I made this one before I had any of the MOSFET gate driver ICs. Of course, you could argue that the BJTs I've used here could make nice gate drivers themselves (the ZTX718 lists that as a primary application on the datasheet). But, I decided to see what I could do with BJTs alone.

I made this from jellybeans, because everyone has jars of them lying around. The ZTX718 doesn't qualify, but I originally had a 2N4403 in there instead :-)

I worked hard to overcome the storage time in the first common-emitter stage, hauling out the big guns: a three-diode Baker clamp. In simulation it appears to work pretty well. Here are the current pulses through the LED (yellow) for supply voltages from 6 to 24V compared with the input pulse (magenta):

On the bench, though, as I turned up the voltage, the storage/fall time got longer and longer - much worse than this simulation. I accidentally deleted the oscilloscope screenshots, so I'll have to update this log later. I have two suspicions. First, the supply voltage may have been sagging due to the heavy current pulses, so there wasn't enough drive through the 220-ohm resistor during the falling edge. Another possibility is bad layout (I know you're eyeing those big current loops). I'm still interested in figuring this out, but I've basically abandoned this circuit for the flash. I had originally thought it would be useful to have a number of these circuits driving individual LEDs in a big array. I might end up back there, but for now, I think a smaller number of drivers each with a handful of LED chips is the way to go, and MOSFETs with commercial driver ICs eliminate the storage time issues entirely.

Baker Clamps

I did take the opportunity to explore various baker clamp topologies in simulation, though. Here are some interesting results. Three circuits I compared:

From left to right, no clamp, a 2-diode clamp, and a 3-diode clamp. They're all driven from a 50-ohm source resistance. Here are the waveforms:

On the bottom is the driving waveform (before the source resistance). The top trace shows the output of the three-diode clamp, which doesn't show appreciable storage time. In the middle is the 2-diode clamp. I didn't expect the exponential tail here. Finally, with no clamp, you see about 500 ns of delay between end of the input and output pulses.

The tail on the 2-diode clamp bothered me - the problem is that the input can't pull the base down through the diode. This gets fixed in the 3-diode version with another (reverse) diode, but you can also add a resistor to pull down the base (right hand side):

This resistor does a good job of eliminating the tail (magenta trace has R4, cyan trace does not):

I find it very interesting that this base resistor is omitted from most discussions of the 2-diode Baker clamp. Maybe because the original circuit was made with germanium PNPs (and possibly Ge diodes) in the 1950s, and they didn't require this resistor?

Schottky Clamps

Of course, you can do even better with a Schottky clamp:

The Schottky cuts off another 50ns or so (cyan trace) of the 3-diode version (magenta):

I knew this when I built the circuit above, but I used the 1N4148s because I couldn't find any of my through-hole Schottky's when I went to assemble it, and I didn't feel like dealing with SOT-23's (I easily found the reel of BAT54s).