Types of diode-connected BJTs

C-B can handle much larger reverse bias than B-E

B-E is used in current mirrors to get temperature compensation for the forward voltage drop

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Chris, I don't know the answer to your question, but are you sure that when using the C-B junction that you short the base and emitter?

George H.

George,

When you ask, no, I am not sure.

That begs the question what happens if I leave the emitter floating. It does seem a rather odd configuration though.

Chris

Hmm, perhaps time to do a bit of curve tracing and see if there is any difference.

George H.

I was thinking the same except that it is going to be with a selection of resistors, a battery and a DVM.

Still, I don't think that I can answer my original question that way.

Chris

Yup, no curve tracer here either.. have to do it by hand. I thought Jason answered your question. The CB junction has a higher reverse breakdown voltage.

George H.

B-E junctions have good compliance with the diode equation, but base resistance can be significant, so shorting C to B is recommended. Breakdown voltage, alas, is usually

7V or so (not always, chopper and oscillator service benefit from higher breakdowns, at the expense of current gain).

B-C junctions have higher breakdown, and low leakage and capacitance, but Rbb (the base spreading resistance) would be a problem if you wanted to use 'em for switching. Shorting B-E helps that.

For really good low leakage, I often use analog FETs (there were LOTS of pFETs in the surplus outlets some years back).

Most diodes-sold-as-diodes, like BAV99 and 1N4148 and such, leak nanoamps, and the glass ones are photosensitive.

I took data on using BFT25A C-B junctions as diodes. They are fantastic. I measured about 20 fA reverse leakage at a few volts, log linearity from 1 pA to 10s of mA, and about half a pF. I didn't try the B-E junction, because it will zener at a few volts so isn't as generally useful. The measurements are tedious.

It leaks less than a PAD-1, has much lower forward resistance and capacitance, and costs a lot less. You can hardly buy a diode that good.

One exception is the Central CMPD6001S, a dual SOT-23 that leaks about

50 fA at -5 volts and room temp. It's probably a bigger junction than the BFT25, because it leaks more and is more like 2 pF. That's two diodes for about 16 cents

John

Many thanks to Jasen Betts, George Herold, whit3rd and John Larkin. I learned something useful.

Chris

This is a good thing to remember when you are looking for Zeners in your junk box. Handy when you don't need a really specific breakdown voltage, just a good sharp knee. Sharper than "real" Zeners, in my experience. (Along the same lines, a forward-biased LED does the same thing in the 1.5 to 2 V range, depending on color.)

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That's great! It would be useful as a over-voltage protection 'diode' on a low noise front end. Any idea how much current it can handle. Seems like if used as over-voltage protection you might want to short the E and B and get a bit of current gain.

George H.

I'll try the BFT25 with the emitter open and shorted to the base, when I get a chance. I'd be interested in comparing both leakage and capacitance. As I mentioned, fA leakage testing is tedious.

John

Reverse beta lowering the voltage drop? Probably so. C would go up.

I'd always assumed that "microwave" transistors would be leaky for some reason. As Phil pointed out, they make good low-leakage diodes because the junctions are so small.

We created PADS schematic and PCB symbols for the BFT25 as a diode. Schematics get weird and ugly when you use a bunch of transistor symbols as diodes.

I did the testing for a couple of projects. One is a photodiode amp where we want to prevent windups and inject some test currents, and the other is an FTMS preamp where we have a kilovolt of transmit RF millimeters away from a nanovolt receive antenna, and we need to recover quickly but add minimal leakage and capacitance. The resulting circuit is cute but un/fortunately too good to publish in the open. I

*did* Spice it because I *didn't* entirely understand how it would work; too damned nonlinear, too diode dependent, no hard definition of "best."

John

hen

I was just thinking that if E and B were shorted then not all the current would have to flow through the base. There must be some small amount of current gain in this 'backwards' transistor.

George H.

My Fluke seems to output 0.6 mA on the "diode" range. A BFT25A reads

0.843 for the C-B diode, down to 0.771 if I short the base to emitter, so there is some advantage from reverse beta. Capacitance is 0.55 and 0.83 pF respectively. Those are the easy measurements; maybe I'll get around to doing leakage, too.

The high voltage drops at low current suggest a very small chip, no surprise.

Hmmm, delta-V is about 70 mV. Does that imply a reverse beta around

10?

John

If you go collector-emitter as a zener, you get the forward c-b junction in series with the zenered b-e. Some transistors hit the magical "reference zener" voltage, 6.2, where TC is nearly zero.

Zenering the b-e junction wrecks the beta of course. And some transistors seem to creep in b-e zener voltage over time, probably related physics.

John.

ion but then

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n 'diode'

andle.

That's Great! I'll try and remember to measure a 2N3904/6 on Monday. How did you measure the capacitance? We've got an SRS RCL 'meter' (box) but I've never tried it on an active device.

70mV looks like a bit more than ten at room temp, but at low gain there must be an 'extra' factor of one floating around somewhere.

Can I 'measure' the forward current gain the same way? I've got some old BK meter in the shop.....(I won't know the current)

gonna have to find a transitor

Thanks, another hammer looking for a nail.

George H.

I used my AADE capmeter, with the surface-mount adapter. I'm not sure what the drive volage is, so maybe I should check it with one of my old pale green Boonton analog c-meters; I know it runs about 0.1 volts.

John

That '2N3904' only means a JEDEC specified part, it could vary widely from one manufacturer to another (and from one year to another) because any transistor that meets the loose specifications can be so labelled. If you want a meaningful result for future guidance, it's better to pick a manufacturer-specific part, and maybe even one with some kind of premium specifications, like low noise or high bandwidth.

Hopefully, if NXP or Zetex redesigns, the 'newer' parts don't have the same ID number as the ones they replace. Unless the part is a JEDEC standard and they want to bid on the big jobs that use those standard parts.

BFT25A is Philips/NXP part with 5 GHz bandwidth, only available in surface mount SOT-23 (low stray capacitance due to leads). I'd expect its character to be more consistent than that of 2N3904.