Re: BJT selection for noise generator

C:\tmp\archive\www.xxxcomfort.com\xm301\30On Sun, 30 Sep 2007 10:28:39
-0700, Wimpie <wimtel@xxxxxxxxx> wrote:

On 30 sep, 08:09, Allan Herriman <allanherri...@xxxxxxxxxxx> wrote:
On Sun, 30 Sep 2007 03:48:16 +0100, Eeyore

<rabbitsfriendsandrelati...@xxxxxxxxxxx> wrote:

Allan Herriman wrote:

Hi,

I was wondering if anyone could help with the selection of the
transistor Q1 in the noise generator circuit below?

It's an NPN, used with the EB junction reverse biased to breakdown.
Noise is generated by this breakdown, and amplified by Q2.

Seems like an odd way to generate noise to me.

It's used in a large number of professional (i.e. FIPS certified)
entropy generators.

It's an old design (not one of mine), and I'm interested in knowing
whether anyone has insights into the cause of a field failure.
So far we've had 10 replies to this thread, and not one has mentioned
BJTs.

Regards,
Allan
Hello Allen,

You are focusing on the BJT. Are you sure there are no other
components in the circuit that may show aging effects (like
electrolytic capacitors and certain ceramic types).

The failure can also have a complete other origin (like surge
phenomena or cumulative radiation effects).

If you are sure the BJTs are to blame, you might get document "Hot-
carrier Degradation in bipolar transistors at 300 and 110K - Effect on
BiCMOS Inverter performance". It discusses the effects of reverse
breakdown on forward beta. Although you are not interested in forward
beta, it may provide you some insight.

All components in that circuit were replaced and the repaired unit
(now passing its FIPS tests) was returned to the customer, so I guess
we'll never know the exact cause.

Thanks for the pointer to that document.

Regards,
Allan
.

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