Darlington

[Unlikekurt]

I have a circuit that uses a 2n3904 Darlington pair.
Does anyone know off hand what an appropriate single darlington transistor that would be a good replacement is?  Or , better yet, would anyone have the time to spend to explain how one would select a suitable replacement.  That would also, of course help in sourcing an appropriate pair when a Darlington up and fails and is found to be obsolete.

Thank you for any help

[Kevin Mitchell]

I would try MPSA13 or similar

I believe the main reason folks use darlington configurations is because it can handle more current.

[antonis]

What exactlly is a "appropriate single darlington resistor"..??

EDIT: OP, ignore the above query..!!


Now, about the "suitable replacement":

There isn't nothing different from "sigle" BJT replacement here other than discrete BJT Darlington pair DC current gain (β or h_FE) is the product of individual trasistors gain..

[Kevin Mitchell]

What exactlly is a "appropriate single darlington resistor"..??

I assumed they meant transistor rather than resistor

EDIT - OP fixed it

[iainpunk]

I would try MPSA13 or similar

I believe the main reason folks use darlington configurations is because it can handle more current.

I second the MPSA13, but the reason a cascaded pair (be it the Darlington or Sziklai pair [i recommend the Sziklai pair on completely arbitrary reasons]) is because they have a ß which is the two ß's of the used transistors multiplied. Most of the times the gain is HUGE, which is beneficial for SOME fuzz circuits and most buffers.

[kingswayguitar]

Of course the Base-emitter drop is double so u need to re-bias.

[Rob Strand]

What exactlly is a "appropriate single darlington resistor"..??

While it's not what was meant by the OP, some Darlingtons actually have a resistor across the base and emitter of the second transistor.  If you run these at low collector currents the second transistor stays cut-off and the Darlington ends up running as a single low gain transistor (the gain of the first transistor) with a resistor in series with the emitter.

[antonis]

some Darlingtons actually have a resistor across the base and emitter of the second transistor.  If you run these at low collector currents the second transistor stays cut-off and the Darlington ends up running as a single low gain transistor (the gain of the first transistor) with a resistor in series with the emitter.

That's for sure Rob (almost all of "old fashion" power supplies Emitter followers have got such a "trigger" resistor) but, that should also be the point for discrete pair design..

[antonis]

Of course the Base-emitter drop is double so u need to re-bias.

Double than that of a single BJT..

In case of Darlington discrete pair (2 X 2N3904) it should be the same as for 1 X MPSA13.

[Unlikekurt]

Thanks to all for their replies.  I had assumed it would be as simple as calculating the gain of the pair and replacing them with a single Darlington of that gain, but wasn't sure if there might be some other merit to the two transistor arrangement over the single.

[PRR]

Small-signal Darlingtons, packaged or assembled, all have "infinite gain" for nearly any audio purpose. >50,000. Aside from setting-aside your low-gain Germaniums, there is no reason to care what the nominal gain is.

There is some distinction as you move from pea-size Darlingtons to Power Darlingtons. Big devices have lower gain (naturally, and because it makes them more robust) so a TIP120 has nominal gain of 1,000; and a lot less if you try to use it in a low-current application. (However I have run TIP120 at 18mA with good result.)