current regulator -- explanation?

The circuit in question is a current mirror. The idea is that the current through the first transistor sets the same current in the second transistor as a result of Vbe amtching.

As shown it's a poor way to drive LEDs ! Not least because you have to 'waste' as much current through the resistor as flows in the LEDs.

Graham

Hi, Rex. Current mirrors are a good choice if you're making and IC, because you can match the transistors well, and temperature tracking between transistors is easy.

If you want an easy current source that has pretty good compliance over a fairly wide range, you could do worse than this (view in fixed font or M$ Notepad):

| | VCC | + | | | .-. | | | | | | | '-' | | | | | Vb |< |

matched transistors are required to be truly accurate, might find an ic with four matched pairs in some older manufacturer catalog

i prefer using those voltage regulators in current mode configuration,

many other voltage regulators can be configured to do this very easily with only a single resistor

I've had good results with another 2-transistor circuit, found here:

No transistor-matching required. And the exact type of transistor isn't too important, I use 2N3906. With that circuit, I can run two

Mark

here:

Also, while that circuit calls for pnp transistors, you can build an npn version if that's what you happen to have lying around.

If you reverse the polarity of the source, then just use two npn's in place of the pnp's. Use the same configuration of the base, collector, and emitter.

Mark

here:

here:

Yes! Thanks John. -- Mark