Darlington pair, second transistor base resistor?

[ElektroQuark]

Very basic question here 

Why there is not a resistor from the first transistor's emisor to the second transistor's base? If the first one is in saturation, how is the base current of the second transistor controlled?

[Simon]

I'm going to guess you damage it the same as you damage any transistor that is over driven although your second transistor should need more current than the first so less likely to go over limit. Also your voltage on the resistor can ultimately limit total voltage across the two bases.

If too much current tries to go through the second transistor it will try to saturate at 0.2-0.3V which could limit current into the base because it wants 0.7V.

I'm no expert on BJT's but at the end of the day a darlington should perhaps be treated as a single transistor with a larger Vce.

[rob77]

if the first one is in saturation,

then the second one is in deep saturation as well. you have to think of a darlington as a single transistor with higher beta and double Vbe drop.

[ElektroQuark]

That's why I don't see it. The second one must die in that configuration.

[ElektroQuark]

I think I see it now: the Q2's base current is limited by the load.



O.T.: "Q2's base current" is a correct English expression?

[Simon]

O.T.: "Q2's base current" is a correct English expression?

Yes

[ElektroQuark]

Thank you Simon.

[LvW]

I think I see it now: the Q2's base current is limited by the load.

...limited by the load? Which load?

[mikerj]

I think I see it now: the Q2's base current is limited by the load.

...limited by the load? Which load?

The load that would be in the collector or the emitter.

[ElektroQuark]

Quote from: mikerj on Yesterday at 11:40:41 PM

The load that would be in the collector or the emitter.

That's it.

[Mark Hennessy]

Simon, your guesses were pretty good 

As you say, the second transistor in a Darlington pair cannot saturate. Even if Q1 was able to saturate to 0V across its collector-emitter, you need to have ~0.6V across the B-E junction of Q2. In practice, Q1 is likely to saturate to around 0.3V, so the lowest saturation voltage you should expect from a Darlington pair is around 1 volt (~0.3 plus ~0.6). Approximately...

Q2 is not damaged by too much base current because too much base current would (in theory) turn off Q2 by starving it of base current (by reducing Vbe). To make analysis easier, replace Q1 with a short, so the base of Q2 is connected to the collector. In that situation, the total current entering will divide - a little goes into the BE junction, the rest goes via the CE junction. Funnily enough, the ratio of this current division will be Hfe (or Beta if you prefer).

Funnily enough, I've met plenty of really quite experienced engineers who don't appreciate these facts, and assume that a Darlington will saturate to ~0.3V like a singe transistor...

If you want to think of a Darlington as being a single "transistor" - not a bad idea sometimes - then you just need to remember that the Vbe is now ~1.2V, and that the Hfe is the product of the individual transistors. The higher saturation voltage is rarely a big deal in linear applications (e.g. audio amplifier), but if using them to switch loads, then yes, they might run warmer than you expect.

If working quickly, a resistor across the BE junction of Q2 helps. Values vary according to application - perhaps 100 ohms might be a good start...