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Old 25th January 2006, 12:53 AM   #101
lineup is offline lineup  Sweden
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Quote:
Originally posted by hugobors
Hello
What's the best way to make a low noise adjustable current source.
In fact I'm looking for a low noise adjustable voltage source to drive a mosfet gate.
I've tried 9V1 zener diode paralleled by a 10K multiturn adjustable, but what a noise.......

Do you have some ideas?
LED, TL431?

Thanks
What MOSFET transistor is it?
Is it in the output stage of a power amplifier?

TL431 might be a good option.
Because you can change voltage of TL431 using a trim potentiometer.
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Old 25th January 2006, 01:02 AM   #102
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Quote:
s it in the output stage of a power amplifier?
No it's the current source of a preamplifier with IRF9610 as current source.

Quote:
TL431 might be a good option.
Because you can change voltage of TL431 using a trim potentiometer.
Isn't the TL431 too noisy for this application?
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Old 26th January 2006, 01:48 AM   #103
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I've tried something that work well but I don't have the tools to measure it. It's a 2SK30 mounted in current source (gate directly connected to source) feeding a trim pot.
Simple, adjustable and seems to be low noise (in my shematic and with a stax for listening test)

Thanks
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Old 26th January 2006, 12:39 PM   #104
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Quote:
Originally posted by jwb
Not sure why you would measure the noise bandwidth over such a wide band. Surely a voltage reference has a bandwidth of interest only at DC. Normally you would put a capacitor across the reference to reduce the bandwidth to some tiny amount as has already been noted in this thread.

Noise = probability that the reference voltage isn't what you expect it to be. For a reference you have to look at it over time to estimate the "confidence" of a measurement.

You can do the math involving the sampling rate and probability distribution function.

Want a really low noise reference ? -- put a low noise integrator after the device, not "just" a capacitor. When you simply put an RC filter on a reference, then connect to an opamp the input bias current causes an offset error.
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Old 11th March 2006, 12:14 AM   #105
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Great work, Christer! How about different color leds from the same family of devices? Did I miss this in reading this thread? You have a great test rig. Just plug in a red, yellow and green one at two ma, and compare. You will have my appreciation, (to save me from doing this).
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Old 11th March 2006, 07:53 AM   #106
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John, you may want to do it anyway. Christer mentioned that he hadn't really determined the relative noise contribution of the current source versus the LED (BTW, major props to Christer for both the measurements and his directness about possible limitations and errors!). Now, his measurements may be correct, but I'd like to see some confirmation with a different test setup- after all, don't you think it's odd (albeit interesting) that there's a minimum to the noise at a current well below where the impedance of the LED is at a minimum?

I'm attaching a Vf versus current graph of a cheap red LED, courtesy of Morgan Jones. I've done some single point measurements which give me very similar results. Naively, you'd think that there would be lower noise at 10mA than 3mA since the dynamic impedance is lower. If indeed the minimum is real and not a measurement artifact, that's a VERY interesting result.

When I get back to the US later this month, if you've got time, are you interested in trying a couple of experiments?
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Old 11th March 2006, 12:59 PM   #107
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I post here again
link to Christer latest version of noise test figures.

It is a valuable and useful paper.
It gives some hints of level of noise in voltage references we commonly use.
Like Zener diodes, 1N4148, TL431, different colour LEDs and small signal transistors.

Very useful!
I see such an experienced man as John Curl can see the value in
this good work done by Christer.

Christer Noise Test Paper 1.4

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Old 11th March 2006, 01:50 PM   #108
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it would be helpful to baseline the measurements -- short the input of the LM1115 and measure for 10s.

one way to determine the noise would be to take a very low level , very pure sine wave signal, amplify it and then use the averaging function to determine how many averaging periods are necessary to return it to a pure sine wave. Bonnie discussed this in EDN from a different angle in the most recent issue.

now if I can get my son who just took stochastic calculus to come up with the math we are all done
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Old 19th April 2006, 06:53 AM   #109
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Well, it seems that I have used the noisiest possible zeners for my Class-D prototype input stage opamps: 6v8 zeners, biased at 5mA (showing 13uV noise)
They are connected to the base of a NPN transistor (PNP for the negative rail), with a series collector resistor to share dissipation, to regulate the supply rails to the required +/-6.3V, and the zeners themselves are bypased by a 100nF ceramic cap.

Should I expect to improve the audio output noise floor noticeable if I use a lower noise zener configuration (such as 5V6, that shows 2.9uV noise)?

Thanks!
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Old 22nd June 2006, 12:03 PM   #110
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Quote:
Originally posted by john curl
Great work, Christer! How about different color leds from the same family of devices? Did I miss this in reading this thread? You have a great test rig. Just plug in a red, yellow and green one at two ma, and compare. You will have my appreciation, (to save me from doing this).

Sorry for not answering earlier John. I have been away from the forum for some time.

I did deliberately try to use the whole spectrum of colours from the same LED family, in this case the Everlight EL204 series. One exception was the 697 nm red, where I used EL202 instead. I don't quite remember why, but probably the 204 version was out of stock when I bought the LEDs. Anyway, this means I tried the EL204 all the way from reddish orange up to blue. I also used the IR204 IR LED, but I am not so sure if that one has much relation to the EL204 series. In addition to these I threw in some other LEDs for comparison. I tried some EL1254, which were a bit noisier than the corresponding EL204 ones. I also tried a red LED of a different brand, Kingbright, which had approximately the same noise figures as thered EL202. Since the EL202/204 and EL1254 have approximately the same max If and it seems that noise is not only correlated to wavelength and max current. This obviously means one cannot just take my figures and hope they are the same for all LEDs. You definitely whoudl make your own measurements with the devices you use. The test rig is very simple but surprisingly good, and I am sure you could easily come up with something better.

I never thought the day would come when you would learn something about electronics from me, especially not empirical knowledge.
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