Measuring low noise - Page 2 - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 5th September 2008, 11:20 PM   #11
fotios is offline fotios  Greece
diyAudio Member
 
fotios's Avatar
 
Join Date: Feb 2007
Location: Δραμα - North Greece
From the photo of S. Palin as your avatar, can suppose that you are with J. McCain and Republican?

Regs
Fotios
__________________
Best Regards FOTIS ANAGNOSTOU
  Reply With Quote
Old 5th September 2008, 11:38 PM   #12
diyAudio Member
 
unclejed613's Avatar
 
Join Date: Dec 2006
i have seen a technique where a bipolar input op amp was preceded by a JFET diff amp (using hand selected {for low noise as well as Idss matching} JFETs).
__________________
Vintage Audio and Pro-Audio repair ampz(removethis)@sohonet.net
spammer trap: spammers must die
  Reply With Quote
Old 5th September 2008, 11:42 PM   #13
diyAudio Member
 
Juergen Knoop's Avatar
 
Join Date: Jan 2007
Blog Entries: 3
John what are you up to?
Wouldn't be feedback impedance in a high gain noninverting amplifier quite low?
regards
  Reply With Quote
Old 6th September 2008, 12:18 AM   #14
diyAudio Member
 
scott wurcer's Avatar
 
Join Date: Jan 2004
Location: cambridge ma
Quote:
Originally posted by john curl
What about resistor values, Scott?
I'm into capacitors these days. The circuit I posted was transformer feedback a concept pioneered by Ulrich Rhode, you might know him as 1/2 of a pretty famous instrument house. For lowest noise you can't use resistors in the feedback at all.
__________________
"The question of who is right and who is wrong has seemed to me always too small to be worth a moment's thought, while the question of what is right and what is wrong has seemed all-important."
  Reply With Quote
Old 6th September 2008, 01:41 AM   #15
KSTR is offline KSTR  Germany
diyAudio Member
 
KSTR's Avatar
 
Join Date: Jul 2007
Location: Central Berlin, Germany
Quote:
Originally posted by scott wurcer
The circuit I posted was transformer feedback a concept pioneered by Ulrich Rhode, you might know him as 1/2 of a pretty famous instrument house.
The circuit shown reminds me a lot to the ones found in European Patent # EP0157187 by Studer (of Switzerland) from 1985, although the goals seem to be a little different, aiming to lowest distortion and best performance from smaller xformers running in current mode (known as "zero field technology"). But good noise performance was also a design goal, obviously.

- Klaus
  Reply With Quote
Old 6th September 2008, 05:52 AM   #16
diyAudio Member
 
john curl's Avatar
 
Join Date: Jul 2003
Location: berkeley ca
The problem here is a matter of communication.
IF a serious audio amateur wants to make a gain step-up device that is low noise and practical, we can't play with exotic transformers, and even cap dividers are a bit exotic.
Any resistance that is in SERIES with the input, will add its noise.
Now, let us say that Scott's AD797 has an equivalent noise of 60 ohms. Then another 60 ohms in series with the input will add 3dB to the noise. 100 ohms will do even worse, which is the case in one of the examples.
Another factor is that a non-inverting input can be just as quiet as an inverting input, AND you can have high input impedance as well. This is more practical.
  Reply With Quote
Old 6th September 2008, 02:26 PM   #17
diyAudio Member
 
scott wurcer's Avatar
 
Join Date: Jan 2004
Location: cambridge ma
For maximum versatility you could use an instrumentation amplifier. There are many already available or you could build this little circuit. The FET's need to be matched or you need a big AC coupling cap in series with the gain resistor which goes between the sources of the input FET's.
Attached Images
File Type: jpg nicelittlepreamp.jpg (78.8 KB, 849 views)
__________________
"The question of who is right and who is wrong has seemed to me always too small to be worth a moment's thought, while the question of what is right and what is wrong has seemed all-important."
  Reply With Quote
Old 6th September 2008, 02:53 PM   #18
diyAudio Member
 
jackinnj's Avatar
 
Join Date: Apr 2002
Location: Llanddewi Brefi, NJ
In case you were wondering:

* BF862 SPICE MODEL MARCH 2007 NXP SEMICONDUCTORS
* ENVELOPE SOT23
* JBF862: 1, Drain, 2,Gate, 3,Source

Ld 1 4 L= 1.1nH
Ls 3 6 L= 1.25nH
Lg 2 5 L= 0.78nH
Rg 5 7 R= 0.535 Ohm
Cds 1 3 C= 0.0001pF
Cgs 2 3 C= 1.05pF
Cgd 1 2 C= 0.201pF
Co 4 6 C= 0.35092pF

JBF862 model parameters:

.model JBF862 NJF(Beta=47.800E-3 Betatce=-.5 Rd=.8 Rs=7.5000 Lambda=37.300E-3 Vto=-.57093
+ Vtotc=-2.0000E-3 Is=424.60E-12 Isr=2.995p N=1 Nr=2 Xti=3 Alpha=-1.0000E-3
+ Vk=59.97 Cgd=7.4002E-12 M=.6015 Pb=.5 Fc=.5 Cgs=8.2890E-12 Kf=87.5E-18
+ Af=1)
  Reply With Quote
Old 6th September 2008, 04:52 PM   #19
matejS is offline matejS  Slovenia
diyAudio Member
 
Join Date: May 2007
Hi, all!

Thanks for the input, I really appreciate it.

Simple and very effective way for me (as you have suggested) is to go with the low-noise op-amp (AD797 or LT1028), non-inverting, keeping Rs low (< 60ohm), +/-9V battery.

This requirement of low input impedance annoys me a little bit (right now I cannot tell what I will measure in the future; I might be over-concerned). Should a discrete JFET (nx 2SK170) input stage + op-amp solve this problem or not?
Want to know out of curiosity - always want to learn. My thinking: FET inputs helps (if low noise due to 1/f), but how much?

I would like to avoid instrumental amplifiers since usually I will do single ended measurements (=> always connecting one input to a GND).

ps I would like to have good 1/f behavior; 0.1Hz to 10Hz Vpp < 0.1uF (clock supplies require good 1/f PSUs; 50nV as I've learned).

But again, everything more than AD797/LT1028 adds complexity - is it worth for my purpose?
I want to keep this a one day build project (also useful for many other hobbyists).
(Although, I've already spent more than couple of days learning lots of 1/f stuff )

Thanks again,
Matej
  Reply With Quote
Old 6th September 2008, 05:01 PM   #20
diyAudio Member
 
Juergen Knoop's Avatar
 
Join Date: Jan 2007
Blog Entries: 3
input impedance could be almost as high as you like, from noise point of view. John was referring to a series resistor, as one would add for current limiting or as RF-stopper/low-pass.
regards
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Low noise transistor and noise figure Real_Macgyver Solid State 28 27th July 2012 10:42 AM
A novice scope user - measuring noise? phrarod Tubes / Valves 15 15th July 2009 04:14 AM
Multisim: Measuring Noise underwurlde Power Supplies 0 29th May 2008 10:32 PM
measuring the noise of a power supply pjpoes Power Supplies 6 31st March 2008 07:19 PM
WTB:PCB for low noise measuring amp Ryssen Swap Meet 4 15th August 2007 04:24 PM


New To Site? Need Help?

All times are GMT. The time now is 08:36 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2