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Old 10th March 2006, 01:33 PM   #621
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Gerhard, my question was more about the 100R shunt resistor, this value is to high if you expect something like 200mA from your current source, because in this case you get 20V across this resistor, and your "unreg" source is 20V... so you need at least 27-28V with a 100R resistor, or lower his value.
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Old 10th March 2006, 01:54 PM   #622
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Quote:
Originally posted by Justcallmedad
Gerhard, my question was more about the 100R shunt resistor, this value is to high if you expect something like 200mA from your current source, because in this case you get 20V across this resistor, and your "unreg" source is 20V... so you need at least 27-28V with a 100R resistor, or lower his value.

Hi, you are right. 100 Ohms was the value where I stopped playing ..
Let's take this one, then the Zener gets enough current, too
Gerhard
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Old 10th March 2006, 05:11 PM   #623
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Quote:
Originally posted by john curl
Richard, what I am most concerned with is the 1K resistor. Looking more carefully at your complete schematic, I see that you have only a 2V differential across the 1K resistor and this implies that all the current from the resistor goes into the transistor to supply the base current. Is the 3V device even turned on? Anyway, why not put the 1K resistor to ground. Wouldn't that work OK?
As I said in my last post, you are absolutely right John, about the 1K resistor (too low to set the zener current), putting it to ground now that you pointed it, is obvious! In this case the value for this resistor must be higher around 2.5K-3K (17V/3K=5.6mA) and for fig1. During all these discussions I wondered more especially about the pro's and con's of the current source transistor "direction" and trying it in an unusual way, so just like you (as my crossover design is fet's based), I forgot Ib!.


Quote:
Originally posted by john curl
...I prefer, myself, to use an active IC regulator in series with a simple high speed fet cap follower...
I am a little confused ??? Didn’t you said that you use shunt regulation after the IC series regulator and before the fet followers?

When I asked for noise values in a previous post, it was about Zener and led devices, have you (or someone here) some noise values on them? (2.2V, 4.7V and 15v zeners and some led's), I would like to find the best ratio between the zener/leds reference in matter of noise vs. Vz-If values and the emitter resistor for the current source compound. According to what you said previously, for an Ic around 200-400mA, I should use a Zener lower than 4V7 (real Zener not avalanche) or 2 leds and an emitter resistor around 10R-20R, right?
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Old 10th March 2006, 05:56 PM   #624
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Voltage references noise : see post # 75 and the whole thread :
Some noise measurements for LEDs and zener diodes
Results are surprising.

~~~~~~ Forr

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Old 10th March 2006, 07:25 PM   #625
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Forr, I looked at #75, but I found it confusing to me. However, I did find it useful.
Justcallmedad, I am following a pretty old data book by Motorola. It was made in the days when the military insisted that they measure these things.

It is called: 'Rectifier and Zener Diodes Data' 1984. It seems that the very first quality zener diodes had a noise curve measured on them. So, if you took a particular zener diode series, you could estimate the noise contribution.
One series is the 1N746-759 and 1N957-1N986.
On page 4-8 they show a 'typical noise density' graph.
Below about 4V, the noise is so low, that it is off the graph. However, the optimum operating current of these devices is about 20ma. 2 ma operation will severely effect the operating impedance, changing it from 10ohms to perhaps over 100 ohms. Not too good. Above about 4.5V the graph turns upward fast! At perhaps 1uV/rt Hz at 4.7V, to 1000uV/rt Hz at 10V! It peaks at about 15V to about 2500uV/rt Hz, and then DROPS to about 50uV/rt Hz at 40V or so, then starts up again and peaks at 1500uv/rt Hz at 90V. What a curve!. Is this typical for today's devices? I wouldn't chance it, and ignore this curve.
In any case, for low noise, stay below 5V or so. I should think that leds would actually be a better choice. This is where #75 could be valuable. Also, any input by Walt Jung would be useful. However, wouldn't noise add as the sqrt of the sums of the squares, rather than linearly? So, 2 identical diodes in series would have 1.4 times the noise.
I only glanced at #75, so this may be the same thing. I'll go back an look.
 
Old 10th March 2006, 07:59 PM   #626
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Thank you forr for the link and John to spend time to seek this information, really thank you . Apparently between the thread on the Zener noises measures and your research, Zener are less noisy when they are fed by a "high" current (about 20mA), but what about fet's current sources at this Id? I suppose they are more noisy too? so an J203 or an SK246 feeding 2 leds would be less noisy?
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Old 10th March 2006, 09:05 PM   #627
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Long gate fets, like the j203 or the k246 are better current sources, because they have LESS transconductance and have generally higher output impedance. However, they are limited in current. I would think that an led or two in series, would be best.
Zeners are very susceptable to operating current, not because of noise, , but because of impedance, which reduces their effectiveness in regulating anything. Noise in zeners is facinating, because it varies so much with operating voltage.
Looking through my book further, I found that the 1N4099 thru 1N4135 series of 1/4W zener diodes gave a slightly different spec.
For example: 4.3V gave 1uV/rt Hz, 4.7V gave 1uV/rt Hz, 5.1V gave 2uV/rt Hz, 6.2V gave 5uV/rt Hz, and 6.8V and above, gave 40uV/rt Hz! This is better than common zeners, but the trend is the same: Use zeners below 5V, or else you must expect noise.
Again, I would stick to leds, but there could be a 3V zener out there that is actually quieter than 2 diodes, but I expect you would have to run it at a higher current than the leds to get the same performance.
 
Old 10th March 2006, 09:06 PM   #628
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Searching at Google "Zener noise" I find this : It could be useful, and some more tests for Christer ;-)

http://www.knoxsemiconductor.com/onlinecatalog.htm#z

Series 1N6082-6091
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Old 10th March 2006, 09:16 PM   #629
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Looks good, justcallmedad. I am surprised that they could go all the way to 10V. I would not try this with just any zener, however! Somebody at Knox or elsewhere did their homework, and note that they test noise at 250uA. Impedance is probably pretty lousy, however.
 
Old 10th March 2006, 10:17 PM   #630
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In fact I need 150 mA for the audio circuit and I think that a current source about 200 -250mA would be appropriate, and would allow a suitable Rds on the shunt mosfet, the problem with this circuit is to get a high current (with low noise) on the pass device (current source), with the requirement of a low noise reference (zener or led) so 3 or 4V and a “high” emitter resistor (but not to high) in my case something between 10-20R. In short I need the highest V reference together with the lowest noise to don't amplify it by a low emitter resistor value.
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