From: John Larkin
Subject: Re: reverse-biased diode noise source?
Date: Mon, 16 Sep 2002 10:09:02 -0700
Organization: Posted via Supernews, http://www.supernews.com
X-Newsreader: Forte Agent 1.91/32.564
On Mon, 16 Sep 2002 15:17:59 +0100, "Tom Faloon"
>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.
a 1N758 (10 volt) zener makes pretty nice gaussian noise at 1 mA; at
much lower currents the waveform starts getting asymmetric and then
oscillatory. At 1 mA, noise density is about 340 nV per root Hz, or
about 60 uV RMS in a 30 KHz bandwidth.