New kind of feedback or just re-inventing the wheel? - 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 11th July 2002, 02:38 PM   #1
diyAudio Member
 
Circlotron's Avatar
 
Join Date: Jun 2002
Location: Melbourne, Australia
Lightbulb New kind of feedback or just re-inventing the wheel?

With normal negative feedback you compare the output with the input and apply the difference as a correction signal. Of course, as you reduce the distortion and nonlinearities you also reduce the ability to reduce them(!) because you haven't got as much error as you had before (good) so in proportion you lose the ability to reduce what error is left. Sort of a 1/x function.

What about this idea I thought of today? Say you have a *unity* gain buffer or maybe an emitter follower half of an output stage. Compare the input and output signals and subtract i/p from o/p to get an error signal. No big deal so far. Put a summing cct upstream of your buffer amp. Feed the error signal to this summer so that what comes out of it and feeds the amp is signal + error. (Error may be pos or neg so may boost or buck signal when added to it).

Here is the big deal part - the feedback does not try to correct the amp per se, the summer merely feeds the amp an "antidistorted" signal that the amp then proceeds to distort into a good signal!! What's more, because the distorting efect of the amp is still present, you are still dealing with error signals of the original size, not vanishingly small ones that have been made so by nfb. Also, nfb can only make distortion smaller, even a squillion times smaller, but never eliminate it. But what I propose might be able to eliminate it completely in theory at least.

Is this something new or just old stuff I've never come across?

GP.
  Reply With Quote
Old 11th July 2002, 02:55 PM   #2
Account Disabled
 
Join Date: Jul 2002
Location: North
Send a message via ICQ to gromanswe Send a message via AIM to gromanswe Send a message via Yahoo to gromanswe
Default The timing is the issue.

When correcting a signal
you take a sample of the input
and compare it to the output.
But the correctionprocess in it self takes time.
In meanwhile there is a new input and a new output.

To be able to correct signal completely
there has to be some sort of delay.

This delay is found in Portable CDplayers.
They have memory buffer for the input signal
and then can correct the signal.

That is why you cannot have ultimate bandwidth
in a coventional NFB device.
The currents are somewhat dealyed through
the transistors.
So if you delay the amplifier a bit
correction has time to take place.

Groman
to much time at keyboard
  Reply With Quote
Old 11th July 2002, 03:00 PM   #3
diyAudio Member
 
Join Date: May 2002
Location: Switzerland
Hi Circlotron

What you describe is basically the idea of error-feedforward, applied to just a part of an amplifier.
The original error feedforward approach was to compare the output signal of a whole amplifier (which might by itself use error feedback) with the input signal and add the inverted error signal at the output.
But what sounds easy in theory can be tough to implement in practice, since the device that sums the signals has to be perfect (as well as the error-amplifier).

Regards

Charles
  Reply With Quote
Old 11th July 2002, 03:20 PM   #4
diyAudio Member
 
MRehorst's Avatar
 
Join Date: May 2002
Feed-forward is commonly used in RF power amplifiers where controlling parasitics in a feedback network becomes extremely difficult.

Nope, not new, but keep trying!

MR
  Reply With Quote
Old 11th July 2002, 07:23 PM   #5
The one and only
 
Nelson Pass's Avatar
 
Join Date: Mar 2001
What you describe with a unity gain follower ends
up being negative feedback in the classic sense.

This scheme can be seen to be operating in my
dynamic bias patent #3,995,5228 and later work
by Hawksford and (separately) Cordell.

There are plenty of other possible variations.
  Reply With Quote
Old 20th July 2002, 01:17 PM   #6
diyAudio Member
 
Circlotron's Avatar
 
Join Date: Jun 2002
Location: Melbourne, Australia
Quote:
Originally posted by phase_accurate
The original error feedforward approach was to compare the output signal of a whole amplifier (which might by itself use error feedback) with the input signal and add the inverted error signal at the output.
Addding the error signal to the output becomes difficult when a power output stage is the one you are trying to linearise! The method I am thinking of is to apply the error signal to the incoming signal before it has reached anything else.

The opamp cct I have attached is deceptively simple. Beware! The opamp stage has a x2 gain. The incoming signal is halved in amplitude by the top left 100k and the one to it's right. The x2 gain brings it back to normal size again at the opamp output. Also anything fed back to the opamp + input is also halved by the top two 100k's. This way the opamp functions as a non-inverting summing cct.

