Voltage coefficient in feed back resistor - 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 17th March 2010, 02:47 PM   #1
diyAudio Member
 
Join Date: Mar 2010
Location: Paris
Default Voltage coefficient in feed back resistor

I folks,
It's my first post on this forum I'm very interested in amp design and I would like to submit this subject (kind of mystery) to you.
I recently discovered the concept of voltage coefficient in resistors: The resistor value decreases with voltage applied.
The change in resistor value is small: metal film according to datasheets can have values as small as 1ppm/V in 0.1% range but most metal film have voltage coef much above 5ppm/V.

Now imagine you have designed the perfect 100W/8Ohm audio amp with 0% THD in open loop (!!!).
You introduce your FB loop using your tremendous (for example RC55Y) part that has only 1ppm/V of VCR and then ...
Considering you get approx 40V peak voltage on the FB resistor : your FB resistor is affected of a 40.10-6 fluctuation factor. Which will give a closed loop DHT of around 0.004%.
OK It is small but it is much more than some figures I've often red.

Can some one help understand this strange fact ?

Regards
Stéphane
  Reply With Quote
Old 17th March 2010, 04:35 PM   #2
h_a is offline h_a  Europe
diyAudio Member
 
h_a's Avatar
 
Join Date: Sep 2006
Location: Graz, Austria
The NFB resistor fluctuates (max) by 40e-6 ppm, which you forgot to divide by 40 to get the ratio, so it's (40e-6/40)*100 in %=0.0001%

Next, these 40e-6 ppm are the peak value when the sine reaches Vpeak; 99% of the time this value will be smaller or even zero. THD measurement averages these values and will give a further reduced reading.

Last, this number will be reduced by negative feedback just like any other error within the amp as well.

Your example sounds like a low NFB design (or big power amp), but these have still 20-30dB NFB which reduces this number by at least another factor 10.

Have fun, Hannes
  Reply With Quote
Old 17th March 2010, 05:09 PM   #3
diyAudio Member
 
Join Date: Mar 2010
Location: Paris
Hi Hannes,
I not exactly agree with your analysis, I did not forgot to divide by 40 ...
40ppm = 0.004% is the actual fluctuation of NFB resistor (if we consider the voltage on negative input as small against output voltage).

It is not a voltage error against output voltage.
It is the modifcation of NFB resistor during a signal period.

If closed loop gain of the amplifier is G in a non inverting configuration. the simplified NFB loop is a voltage divider made of said NFB resistor plus an other R Resistor. The gain of the amplifier when open loop gain is big against G is:
G = 1 + Rnfb/R
So we clearly see that a 0.004 % modification in Rnfb is almost a 0.004% change in G which means a 0.004% DHT.

This cannot be cancelled by feed back while feed back is the core of the issue.

Regards,
Stéphane
  Reply With Quote
Old 17th March 2010, 05:58 PM   #4
diyAudio Member
 
Join Date: Jun 2005
Location: Zürich
Quote:
You forgot to divide by 40 to get the ratio.
I don't understand why you want a factor of 40. I agree that if the amp has 6 dB noise gain (i.e. equal shunt and feedback resistor values) the voltage coefficient will cancel (given resistor value and voltage coefficient match). However for the usual noise gains > 20 dB there is almost no cancellation and voltage coefficient will proportionally affect linearity.

Quote:
This number will be reduced by negative feedback just like any other error within the amp as well.
By definition imperfections of the feedback network will not be reduced by loop gain. If you put a diode in your feedback network the output will be distorted. Otherwise we couldn't make precision rectifiers and clamps using opamps...

Quote:
Can some one help understand this strange fact?
Voltage coefficient is extremely difficult to measure down to 1 ppm/V. 1 ppm/V is likely to represent the measurement residual rather than resistor imperfection of decent metal film parts. I've investigated this myself using special distortion measurement techniques and I've not been able to find distortion above -150 dB at +20 dBu. Surprisingly the $20 Vishay bulk metal parts are actually worse with this respect than most standard metal film parts I've measured...

Of course voltage coefficient *is* a serious problem for carbon and thin/thick film resistors. Another issue (for any resistor except those with near-zero tempco) is distortion from thermal self-modulation at low frequencies. I'm doing some research here at the moment but no usuable results yet.

Samuel
  Reply With Quote
Old 17th March 2010, 06:57 PM   #5
diyAudio Member
 
Join Date: Mar 2010
Location: Paris
Samuel,
thank you for your analysis,
Stéphane
  Reply With Quote
Old 17th March 2010, 07:15 PM   #6
diyAudio Member
 
Join Date: Jun 2005
Location: Zürich
Typo: I should have written -140 dB instead of -150 dB.

Samuel
  Reply With Quote
Old 17th March 2010, 07:28 PM   #7
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Default thermal and voltage ratings

is this why some suggest that peak voltage use <10% of the resistor's power rating for the NFB loop?
__________________
regards Andrew T.
  Reply With Quote
Old 17th March 2010, 08:21 PM   #8
godfrey is offline godfrey  South Africa
diyAudio Member
 
Join Date: Nov 2009
Location: Cape Town
Quote:
Originally Posted by Samuel Groner View Post
Surprisingly the $20 Vishay bulk metal parts are actually worse with this respect than most standard metal film parts I've measured...
There's another discussion about distortion and excess noise in resistors here: http://www.diyaudio.com/forums/analo...rt-ii-393.html

There's some surprising results with distortion measurements there too.
Quote:
Originally Posted by AndrewT View Post
is this why some suggest that peak voltage use <10% of the resistor's power rating for the NFB loop?
I expect that's more to do with temperature coefficient - the increase in resistance with self-heating.

To reduce distortion, perhaps one could connect several resistors in series, so that each one has a smaller voltage across it?

(Not that I expect to design a circuit good enough to tell the difference anytime soon)

Last edited by godfrey; 17th March 2010 at 08:27 PM.
  Reply With Quote
Old 17th March 2010, 08:35 PM   #9
diyAudio Member
 
Join Date: Jun 2005
Location: Zürich
Quote:
There's another discussion about distortion and excess noise in resistors here:
Can't finde the details how these measurements are done; anyone knows the page number..?

Quote:
Is this why some suggest that peak voltage use <10% of the resistor's power rating for the NFB loop?
Well, that won't address any voltage coefficient issues (rather the opposite: most high-power parts actually have higher voltage coefficient) but at least the thermal self-modulation I mentioned because higher power parts typically have slower thermal time constants which moves the onset of this distortion mechanisms towards lower frequencies.

Quote:
To reduce distortion, perhaps one could connect several resistors in series, so that each one has a smaller voltage across it?
Sure. But as I said: It ain't a serious issue anyway.

Samuel
  Reply With Quote
Old 17th March 2010, 08:44 PM   #10
diyAudio Member
 
Join Date: Jun 2005
Location: Zürich
Some literature: http://www.barthelectronics.com/pdf_...lse%20Page.pdf

Samuel
  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
i need some openions....please give me feed back...speaker design... slicemaster101 Multi-Way 219 23rd August 2008 02:34 PM
Feed-back class D mgm2000ro Class D 5 5th July 2008 07:29 PM
10W JFET/MOSFET SE Class A, no Feed Back widowmaker Solid State 9 26th November 2006 01:00 PM
Length of feed back resisitor in NIGC 100db Chip Amps 2 24th February 2006 02:03 AM
Current feed back again.............. ashok Solid State 0 15th November 2004 06:54 AM


New To Site? Need Help?

All times are GMT. The time now is 10:14 PM.


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