Stacking Transistors for higher hfe?

[glasspak73]

I have been wondering about this for a while.Is it possible to stack transistors for higher hfe?Or is there a way to build a circuit that will make this possible? A kind of adapter circuit to use 2 transistors in the place of one for better or different sounds?

[Beaker]

It is possible to do, but you would be making life difficult for yourself. Darlington transistors do it for you - one transistor package containing two piggybacked transistors. This is why their gain gets so high.

BC516 and BC517, and especially MPSA13 and MPSA18 anre commonly used Darlingtons.

[Silver Blues]

This ^. It's certainly possible to do it, but depending on the configuration of your particular devices the resulting hybrid may not be stable and need some variety of additional components. Most often it's a resistor from the emitter of the first to the base of the second. A self-contained Darlington transistor is just two Darlington-paired transistors in one package, correctly stabilized. The way a Darlington pair works is that the final gain of the pair is equal to the gains of both constituents multiplied. This allows enormous final gains (generally 1000-15000 hfe or sometimes even more). The problem you run into eventually is that depending on the gain of the first device, often the second device needs to be fairly high-power-dissipation (i.e. a power transistor) to prevent blowing the whole thing up (figuratively, of course).

Also, Beaker, the MPSA18 isn't a Darlington, it's a small-signal amp/switch like a 5088, etc. MPSA12 and -13 are Darlingtons, as well as most (all?) of the TIP- family.

[Beaker]

Thanks Silver, I presumed the MPSA18 must be a Darlington - serves me right for not reading the datasheet!

[Heath]

You're not alone.  I also thought MPSA18s were Darlington... so much so that I had to go look it up even though I know Silver knows his stuff.