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Old 29th September 2003, 11:04 PM   #81
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This is not necessarily true for extremely low source impedances. Some bipolar transistors have base spreading resistance as low as 1ohm...2ohms (BFW16A and family), and in any case, you can make the total base spreading resistance as low as you like by putting devices in parallel (for AC). For extremely low source impedances and 10mA of total current, one BFW16A still outperforms your ten high-transconductance JFETs (1.3ohm+2ohm<5ohm). Assume a non-negligible source impedance and/or skip the current limit and the JFETs win again.
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Old 29th September 2003, 11:08 PM   #82
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I meant: skip the current and number of devices limit...
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Old 30th September 2003, 02:59 AM   #83
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I agree, BUT most circuits have more than a 2 ohm source. That is the problem. I actually patented the quietest low inherent distortion bipolar input stage for a given current, about 30 years ago. I don't use that circuit anymore, because the FET's do better, overall.
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Old 30th September 2003, 08:33 AM   #84
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Wink actually patented the quietest bipolar input about 30 years ago

Well that's fine ..... but what have you done for us lately?

Sorry John. I couldn't resist, knowing you must have a highly developed sense of humor to keep at this subject this long without a well deserved rant at everyone. I believe Bob Pease wonderfully described it as "being nibbled to death by ducks."

Fred

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We do enjoy a "don't hold back, tell us how you really feel about it" rant once in awhile and I try to have at least one a week.
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Old 30th September 2003, 08:47 AM   #85
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Phred, we are being nice today.

I notice that this thread is about a measurement problem but you talk about noise in general, typical thread jacking.
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Old 30th September 2003, 09:36 AM   #86
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Unhappy Nibbled by Swedish ducks...........

"I notice that this thread is about a measurement problem but you talk about noise in general, typical thread jacking."

I guess I don't quite understand what you are talking about........

The name of the thread is "Question on input noise"

The first post by Crister http://www.diyaudio.com/forums/showt...325#post239325
talks specifically about input noise sources and refers to documents specifically about input noise sources for amplifiers and op amps.

Would you like to start a a thread on measuring noise and the problems involved in making low noise measurements? I am sure that Mr. Curl would have some insight on that and AWL has made many low level measurements on power supplies and should have some advice. It might be a good place to mention the low noise transistors, op amps, and PCBs for low noise phono preamps on your website.

http://home.swipnet.se/~w-50719/forsale/index.html


Fred

PS for a real example of real thread jacking see my previous one in this thread.
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Old 30th September 2003, 09:49 AM   #87
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Ok, but Christers real question was about how the amp would be designed (with low noise) in order to do his job. We came to the conclusion that his goal was rather high.
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Old 30th September 2003, 11:03 AM   #88
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Eh, well, no. The thread isabout noise, input current noise
or noise in general. I started it since thought the noise discussion
in another thread was getting close to threadjacking. I mentioned
the measurement amp because Marcels comment
on noise for non-matched source impedances seemed relevant
here.

PS since I started the thread I suppose I have a say about
what is threadjacking or not, so I decide it is OK to discuss
measurement amps too.
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