Notice that both the - input of the opamp and it's negative supply rail is referenced to the fet source, so any opamp output at all is viewed as an error signal. i.e. if the opamp swings above the source voltage this means the source voltage didn't follow it perfectly and so there is an error (distortion).

Now the good bit. This error voltage - the ugly difference between the gate drive and the source output voltage - is fed back to the opamp input and *added* to the input signal so it now can make the output go where it should, and in theory at least *completely* eliminating any distortion.

In practice? At first it oscillated badly until I put the 47pf in place. Then I capacitor coupled a small 8 ohm speaker and was able to get a 5v p/p clean sinewave out of the thing. On music it sounded nice and clean and clear, if not terribly loud. I think it would make a great headphone amplifier.

Yes, there is feedback to the + input of the opamp, and the schematic is correct. It wasn't simulated, it was built. (I can't yet drive the blasted software to simulate it - PSpice Student.

My next trick will be to see about incorporating it into the CDA.

GP.
  Reply With Quote
Old 20th July 2002, 01:23 PM   #7
diyAudio Member
 
Circlotron's Avatar
 
Join Date: Jun 2002
Location: Melbourne, Australia
Talking Whoops! forgot the schematic

`
Attached Images
File Type: gif error feedforward opamp and source follower.gif (6.5 KB, 1180 views)
  Reply With Quote
Old 20th July 2002, 01:57 PM   #8
Account Disabled
 
Join Date: Jul 2002
Location: North
Send a message via ICQ to gromanswe Send a message via AIM to gromanswe Send a message via Yahoo to gromanswe
Default On FEEDBACK use/non-use

My views are posted elsewhere.
But on the whole,
it depends.
An Adaptor might need feedback.
Amplifier is a bad terminology to use
I think.
Or not.
It is for you to determine
what is best for any given adoptation
using any given materials/components.

Not to adjust to environment/situation
will not give best performance.

So in signal adaptors
So in Nature

gro

or else Nature WILL Revenge
sometimes I have to walk the other way, without feedback
  Reply With Quote
Old 22nd July 2002, 08:55 AM   #9
diyAudio Member
 
Circlotron's Avatar
 
Join Date: Jun 2002
Location: Melbourne, Australia
Default Here's another setup

Here it is as applied to a complementary symmetry setup. Note the zero volt point of the opamp supply rails rides up and down on the loudspeaker output as per the other cct.. This will definitely have to be a new project for me. The only thing about it, is that it looks so conventional it belies just exactly what goes on in the cct. The output at Q11 & Q14 emitters because of normal nonlinearity, does not quite follow Q12 & Q13 bases. The amount the driver bases differ from the output emitters, this same amount is also fed back by R9 and added to the input signal so that *exactly the right amount* is fed to the driver bases to make the output emitters do the right thing.

With all due modesty, I think this is the best thing since sliced bread!

GP.
Attached Images
File Type: gif eff to comp symm.gif (12.9 KB, 1133 views)
  Reply With Quote
Old 22nd July 2002, 11:21 AM   #10
mirlo is offline mirlo  United States
diyAudio Member
 
Join Date: Jul 2002
Location: San Diego
Hmmm.

Two things:

1) You have to make damn sure the negative feedback is greater than the positive feedback, or it will latch one way or the other. Since the resistive dividers are nominally equal and the emitter followers have a gain ever so slightly less than one, this could be a problem.

2) It can reduce the errors but not eliminate them. To see this imagine that the amp you are trying to correct has a bad second order nonlinearity such that if the input is X the output is X^2.

Then to get the output to be X, ie what you want, the input has to be the square root of X. But if your circuit is working to eliminate the errors, the input to the difference amp is some linear combination of the input X and the perfect output X. Can't happen.

On the other hand, playing with how NFB works can have advantages. You might find that you can make an amp faster and remain stable. by some such machination.

On a similar vein, I think there is an interesting app note on the Analog Devices website about improving opamp circuit phase response by putting a replica opamp in the NFB loop.

I am probably rambling incoherently now -- time for me to go pretend to sleep so i can get up and pretend to work tomorrow.

Zzzzzzzzzzzzzz ...........
  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
Theta Transport Drive Wheel g'wyn Digital Source 18 14th October 2012 06:58 PM
Reinventing the N-channel wheel? Eva Solid State 72 7th April 2010 07:05 AM
Ferris Wheel Project mattthegamer463 Everything Else 14 31st May 2009 08:37 PM
H. S. Black: inventing the negative feedback amplifier (1977) Bricolo Parts 6 13th June 2006 02:41 PM


New To Site? Need Help?

All times are GMT. The time now is 06:41 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