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Old 4th September 2003, 03:17 PM   #1
Mambo is offline Mambo  Italy
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Question Use od LED for CCS and cascode stages

From Your knowledge/experience are there any cons in using LEDs for current sources and cascode stages ?

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Old 4th September 2003, 04:39 PM   #2
Zldz is offline Zldz  Italy
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Rod Elliott uses a green led in his famous PROJECT 3A.
http://sound.westhost.com/project3a.htm

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Lorenzo
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Old 4th September 2003, 04:50 PM   #3
Electrons are yellow and more is better!
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Default Re: Use od LED for CCS and cascode stages

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Originally posted by Mambo
From Your knowledge/experience are there any cons in using LEDs for current sources and cascode stages ?
No, not really but it looks cool. The disadvantage is the "unprecision". The voltage is very dependent of brand. It's better to use a normal silicone PN-junction or even better a 431.

But it's not bad when the LED is there and tweaked in, but it hopeless if you want repeatability and precision.
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Old 4th September 2003, 11:32 PM   #4
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Default Re: Re: Use od LED for CCS and cascode stages

Quote:
Originally posted by Mambo
From Your knowledge/experience are there any cons in using LEDs for current sources and cascode stages ?

Thank You

Quote:
Originally posted by peranders

No, not really but it looks cool. The disadvantage is the "unprecision". The voltage is very dependent of brand. It's better to use a normal silicone PN-junction or even better a 431.

But it's not bad when the LED is there and tweaked in, but it hopeless if you want repeatability and precision.

Per is right with regard to variable tolerances from one LED to another.....However one significant advantage is that LED's provide very effective temp. compensation for your current source....compared to ordinary diode-biasing.
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Old 5th September 2003, 06:54 AM   #5
Electrons are yellow and more is better!
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The temperature compensation isn't so important in most cases as long as the current source has sufficient feedback and the environment is "home". IF it's important it's better to use a monolithic pair instead, cheap but hard to solder.

BC847BS at the picture
--- can't reach my homepage right now. But I have a picture at the bottom of the page
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Old 5th September 2003, 08:10 AM   #6
Electrons are yellow and more is better!
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The links weren't right any of them.
Click the image to open in full size.
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Old 5th September 2003, 10:58 AM   #7
bocka is offline bocka  Germany
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Quote:
No, not really but it looks cool. The disadvantage is the "unprecision". The voltage is very dependent of brand. It's better to use a normal silicone PN-junction or even better a 431.

But it's not bad when the LED is there and tweaked in, but it hopeless if you want repeatability and precision.
No, I highly disagree with you. If you're not using bulk LEDs but only from one manufacturer the tolerances are very small, about +/-1% I measured in a lot of 200. The TL431 is not better.

A LED provides very low noise, too. I can't see any reason why using a TL431 instead of a LED. Use a brand like a TLLR3360.
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Old 5th September 2003, 11:04 AM   #8
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I can, the dynamic resistance is under 1 ohms in a 431 and the temperature stability is better and the initial tolerance better, but it isn't the same type of component.
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Old 5th September 2003, 11:24 AM   #9
bocka is offline bocka  Germany
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Without coupling the ref pin to emitter of a (NPN) the temperature stability is worse. Simply couling the TL431 with the ref to a PNP is not possible. Noise is higher. Initial tolerance is not better (refering to the datasheet). 1% of a LED is the same as the resistor used to set the current of the CCS. So I can't really see any reson why using the shunt reg, although it'll make the job.

I'd strongly recommend the LED.
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Old 5th September 2003, 11:28 AM   #10
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I'm talking more from the perspective of series production. You can't really on purpose design in a part which is so special. Parts with no second source is a nightmare.
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