Darlington from two common BJTs

[lrgaraujo]

Hi guys,

I've just ordered some parts and totally forgot to order a darlington (MPSA 13 for the bellows)
Will two cascaded npn BJTs work the same?
What should I look for hfe-wise?  (I have some 2222s, some 2n3904, etc)

Thanks

[midwayfair]

That's all a darlington transistor like the -13 is anyway, so yes, absolutely.

You can multiply the two transistors' gain values together. Shoot for something at least vaguely similar.

[lrgaraujo]

Thanks!
I'll get to breadboarding soon

[drog_trog]

I have a lot of MPSA14's (approx 1000 or more) i got them cheap years ago, I can do you a good deal if your interested?

[lrgaraujo]

I have a lot of MPSA14's (approx 1000 or more) i got them cheap years ago, I can do you a good deal if your interested?

Thanks for the kind offer, but I managed to breadboard the Bellows with the darlington pair from two 2n3904s (even feels a bit more DIY    )

[madbean]

That's all a darlington transistor like the -13 is anyway, so yes, absolutely.

You can multiply the two transistors' gain values together. Shoot for something at least vaguely similar.

Jon, is there anything interesting to mixing different types of BJTs into a Darlington pair? Never tried it myself. Or, maybe mixing a BJT and really low gain NPN Ge.

[midwayfair]

That's all a darlington transistor like the -13 is anyway, so yes, absolutely.

You can multiply the two transistors' gain values together. Shoot for something at least vaguely similar.

Jon, is there anything interesting to mixing different types of BJTs into a Darlington pair? Never tried it myself. Or, maybe mixing a BJT and really low gain NPN Ge.

Leakage is also multiplied, so unless there is some special quality of the low-gain Ge you want to use, it's dangerous (noise-wise) to use them. However, if you're trying to use up low-gain transistors, then thing won't get out of hand.

These both seem fairly thorough:
http://diy.smallbearelec.com/HowTos/BreadboardGeDarlingtonFFs/BreadboardGeDarlingtonFFs.htm (you've probably read this one before)
https://www.electronics-notes.com/articles/analogue_circuits/transistor/darlington-pair-design-example-calculations.php

The first thing that came to mind when you asked was "I wonder how the internal capacitance of a transistor affects the darlington pair?" and lo and behold there's a thread about almost that exact thing:
https://forum.allaboutcircuits.com/threads/darligton-pair-bandwidth.99202/

So what you want to think about is: the first transistor in the pair is  strapped across the base and collector of the second transistor in the pair. This is where the capacitance would come into play. If you breadboard it up and it's too dark, it could be that swapping their positions or using a transistor with lower internal capacitance are the way to go. Or if it's too harsh. But you're only using a Darlington for high gain. So it should make sense that you'd want the Ge first in the pair.

Now, it just so happens that I have unwittingly tested this theory with my ears in the past! It's always nice when a theory bears out in the real world.

http://www.madbeanpedals.com/forum/index.php?topic=6358.0

That's my Buzzaround build from ... holy crap, 6 years ago (!?wheredoesthetimego!?). The first two transistors in that circuit are a Darlington, and while I was playing around with all the configurations for it, I settled on silicon in Q2 sounding best.