Transistor current source (sink), high current, low dropout

[741]

I'd like to realise a fast acting current source. It might well be saturated at times, but must recover ASAP.

About 10A, about 30V max.

For reasons of speed (esp saturation recovery), stability and simpler power supply, I assumed a standard NPN current sink with a set voltage on the base, then (minus a vbe drop) across the sense resistor on the emitter.

If the equation re ~= 25/(1000.I) is about right, I guess the sense resistor has to be at least 10 times that. re = 0.0025 Ohms (2.5 mOhms) at 10A; RE = 25mOhm, sense-resistor dropout = 0.25V

But 0.25 is small compared with a hard-to-define Vbe, maybe 0.7..0.9V at this current, so its rather inaccurate. Normally, when sense dropout is no concern, and lower currents, Vbe inaccuracies can be brushed aside.