Re: Types of diode-connected BJTs
John Larkin
<ege...@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)?
>
>Naīvely, 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
Grant
>On Thu, 8 Jul 2010 00:53:14 -0700 (PDT), C Egernet
><ege...@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.
Does E connect anywhere? Or leave open?
Grant.
Jim Thompson
>On Thu, 08 Jul 2010 17:35:30 -0700, John Larkin <jjla...@highNOTlandTHIStechnologyPART.com> wrote:
>
>>On Thu, 8 Jul 2010 00:53:14 -0700 (PDT), C Egernet
>><ege...@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)?
>>>
>>>Naďvely, 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.
>
>Does E connect anywhere? Or leave open?
I'll leave it for Larkin to pontificate, then I'll say :-)
>
>Grant.
>>
>>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
>>
...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
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Obama isn't going to raise your taxes...it's Bush' fault: Not re-
newing the Bush tax cuts will increase the bottom tier rate by 50%
C Egernet
learned something useful.
Chris
George Herold
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> On Thu, 8 Jul 2010 00:53:14 -0700 (PDT), C Egernet
>
>
>
>
>
> <eger...@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)?
>
> >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 -
John Larkin
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
John Larkin
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
George Herold
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 Larkin
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
George Herold
John Larkin wrote:
> On Fri, 9 Jul 2010 10:00:08 -0700 (PDT), George Herold
> <ghe...@teachspin.com> wrote:
>
> >On Jul 9, 11:20�am, John Larkin
> ><jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> >> On Fri, 9 Jul 2010 06:48:53 -0700 (PDT), George Herold
> >>
> >>
> >>
> >>
> >>
> >> <ggher...@gmail.com> wrote:
> >> >On Jul 8, 8:35�pm, John Larkin
> >> ><jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> >> >> On Thu, 8 Jul 2010 00:53:14 -0700 (PDT), C Egernet
> >>
> >> >> <eger...@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
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.
John Larkin
<ghe...@teachspin.com> wrote:
>
>
>John Larkin wrote:
>> On Fri, 9 Jul 2010 10:00:08 -0700 (PDT), George Herold
>> <ghe...@teachspin.com> wrote:
>>
>> >On Jul 9, 11:20?am, John Larkin
>> ><jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
>> >> On Fri, 9 Jul 2010 06:48:53 -0700 (PDT), George Herold
>> >>
>> >>
>> >>
>> >>
>> >>
>> >> <ggher...@gmail.com> wrote:
>> >> >On Jul 8, 8:35?pm, John Larkin
>> >> ><jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
>> >> >> On Thu, 8 Jul 2010 00:53:14 -0700 (PDT), C Egernet
>> >>
>> >> >> <eger...@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.
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
whit3rd
> [John Larkin wrote:]
> > 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.
> That's Great! I'll try and remember to measure a 2N3904/6 on Monday.
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.