Question on input noise current - Page 9

MarcelvdG · 29th September 2003, 11:04 PM

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.

MarcelvdG · 29th September 2003, 11:08 PM

I meant: skip the current and number of devices limit...

john curl · 30th September 2003, 02:59 AM

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.

Fred Dieckmann · 30th September 2003, 08:33 AM

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

PS

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.

peranders · 30th September 2003, 08:47 AM

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.

Fred Dieckmann · 30th September 2003, 09:36 AM

"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.

peranders · 30th September 2003, 09:49 AM

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.

Christer · 30th September 2003, 11:03 AM

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.

30th September 2003, 08:47 AM	#85
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	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. __________________ /Per-Anders (my first name) or P-A as my friends call me

30th September 2003, 09:36 AM	#86
Fred Dieckmann diyAudio Retiree Join Date: Oct 2002 Location: Spain or the pueblo of Los Angeles	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.

30th September 2003, 09:49 AM	#87
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	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. __________________ /Per-Anders (my first name) or P-A as my friends call me

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29th September 2003, 11:04 PM	#81
MarcelvdG diyAudio Member Join Date: Mar 2003 Location: Haarlem, the Netherlands	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.

29th September 2003, 11:08 PM	#82
MarcelvdG diyAudio Member Join Date: Mar 2003 Location: Haarlem, the Netherlands	I meant: skip the current and number of devices limit...

30th September 2003, 02:59 AM	#83
john curl diyAudio Member Join Date: Jul 2003 Location: berkeley ca	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.

30th September 2003, 11:03 AM	#88
Christer diyAudio Member Join Date: Sep 2002 Location: Sweden	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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