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Fundamental Noise Basics and Calculations

Fundamental Noise Basics and Calculations 71

of bandwidth, or other distributions. Here the power per unit bandwidth rises
at 10dB/decade toward lower frequencies.
For highly repeatable, calibrated measurements, it is still of interest to use a
thermionic (vacuum) diode operated under so-called temperature-limited con-
ditions, where current through the diode changes only slowly with voltage, and
there is no space charge. However, the relatively high voltages and the filament
heaters needed for valve operation don’t fit in well with current electronic
equipment.
In principle, any semiconductor device or even resistor can be used as a gen-
erator, with sufficient amplification, using their shot or Johnson noise spectral
densities. The difficulty is in ensuring a flat frequency spectrum. Devices that
produce much higher amplitudes are useful to reduce the problems of 1/f noise
in the amplifier. The zener diode is often used for this task. It may be better to
use high-voltage types because above breakdown values of about 5.7V Zener
diodes operate via avalanche processes and have a positive temperature coeffi-
cient. Below this value they operate with the zener effect and have a negative
temperature coefficient. Any ordinary semiconductor device operated in reverse
breakdown mode may also work well. I have not been able to find in the liter-
ature any consensus on this. Published circuits typically use a 12V zener diode,
operated at a current of the order of 1mA with AC coupling to a low-noise ampli-
fier. I tried instead a small npn silicon transistor (ZTX453), with the base-
emitter junction reverse biased to breakdown, operated into a transimpedance
amp (Fig. 3.20). Below the breakdown voltage of the junction, the noise will be
just that of the amplifier and its load resistor. Just at breakdown, about 8.30V
in my example, the noise level increased to an enormous value. In fact it is not
even necessary to use the transimpedance amp. With a 100k load the scope
input is sufficient to see the noise. In the time-domain the waveform looks like
a slow sawtooth with fast jumps between two voltage levels and stable stretches
of many tens of microseconds. As the bias current is increased, the slope of the
sawtooth increases, putting more energy into higher frequencies (Figs. 3.21,

+0–12V
100k
100k

2x -
ZTX453 LMC7101
+
100k

-0–12V
Figure 3.20 High-level noise can
be conveniently generated using a
bipolar transistor base-emitter junc-
tion biased to breakdown. The use of
two devices can avoid AC-coupling.

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