Using a transistor as a diode.

[blaren]

I'm a noob here and pretty much in pedal building. Mod/repair all my tube amps, all my wahs, most of my ODs, have built a few fuzzfaces.
I am WICKED with an iron, work safely in old high voltage holding tube amps, and can follow a layout like NObody's business lol. Ask me what is doing what or why whats where...or show me a schem and I'll go...uhhhhh...wha...? slobber...drool...

SO...lol...I'm gonna build me an Eternity clone (from a kit...slobber...hun??) and only have the one type of diode coming. I might have one or two others kicking about, I have some LEDs, but I also have some Ge and Si transistors. I know you can use a transistor as a diode by connecting the (iirc) B&E together and using that as one leg with the Collector as its own leg (I might have the C&E mixed up) but...I don't know where you would and wouldn't want to use a tranny as a diode, whether to use a Ge or Si, what gains, how much leakage is acceptable when used as a diode...all that stuff.
I could just start experimenting but man...that's a LOT of options/combinations. Oh...and I assume you'd wanna use NPN transistors? Or could you use PNP ..or use pnp backwards? ..so yeah..
I searched and didn't find much on google. I wouldn't know where to start "playing"..NPN? PNP? one of each but the pnp backwards? Si? A Si and a Ge? ...you get my drift. I could be testing for WEEKS! And who knows, if it's a good idea and if I find I like the sound of them I might WANT to be messing with them for weeks.

Have you fellas tried using transistors as diodes in a bufferless tubescreamer type pedal (Eternity flavor)? And if so, how did you like them? what kind did you use? their gains and leakage and polarity and construction....
Thanks in advance. and I'll be back with results of course if/when I mess with any.

[PRR]

It is not important to connect any legs together.

Ignoring polarity, there are only 3 ways to put a 3-leg device into 2 holes. If you socket (temporarily), you can stuff a transistor all three ways faster than you can think about it.

It is very likely the CB and EB diodes are "the same" for practical purpose (and low voltages) but you should try.

PNP, NPN, FET... they all have diodes.

Ge diodes give smaller voltages than Si diodes.

[blaren]

Thanks for the reply.
Wouldn't there be a whole bunch of ways to plug a 3 leg device into two holes? E into hole 1 and B into hole 2, E into hole one and C into hole 2, B into one and  C into 2, B-1 and E-2, etc...and then E&B shorted and C into the other hole, E&C shortedinto hole 1 and B into hole 2...etc etc..??

But anyway...so you don't have to short 2 legs of the transistor together to use it as a diode? Hmm.
Like I said...NOOB here. So use any 2 legs of the transistor into any 2 holes (I say holes...you know..) that would accept the diode's legs? And no shorting...just leave the unused leg alone or chop it off or whatever. Hmm.
Thanks again for the reply.

[Seljer]

A diode is a PN junction. P and N being positively or negatively doped silicon. Silicon by itself isn't conductive, so you add some impurities that either have an excess or shortage of electrons and the knocks off the balance of charge carrier. Now just one type alone doesn't really do anything interesting, but if you put the two types of material right next to each other they only let current flow in one direction.

A bipolar transistor is two PN junctions, either PNP or NPN. Sometimes transistor are even described as "two back to back diodes" however if you make up such a circuit out of diodes, it doesn't function because there are other phenomena going on inside the transistor. So yes you can use the collector-base junction or the base-emitter junction on their own as a diode. You can leave the third leg of the transistor floating or short it to the base, in a clipping circuit it shouldn't make any difference.

[PRR]

> a whole bunch of ways to plug a 3 leg device into two holes?

Think of a 2-note chord. Say you only have 3 fingers. How many ways can you hit two fret-points with three available fingers? (Pretend fingers are infinitely flexible.)

I get three ways.

If parity (polarity) matters, six ways.

[R.G.]

I'm not going to get into the ways of counting leads and holes, but a lot of circuits practice conventional wisdom holds that a bipolar transistor shorted base to collector makes an equivalent "diode" that obeys the theoretical diode equation better than a single junction. The transistor action minimizes certain of the non-ideal-nesses.

That being said, a base emitter in modern silicon stuff is a pretty good heavy-doped junction with a reverse breakdown in the 5-8V range and the base-collector is a good high(er) voltage, low leakage junction with a breakdown equal to the device's BVcbo.

I think that's what the OP was after first.

[blaren]

Wow interesting. Thanks RG and everyone.

[PRR]

> shorted base to collector makes an equivalent "diode" that obeys the theoretical diode equation better

True. But this is a distortion, not a logarithm computer.

Also some of that accumulated wisdom came from "worse" transistors than we get today. Recombination 'leakage' skews the low-end; transistor action tends to mask that down to lower current. The low-end fall-off is much less every decade, as generic Silicon and its processes get cleaner.

If it really matters you buffer the junction with an opamp.

But there is a phrase about polishing cow-pies. Not that the music is cow-pie, but when you want *distortion* you may not want any sort of theoretical perfection.

But whatever. Feel free to jam a needle-point screwdriver in there to short the 3rd leg to something. In most clippers the worst can happen is you get no output until you take the screwdriver back out. If you do hear a difference, hot-up the soldering iron.

[duck_arse]

rg, while you're here talking transistors as diodes, can I use the B-E connection instead of B-C as the no-leak diode in a mill II? using smt on vero, it would be an easier layout.

[PRR]

> can I use the B-E connection instead of B-C as the no-leak diode in a mill II?

R.G. knows his stuff. Wait for him. But to stop you from leaping ahead, as he already said, B-E is typically a 7V breakdown. Does this diode have to stand-off the full "9V" supply (or most of it)? If so, it probably won't.

[R.G.]

And so does Paul. Won't work, the B-E junction will break over.

And B-E junctions are not particularly low leakage when used as reverse diodes. The B-C junction usually is. So sorry - no, don't use the B-E in the Mill 2. Not good on two different counts.

[duck_arse]

thank you both. leaping ahead is not my forte. in this case, I'll stay with leaving that diode off for a while.

[blackieNYC]

Im not clear on this -the conclusion is to use the B and C as your diode cathode-anode?  Is in important to tie the emitter to something?  Never heard of that before.
I have a fistful of Ge trans with gains in the single digits.  Hoping to use them as clipping diodes.

[PRR]

> Is in important to tie the emitter to something?

It is not important to connect any legs together.

For fuzz-clipping diodes: Do it, or not, your choice.

[deafbutpicky]

btw. if you connect the open  leg with the base you get lower Vf (at least with ge-Ts) and if you place the "clipping diodes" between Cs you don't need to worry about any breakdown voltages. As PRR said, just go for it and listen to what it does; there's a chance parasitic capacitance is more influential on the the sound than Vf and you're not going to make a bulk of 'em but a good one, right?!