Some noise measurements for LEDs and zener diodes

[jackinnj]

wouldn't the Vbe multiplier noise be dominated by the resistors, and can you analyze it with/without a capacitor between collector and emitter.

[lineup]

Originally posted by plovati
Sorry if some of You already discussed this point, I wasn't able to found it in the present thread:
what about the Vbe multiplier noise?

Originally posted by jackinnj
wouldn't the Vbe multiplier noise be dominated by the resistors, and can you analyze it with/without a capacitor between collector and emitter.

Christer has provided noise figures for BC550 transistors, as well.
At different currents. Something like 2-3 mA was found to be a good value to use
for minimal noise figures.

Now an Amplified Diode using for example BC550
would probably have similiar noise values, but a bit higher
depending how high the voltage would be.

The Amplified diode has got a total voltage
which is a factor of the transistor vBE, set by 2 resistors.


lineup
Lineup Transistor Lab
http://lineup.awardspace.com/ ....... ( under construction )

[plovati]

A Vbe multiplier or amplified diode to be used as cathode bias has the Ic noise + the two Resistor noise amplified by transistor Gm. I'm not able to understand how much will be the multiplication factor of these resistor noise:
Resistors are in the 100ohm range, not so noisy itself.

I'd like to see how an amplified diode of let's say 2,2V compares to LEDs in respect to noise.

[TNT]

How does noise from batterys vary depending on their level of charge?

http://www.tnt-audio.com/clinica/reg..._noise4_e.html
/

NB: i have nothing to do with TNT-audio - my username has relation to explosives rather ;-).

[jackinnj]

why don't we just call you "tri-nitro"

[jwb]

This guy has also made noise measurements for various diodes, zeners, LEDs, etc. Although his purpose is to get a large noise source, the information is still useful.

http://www.ciphersbyritter.com/RADEL...S/NOISMEA1.HTM

[dalton95070]

Long ago (around 1981) someone that I respect in audio pointed out that a much-lower-noise reference could be made by attaching ordinary NPN transistors together base-emitter-base-emitter. With the emitter side pointed to the higher voltage, and a suitable resistor in series to produce about one milliamp of current, the equivalent of a 15V zener is formed. It is indeed far less noisy than any zener (especially low frequency noise, which is very hard to filter out), and I have used it as a reference in power supplies ever since.

I read about in the Analog Devices App Note #AN-211
"The Alexander Current-Feedback Power Amplifier"

Look at the end of page #5 where he talks about NPN Bipolar transistors used as Zener diodes. The transistors appear as Q14-Q17 on the schematic on page #6.

http://www.analog.com/static/importe...17334AN211.pdf

[bcarso]

This is a great thread and I'm grateful to find it. One thing I didn't see (may have missed) is that the temperature coefficient of at least the standard red LED (GaAsP earliest technology parts) is a fair match to the silicon transistor base-emitter forward voltage tempco, but with the net voltage difference of order 900mV. Thus one can make temp-stable current sources with them (the resistor in the emitter determined by the drop of 900mV for the desired collector current). I think the first place I saw this was in a Precision Monolithics app note (so that goes back). A friend complained about light sensitivity being a reason he didn't do this, but although LEDs are indeed somewhat reciprocal, they are pretty lousy photodiodes and it takes an awful lot of light to be anywhere bothersome compared to milliamperes. And as mentioned you can paint them black or enclose in opaque tubing.

Another remark about "zeners": the observation about the high noise around 5V where the breakdown phenomena are a mix of zener and avalanche happens (undoubtedly due to the same underlying physics) at or near the zero temperature coefficient of voltage operating point. I used to test nominal 4.7V and 5.1V parts to make one-off references with low tempco, adjusting the current for optimal voltage stability, and usually the voltage wound up being around 4.9V. But oh were they noisy.

Brad Wood