A friend of mine recently discovered that bipolar transistors work in inverse mode. He told me of his discovery and was dismayed to know that I had not only heard of such a thing but had used it in designs eons ago.
However, I do not recall the reasons that I did so (it was 25 years ago) and what the characteristics of inverse BJTs are.
Can anyone lead me to a link that explains it all, for me and and my friend?
TIA
John
No, but when I used inverse BJT's back then it was to take advantage of the really low saturation voltage you get when you use an inverted BJT as a saturated switch.
-- Bill Sloman, Nijmegen
You'll probably have to paste the URL into your browser... Agent doesn't seem to like the parentheses and may not pass that information on to your browser.
...Jim Thompson
the gain is also pretty low. I "discovered" this by having a tech flip a part, swapping C & E, and it still worked. but the reduced gain played havoc with EMC/ESD susceptibility, which is how I found it. Then I read the paper from Jims website.
Cheers Terry
Vacuum tubes work in inverse modes also..how does a "plate follower" grab you?
Rather common trick used in choppers in the old daze...
If you need decent gain / beta, check out transistors by Zetex.
Not really, just grid and plate swapped.
Meh, negative screen current is more interesting. Don't see that too often.
Tim
-- Deep Fryer: A very philosophical monk. Website @
I think I remember reading somewhere that a bipolar run inverted will slowly get a lower and lower gain, but that the gain can be brought back up by heating it for a few seconds.
-Lasse
I think the beta reduction occurs when the base emitter junction has current driven through it in reverse breakdown. As long as the inverse transistor circuit applies less than the rated base emitter junction voltage I don't think there is any adverse effect on the gain.
Huh? Beta is largely dependent on the ratio of the doping levels of the (apparent) emitter and the base. There is no breakdown involved _unless_ you exceed "collector"-base greater than VEBO.
...Jim Thompson
If you put enough forward current on the control grid, you can operate a tube in a mode that makes a very high gain device. It is a grid vapor amplifier for a little while. The plate will carry many amps.
There was a tube that had the suppressor grid brought out on its own. It was intended for allowing it to be at AC ground when the cathode wasn't. On one of those you could get gain from the secondary electrons if you pulled it positive.
This is not exactly the same thing, but it's interesting:
Vc | | | c Vb--------Rb----------b npn e | | +--------Ve | | Rl | | gnd
When Vb is above ground but below Vc, it's a regular emitter follower. But as Vb increases above Vc, the transistor eventually saturates. In many situations, there's a value if Vb (of Ib, actually) where the saturation voltage is precisely zero. At higher Vb's, Ve goes above Vc, sort of over-saturation.
I once built a bunch of 16-bit dacs using switches like this, driving r-2r ladder networks made from wirewound resistors.
John
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