Dumb question on noise source

The zener won't care, but a 'constant current source' might be bandwidth-limited with an internal op amp. That would rather complicate the noise spectrum; resistor (noninductive, metal film) would be preferable.

"zener" diodes are actually field-effect diodes below about 5 volts, and true avalanche diodes, with short microplasma breakdowns, above 7 volts. In the transition region, zener diodes have low Rac, low dV/dI, low noise and other "good" properties. So 8 volts or higher is a better voltage for high noise generation. The noise is related to current, whatever the source. You have to empirically determine the best parameters, and the diodes are all different.

The current source will need to have very large gain to be effective, because if the zener is operating in the avalanche region (where you want it to be to get the most shot noise) it's operating in the steepest part of its I/V curve. A simple one or two transistor current source might not be that much better than a resistor, maybe worse depending on bandwidth.

Reverse-biased BJTs apparently make better noise sources at low voltages than regular Zeners; well that's what everybody else seems to use, anyway.

Am 11.01.2018 um 22:24 schrieb bitrex:

Probably you can expect less shunt capacitance for a BFR93 BE junction than for the average Z-Diode. That type actually works quite good to

Gerhard

Where the TC goes through 0 at 6V or 7V , both effects are "equally strong". You can see the onset of avalanching already at low voltages. See for the BZX84 family, how the voltage noise gets more from 2V7, 3V3, 4V7, 6V8. That grows faster than the Zener voltage itself. All diodes were powered with 1 or 2K from a 14V NiMH battery. The battery adds nothing, as has been tested by using a 47 Ohm resistor instead of the Zener. It shows just the expected thermal noise. 0 dB is 1nV/sqrt Hz.

@David: A 3K3 resistor should do no harm. The noise depends on the current, more current is usually quieter with wider bandwidth. I have not done enough tests to be statistically convincing.

Gerhard

Correct; even the same1N between various manufacturers, or the same

ok - and it simplifies things. A cc source might better deal with noise and fluctuatons on the power supply?

more than 8 volts and up the current. gotcha. I was planing on a vca810 or for a low speed version a lm13700 as a variable gain stage. The worst possible outcome would be trusting the end user to make adjustments.