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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 -

From: John Larkin on 9 Jul 2010 17:49
On Fri, 9 Jul 2010 10:00:08 -0700 (PDT), George Herold
<gherold(a)teachspin.com> wrote:

>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)?
>>
>> >> >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 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 -

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


From: George Herold on 9 Jul 2010 21:32


John Larkin wrote:
> On Fri, 9 Jul 2010 10:00:08 -0700 (PDT), George Herold
> <gherold(a)teachspin.com> wrote:
>
> >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)?
> >>
> >> >> >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 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 -
>
> 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

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.
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