Darlington setup, total hfe?

[Eirik]

I have a bunch of low hfe, low leakage germaniums that would like to use for darlington setups.
However, it would be nice to know how to make out the total hfe without actually measuring.
Can anybody help me?

I did search, but wasn't able to fint anything covering that specific topic.


Eirik

[PenPen]

You'd have to know the actual hFE of both transistors first. After that, my understanding is that the gain is hFE1*hFE2 for a darlington. Since you are dealing with germs, regardless of leakage, you'd still need to measure it with the Geofex hFE tester, I would think. You could probably get into the ballpark with just the straight hFE measurement, but to be truely accurate you'd still need to get the real gain for them.

I'm in no way an expert, especially in germ transistors, but that is my understanding of the subject.

[Eirik]

Ok, thanks! I suppose I will be able to figure out what happens just by measuring one pair.
If the leakage multiplies aswell I guess I would have problem

[petemoore]

  If you have an Hfe checker DMM, you can build a little Darlington rig for it.
  just a piece of perf with a 6 pin IC socket in it. three leads [in a row] soldered to the bottom coming straight out, can be inserted into the DMM's Hfe Checker socket.
  The perf is wired with the 6 pin IC socket in darlington configuration. Plan what is a small amount of wiring so the 3 leads [for E/B/C darlington connections] will align with the DMM's Hfe socket.
  This way you can leave the darlington adapter board [about 1/2'' x 1/2''] in the DMM, insert different transistors in Sockets 1 and 2, and read the Hfe.
  Note that this doesn't test for leakage.
 

[Eirik]

Thank you for the idea Pete
I thinkI'll make a rig like that and build it into R.G.'s little testing circuit. That way I guess I would be able to test
the leakage as well.


Eirik

[nelson]

Germanium darlingtons are notoriously problematic, from what I have read.

Yes, you increase gain Hfe1xhfe2, but you also increase the leakage, uA1xuA2.

Consider petes "rangepig" "darlingtonising" one low gain germ with a silicon transistor.
Unless your germs are really really low leakage.


Correct me if I am wrong, but I believe this to be correct.

[Eirik]

Consider petes "rangepig" "darlingtonising" one low gain germ with a silicon transistor.

You're probably right... I will try to make at least one circuit with germs in darlington setup, just to give it a shot.
I tried to measure one pair but for some reason I didn't get any numbers that made sense at all, must've done something wrong.

On the other hand, the rangepig is really nice sounding (just boxed mine  ) and I guess it would be useful in other circuits as well.
Maybe low gains will work in some of Joe Gagan's stuff?
I have to try everything. It would be a shame to use all those low leakage germs for diodes.

[R.G.]

The darlington connection was used in early germanium transistor days just because they couldn't get high enough gains.

They found a world of problems with them. The leakage of the germanium devices is bad, and the leakage of the first transistor is multiplied by the leakage of the second one. So you get a normal-to-high gain transistor with gain2 times the leakage of Q1. The thermal stability is also made worse by the gain of Q2 multiplying Q1's drift. Likewise Q1's noise.

It is possible to get OK results with later germaniums after many of the problems of early devices had been fixed by better processing. But you have to always be aware of what you're playing with, or on hot days, your circuit will drift off into nonfunction.

To measure the gain of a germanium darlington, you do the same as with a single: you measure the open-base leakage current, then the current from a fixed base current, subtract the two to get the increase with base current, then divide the base current into the difference current to get the gain. You have to do this without touching the two transistors because germanium is perfectly temperature sensitive enough to drift from your finger touching the can.

It's a fascinating setup to play with, but you can rapidly appreciate why the electronics industry got off germanium as soon as they could.