Re: Vbe multiplier?
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 said:
jackinnj said:
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 )
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.
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.
How does noise from batterys vary depending on their level of charge?
http://www.tnt-audio.com/clinica/regulators_noise4_e.html
/
NB: i have nothing to do with TNT-audio - my username has relation to explosives rather ;-).
http://www.tnt-audio.com/clinica/regulators_noise4_e.html
/
NB: i have nothing to do with TNT-audio - my username has relation to explosives rather ;-).
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/RADELECT/MEASNOIS/NOISMEA1.HTM
http://www.ciphersbyritter.com/RADELECT/MEASNOIS/NOISMEA1.HTM
LED noise
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/import...tes/58052492001115525484056221917334AN211.pdf
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/import...tes/58052492001115525484056221917334AN211.pdf
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
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
From this, and other discussions on this, my take home message is that it some zeners are quieter than we may have thought and some LEDs can be very quiet BUT it depends what particular make & type zener & LED so without specific manufacturer & model figures our knowledge remains incomplete.
But we seem to have learn't what zeners we have to avoid
But we seem to have learn't what zeners we have to avoid
Bob Moog used to use reverse-biased b-e junctions as zeners a lot in his synthesizers. The example of two such in series is perhaps a bit misleading---as one will suffice, at half the voltage of course.
Walt Jung has done some recent work using LEDs and low rbb' transistors to make shunt regulators with terminal voltages approximating the TL431 with cathode tied to reference, i.e. at minimum voltage. I haven't checked to see if he has published the results on his website yet.
What does surprise me is the amount of discussion in this thread about noise, but hardly a mention of temperature coefficients, low values of which are usually required for anything called a reference. That's why bandgap references are prized, despite as pointed out by one that they require large silicon areas for low noise.
Walt Jung has done some recent work using LEDs and low rbb' transistors to make shunt regulators with terminal voltages approximating the TL431 with cathode tied to reference, i.e. at minimum voltage. I haven't checked to see if he has published the results on his website yet.
What does surprise me is the amount of discussion in this thread about noise, but hardly a mention of temperature coefficients, low values of which are usually required for anything called a reference. That's why bandgap references are prized, despite as pointed out by one that they require large silicon areas for low noise.
< http://www.hoffmann-hochfrequenz.de/downloads/Led_tk.pdf >
but that is not complete and won't stay in this form on my website forever.
Walt's diode is missing and the hlmp6000 that was proposed by you IIRC.
I have bought that stuff already.
also see
< https://www.flickr.com/photos/137684711@N07/24411798996/in/album-72157662535945536/ > and the pictures to the left & right.
disregard noise density @ < 5 Hz or so. There is some misbehaviour of my preamp
thanks to small input capacitor. I skimped on the $400 wet slug tantal, will soon be corrected.
regards, Gerhard
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Also known as the subsurface zener, as found in the LM329.
These are little more than a reversed transistor be junction. Integrated transistors break down below the surface. This avoids contamination, crystal defects and other factors that are the causes of1/f noise
The blue LED tempco data is useful, even if we would not use such parts for these applications.
I like using reverse biased transistor bases as zeners - it takes very little current to get a solid reference, unlike conventional zener diodes of the same voltage range.Voltage is somewhere between 7.5 to 8V. The earliest I saw this was in a magazine back in the 70's, using the B-E junction of a 2N3638.The article touted them as a good, sharp low current reference. 2N3904s work ok for this. I haven't done a comparison between various transistor types in this application. This might prove instructive.
I believe he's trying to describe the circuit used by Mark Alexander in his current-feedback power amplifier - linked from post #127. See Figure 3 (pg 6) of the Application Note, and the discussion in the last third of pg 5.
Dale