Bizzare Darlington Accident

[Joe Kramer]

Hi Friends,

While playing around with some Ge transistors in Darlington configuration, I mistakenly hooked one up backwards and -- it sounds really good.  I can't say I've "discovered" it because it was an accident, but I also don't find any reference to this type of connection in any book I have.  Here's what happened:

Code Select Expand

     Conventional Darlington

                       +V
                        |
            c___________|
      _____|/       c___|
          b|\______|/       two NPN xstrs
            e     b|\_____
                    e     |
                          |
                          G


      Unconventional Darlington?

                       +V
                        |
            c___________|
      _____|/       e___|
          b|\______|/       two NPN xstrs
            e     b|\_____
                    c     |
                          |
                          G






The top example is the "right" way to make an NPN darlington, the bottom one is the mistake I made.  The odd thing is, with the "right" way, I had a lot of trouble finding two transistors that worked within the bias range I needed.  But with the "wrong" way (the second xstr backwards), the bias range is just perfect, about exactly half the supply voltage, no matter which transistors I use.  Not only that, but it sounds like a normal darlington, with the extra output drive and slightly rolled-off top end.  Lastly, it does not seem to matter which transistor is "backwards."  I have not tried this with Si transistors yet.

I have no idea what is going here, but it sounds good, so I'm ready to hardwire it.  I'm posting this in hopes that folks more knowledgeable than me might take the time to analyze and possibly explain it.

Joe

   

[R.G.]

The base-emitter junction of normal silicon transistors breaks over at a few volts. Generally this is in the region of 5-7V. Presumably they leak before they break.

In the circuit you show, one or the other will be breaking over the B-E junction if the applied voltage is more than  a few volts.

NPN devices do work in the inverted mode, subbing E for C. But the standoff voltage falls to the B-E junction rating, and the gain falls a lot. There is still gain, but lower.

There's nothing like finding a gold nugget to make your day though! If it sounds good, it is good. Now go figure out how to make it repeatable.

[mac]

at first sight it seems that the B-C voltage of the transistor at the left should be near cero... if both are of the same kind... if not?

what is the roll of leakage?

mac

[brett]

Hi.
Mac, I think you might be imagining that the B-E junction of transistor 2 is forward biased, not reversed.

Sometimes these simple ideas are very cool.  A long while ago I posted a note about using an NPN and a PNP of similar gain, working in opposition (I called it the tug-of-war).  They were connected base to base, emitter to collector, and emitter to collector.  Mr Hermida even simulated it for me (and I think posted the results).

So there's at least 3 unusual double-transistor configurations available: (1) this twisted Darlington, (2) the piggyback and (3) the tug-of-war.

Rich opportunities.....

[bioroids]

Have you tried this with silicons?

"So there's at least 3 unusual double-transistor configurations available: (1) this twisted Darlington, (2) the piggyback and (3) the tug-of-war"

Don't forget the Sziklai! (complementary Darlington some call it)

Miguel

[Joe Kramer]

Thanks for your responses RG and all.

About repeatablility: it seems to be working with all the odd varieties of Ge's I have, leaky, low Hfe, whatever.  In each case, the bias point is remarkably similar: about half the supply voltage at nine-or-so volts.  Nothing seems to be getting warm or (God forbid) smoking.  Since I'm using this as an input buffer, the gain is set at about unity, and that's all I need.  As I said before, it sounds like a conventional darlington, but has the advantage of not being picky about which transistors I pop in there.

Have not tried this with silicon xstrs yet.

What to name this connection?  Goofy-footer?   

Lab note: After playing through this set-up for a while, I'm noticing a very subtle compression/sustain effect.  A welcome side-effect, at least for my application.

Joe
   

[mac]

Hi.
Mac, I think you might be imagining that the B-E junction of transistor 2 is forward biased, not reversed.

Doh!   
(I should read more about electronics and forget about physics...)

Sometimes these simple ideas are very cool.  A long while ago I posted a note about using an NPN and a PNP of similar gain, working in opposition (I called it the tug-of-war).  They were connected base to base, emitter to collector, and emitter to collector.  Mr Hermida even simulated it for me (and I think posted the results).

So there's at least 3 unusual double-transistor configurations available: (1) this twisted Darlington, (2) the piggyback and (3) the tug-of-war.

Rich opportunities.....

I read your post. I have Hermida's post at alt.guitar. I pasted the full text in Joe Davisson's 'A new fuzz for the new year' topic for those interested.

mac

[David]

Thanks for your responses RG and all.

About repeatablility: it seems to be working with all the odd varieties of Ge's I have, leaky, low Hfe, whatever.  In each case, the bias point is remarkably similar: about half the supply voltage at nine-or-so volts.  Nothing seems to be getting warm or (God forbid) smoking.  Since I'm using this as an input buffer, the gain is set at about unity, and that's all I need.  As I said before, it sounds like a conventional darlington, but has the advantage of not being picky about which transistors I pop in there.

Have not tried this with silicon xstrs yet.

What to name this connection?  Goofy-footer?   

Lab note: After playing through this set-up for a while, I'm noticing a very subtle compression/sustain effect.  A welcome side-effect, at least for my application.

Joe
   

Joe:

Any chance you can try it with a bass?

[Joe Kramer]

Joe:

Any chance you can try it with a bass?

I haven't tried it with everything yet because I'm in the middle (supposedly) of a rather big building spree.  But I did run a drum machine through it and it was clean; or at least as clean as could be expected with line level running through a nine-volt-powered device.  Again, just a hint of compression.  I'll eventually get around to trying with bass, and then post what I find here.  In the meantime, maybe you or someone else will beat me to it on the breadboard. . . .

Joe

[David]

Alas... my breadboards are already populated with my MIDI bass pedal prototype and associated LED readouts. 
I'm not in a position to purchase more breadboard real estate at the moment.  It seems that the Juice Goose, Alesis 3630, banjo and rack case I purchased or laid away within the past week have tried wifely patience to its limits...