From: "Tom Faloon"
Subject: Re: reverse-biased diode noise source?
Date: Mon, 16 Sep 2002 15:17:59 +0100
NNTP-Posting-Date: Mon, 16 Sep 2002 14:14:49 +0000 (UTC)
X-Newsreader: Microsoft Outlook Express 5.00.2615.200
Any silicon or germanium rectifier diode, transistor base emitter junction,
or zener diode will act as a noise generator when reverse biased to
breakdown (avalanche) region.
However, this is generally an unwanted effect, and manufacturers do not
It is possible to buy diodes with published characteristics, but these are
and cost and minimum order quantity put them in reach of business users
You can obtain useful output from everyday devices, but you will not be able
to predict results accurately in advance, or assume repeatability from
component to component.
In the simplest application, simply reverse connect the diode to a supply,
via a current limiting resistor, increase the supply voltage until reverse
and a noise voltage will then be produced across the diode.
You can expect a fairly 'white but not perfect' noise spectrum over a wide
Non perfection is most likely to be due to secondary noise functions in the
most pronounced being flicker noise. (1/f) which will be present below about
The bandwidth will depend on the components and layout used. (Junction
capacitance, lead inductance, PCB track capacitance & inductance.)
Bandwidths into the GHz region are possible.
The noise level generally increases with reverse current. Level will depend
on the device used
As a rough figure think fractions of a nanoVolts / root Hz. (Perhaps X 10 to
A standard rectifier diode. (Not always practical, because most diodes have
a fairly high breakdown voltages.)
Base emitter junction of a bipolar transistor is more practical. (Reverse
breakdown is often around 5V)
(Leave the collector unconnected.
Zener diode, but use devices above about 6 or 7 Volts.
(We call all these devices Zener diodes, but in fact they operate in two
different modes. Devices operating below about 5 Volts rely on the Zener
voltage devices use the avalanche effect.)
remove 'z' from address to email direct.
Walter Harley wrote in message
> I see many references to the fact that a reverse-biased zener diode
> functions as a wideband noise source. However, I can't find much
> information about how to put this into practice. How much noise does it
> generate, for how much current? Does it have to be in avalanche, or
> does it generate noise even at leakage-current levels? Is there any
> difference between different diodes, e.g., different voltages? What is
> the frequency spectrum like?