Types of diode-connected BJTs

[C Egernet]

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

 

[Jasen Betts]

On 2010-07-08, C Egernet <egernet@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)?

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

NaĂŻvely, I have assumed that one always uses the B-E junction but then
I saw this post from s.e.d by Phil Hobbs:

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


--- news://freenews.netfront.net/ - complaints: news@netfront.net ---

 

[George Herold]

On Jul 8, 3:53 am, 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)?

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

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.

 

[C Egernet]

George,

 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?

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

 

[George Herold]

On Jul 8, 9:34 am, C Egernet <eger...@hushmail.com> wrote:

George,

 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?

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.

 

[C Egernet]

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

 

[George Herold]

On Jul 8, 9:52 am, C Egernet <eger...@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.

 

[whit3rd]

On Jul 8, 12:53 am, 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)?

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).

 

[Bob Masta]

On Thu, 8 Jul 2010 14:49:43 -0700 (PDT), whit3rd
<whit3rd@gmail.com> wrote:

On Jul 8, 12:53=A0am, 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)?

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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[John Larkin]

On Fri, 09 Jul 2010 11:44:51 GMT, N0Spam@daqarta.com (Bob Masta)
wrote:

On Thu, 8 Jul 2010 14:49:43 -0700 (PDT), whit3rd
whit3rd@gmail.com> wrote:

On Jul 8, 12:53=A0am, 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)?

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.)

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.

 

[George Herold]

John Larkin wrote:

On Fri, 9 Jul 2010 10:00:08 -0700 (PDT), George Herold
gherold@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.
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.