Transistor backwards... But it works ?_?

[Slater]

I built this EasyI2C circuit:

http://www.lancos.com/prog.html#easyi2c

I think I put the transistors backwards, but the circuit reads and
writes EEPROMS flawlessly.

How can this be?

 

[[email protected]]

I built this EasyI2C circuit:

http://www.lancos.com/prog.html#easyi2c

I think I put the transistors backwards, but the circuit reads and
writes EEPROMS flawlessly.

You sure it's not reading them out backwards?

How can this be?

Bipolar transistors are somewhat symmetrical; it's an N-P-N structure so the
base-collector and base-emitter junctions are similar. The transistor won't
work as well backwards but it's still a transistor. The "reverse" beta isn't
zero.

 

[Fred Bartoli]

John Larkin a écrit :

Transistors have reverse beta, current gain with emitter and collector
swapped. Reverse bata can be as high as 5 or more, and your circuit
only needs about 1.

Some old transistors were symmetrical, no difference between emitter
and collector. Some on purpose. Ditto jfets.

I once measured one BC547C(or 548C) from old stock that was pretty good:

Forward biased it still had beta > 800 @ 1nA Ic (yup, no mistake, that
hardly more than 1pA base current) and reverse biased it still had circa
300.

I think I still have it, carefully stored, somewhere. Well, somewhere...

 

[Spehro Pefhany]

John Larkin a écrit :

I once measured one BC547C(or 548C) from old stock that was pretty good:

Forward biased it still had beta > 800 @ 1nA Ic (yup, no mistake, that
hardly more than 1pA base current) and reverse biased it still had circa
300.

The old Ge ones must have been even better- you could get tens of uA
collector current with 1pA of base current.

 

[Fred Bartoli]

Spehro Pefhany a écrit :

The old Ge ones must have been even better- you could get tens of uA
collector current with 1pA of base current.

LOL.
It was of course checked for leakage

If can I can find it I'll check it and post the results...

 

[Tauno Voipio]

I built this EasyI2C circuit:

http://www.lancos.com/prog.html#easyi2c

I think I put the transistors backwards, but the circuit reads and
writes EEPROMS flawlessly.

How can this be?

NPN is NPN even in reverse. The properties usually differ from
the forward case.

In the time before FET's, the reversed 2N3704's were used as
analog switches, as the saturation voltage was very low, at the
cost of a lousy beta. For more information, look for Ebers-
Moll transistor model, which explains the behavior pretty well.

 

[Tim Williams]

I once put in a 2N3904 emitter follower "backwards" (the EBC pinout is
nicely symmetrical). Didn't even notice a difference.

Occasionally, it's intentionally useful. You can't get a >30V input
differential any other way. (All the classic 30V bipolar analog chips, such
as LM358, LM393, etc., with 30V differential inputs, essentially did this.
In fact, their PNP input transistors were composed of two back-to-back
collectors -- go figure!)

Tim

 

[Fred Abse]

Bipolar transistors are somewhat symmetrical; it's an N-P-N structure so the
base-collector and base-emitter junctions are similar. The transistor won't
work as well backwards but it's still a transistor. The "reverse" beta isn't
zero.

I've used that to identify B and E on an unknown pinout. The way round
that gives the highest beta, and highest BVceo is the correct one.

 

[legg]

I built this EasyI2C circuit:

http://www.lancos.com/prog.html#easyi2c

I think I put the transistors backwards, but the circuit reads and
writes EEPROMS flawlessly.

How can this be?

As with other posters, the parts will work inverted. I see a funtional
gain requirement of slightly more than 0.5 for basic function, though
the output of the parallel port may be unpredictable, given hardware
designer's leeway.

Of greater importance is the limited supply voltage, below the
normally-expected Veb breakdown values.

I'm not sure you'd get usefull inverted beta if the EB jn were
avalanching.

RL

 

[[email protected]]

Where? I do not see any transistor in "backwards".
Swapping emitter and collector functions results in lower beta and
lower saturation voltage (which can be useful).
Ain't happening in ANY of the circuits on that page.

It is if they're inserted backwards.

 

[Spehro Pefhany]

The CB junction usually has lower forward voltage, too.

Cheers

Phil Hobbs

That's the way you can usually tell C from E with a handheld
multimeter (on the diode range). If it has an HFE function, that's a
confirmation (but not so convenient to use on SMT transistors).

 

[Fred Abse]

That's the way you can usually tell C from E with a handheld multimeter
(on the diode range).

If it has a diode range.

I was spoiled, I had access to curve tracers before meters with diode
ranges became common.

 

[Jamie]

NPN is NPN even in reverse. The properties usually differ from
the forward case.

In the time before FET's, the reversed 2N3704's were used as
analog switches, as the saturation voltage was very low, at the
cost of a lousy beta. For more information, look for Ebers-
Moll transistor model, which explains the behavior pretty well.

I kind of thought that destroyed the beta in the device when using
it in forward mode afterwards? This was something that took place over
time of use in the reverse mode.

I guess it wouldn't matter if you never intend to extract the unit for
another job.

I remember a circuit years ago that had a R on the emitter side and if
the R got opened and the circuit had been operating for while in reverse
state, the transistor had to be replaced because the beta was no longer
where it should of been.

Jamie...

 

[Fred Abse]

Zenering the b-e junction gradually destroys beta. I don't think reverse
operation is a problem otherwise.

The classic cross-coupled astable multivibrator drives each base negative
wrt each emitter while each coupling capacitor discharges. Not a problem
unless the junctions zener, just something to watch out for.