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From: Bob Masta on 9 Jul 2010 07:44
On Thu, 8 Jul 2010 14:49:43 -0700 (PDT), whit3rd
<whit3rd(a)gmail.com> wrote:

>On Jul 8, 12:53=A0am, 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

<snip>

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.)

Best regards,



Bob Masta

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From: George Herold on 9 Jul 2010 09:48
On Jul 8, 8:35 pm, John Larkin
<jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:
> On Thu, 8 Jul 2010 00:53:14 -0700 (PDT), 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)?
>
> >Naïvely, 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.

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.
>
> 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- Hide quoted text -
>
> - Show quoted text -

From: John Larkin on 9 Jul 2010 11:04
On Fri, 9 Jul 2010 00:51:51 -0700 (PDT), C Egernet
<egernet(a)hushmail.com> wrote:

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

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

From: John Larkin on 9 Jul 2010 11:20
On Fri, 9 Jul 2010 06:48:53 -0700 (PDT), George Herold
<ggherold(a)gmail.com> wrote:

>On Jul 8, 8:35�pm, John Larkin
><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:
>> On Thu, 8 Jul 2010 00:53:14 -0700 (PDT), 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)?
>>
>> >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.
>
>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.

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


From: George Herold on 9 Jul 2010 13:00
On Jul 9, 11:20 am, John Larkin
<jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:
> On Fri, 9 Jul 2010 06:48:53 -0700 (PDT), George Herold
>
>
>
>
>
> <ggher...(a)gmail.com> wrote:
> >On Jul 8, 8:35 pm, John Larkin
> ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:
> >> On Thu, 8 Jul 2010 00:53:14 -0700 (PDT), 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)?
>
> >> >Naïvely, 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.
>
> >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.
>
> Reverse beta lowering the voltage drop? Probably so. C would go up.

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.
>
> 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- Hide quoted text -
>
> - Show quoted text -

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