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From: C Egernet on 8 Jul 2010 09:52
> Hmm, perhaps time to do a bit of curve tracing and see if there is any
> difference.

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
From: George Herold on 8 Jul 2010 16:38
On Jul 8, 9:52 am, C Egernet <eger...(a)hushmail.com> wrote:
> > Hmm, perhaps time to do a bit of curve tracing and see if there is any
> > difference.
>
> 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.
From: whit3rd on 8 Jul 2010 17:49
On Jul 8, 12:53 am, C Egernet <eger...(a)hushmail.com> wrote:
> What are the relative merits of diode-connecting a BJT by shorting
> base and collector (using B-E junction) versus shorting base and
> emitter (C-B junction)?

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).
From: John Larkin on 8 Jul 2010 20:35
On Thu, 8 Jul 2010 00:53:14 -0700 (PDT), C Egernet
<egernet(a)hushmail.com> wrote:

>What are the relative merits of diode-connecting a BJT by shorting
>base and collector (using B-E junction) versus shorting base and
>emitter (C-B junction)?
>
>Na�vely, I have assumed that one always uses the B-E junction but then
>I saw this post from s.e.d by Phil Hobbs:
>
>> BFT25A C-B junctions are at least as good as 2N4117As as diodes.
>>
>> Cheers
>>
>> Phil Hobbs
>
>What gives?
>
>
>Chris

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


From: C Egernet on 9 Jul 2010 03:51
Many thanks to Jasen Betts, George Herold, whit3rd and John Larkin. I
learned something useful.


Chris
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