too high in value. \$\endgroup\$ Commented Jun 20 at 19:22
  • \$\begingroup\$ In your first drawing Q600 have the wrong orientation. Guess your Q18 is correct. \$\endgroup\$ Commented Jun 20 at 19:26
  • \$\begingroup\$ Hi, thanks for the reply. I agree C3 should be removed. The circuit, however, is working fine and the voltage pulse is correctly generated. It is just that I dont know how to interprete the negative base current. \$\endgroup\$ Commented Jun 20 at 19:27
  • \$\begingroup\$ @AlejandroFernández - Hi, It seems you're asking how the given schematics work, which suggests they aren't your design. To comply with the site referencing rule you should add a reference (citation) for any copied/adapted material from elsewhere, which you copy onto the site. So where did the schematics come from? If it's from another website, then please edit the question & add the website name & webpage/pdf link. It the source is offline, then add a reference "to the best of your ability" e.g. title, authors, publisher etc. TY \$\endgroup\$ Commented Jun 20 at 19:42

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I would get rid of C3 in your simulation. It's way beyond the acceptable capacitive load for stable operation.

Before you turn the 200V high-side switch on, the lower current sink will be hard 'on' trying to draw that current. Op-amp output will be railed at the positive side. It will take time to draw the charge out of the base and for the op-amp to slew back to near ground, so you're getting nasty (almost 3:1) overshoot of current.

The negative base current is real. Think of the E-B and C-B capacitances.

You might be better off switching the current sink.

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