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From: C Egernet on 8 Jul 2010 03:53
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
From: Jasen Betts on 8 Jul 2010 05:22
On 2010-07-08, 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)?

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

> 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:

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


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From: George Herold on 8 Jul 2010 09:29
On Jul 8, 3:53 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)?
>
> 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

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.
From: C Egernet on 8 Jul 2010 09:34
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
From: George Herold on 8 Jul 2010 09:38
On Jul 8, 9:34 am, C Egernet <eger...(a)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.
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