Current feedback - Voltage feedback, how do I see the difference? - Page 52 - 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 24th March 2013, 06:12 PM   #511
Waly is offline Waly  United Kingdom
diyAudio Member
 
Waly's Avatar
 
Join Date: Jan 2011
Location: London
Quote:
Originally Posted by jan.didden View Post
Did you actually look into that instead of just following your unreliable gut? Do you realise that the current into the low impedance node is NOT determined by its low impedance?
I did.

Focus Mr. Didden, I would appreciate if you refrain in putting words in my mouth. Did I mention anywhere that "the current in the low impedance node is determined by it's low impedance"? Or, as a matter of fact, did I mention any "low impedance" at all?

All I said is that a CFA is an amplifier with VFB, where the FB network load at the input node cannot be ignored. As Michael says, this affects the open loop gain, which in turn affects the open loop crossover frequency. Lowering the crossover frequency, combined with higher reverse feedback transfer leads to bandwidth independent closed loop gain.

If Michael doesn't like CFA's for audio (for still unknown reasons), this is his own problem. If Mr. Didden really wants (for still unknown reasons) to consider CFA a marketing created brand, that's also his own problem. I'm trying to show that this entire debate is about semantics only.
  Reply With Quote
Old 24th March 2013, 06:53 PM   #512
RNMarsh is offline RNMarsh  United States
diyAudio Member
 
RNMarsh's Avatar
 
Join Date: May 2012
Location: 2457 Cascade Trail; Cool, CA. 95614
Quote:
Originally Posted by michaelkiwanuka View Post
For a first order analysis, the values of the feedback components appear in the equation for the forward path gain in so-called "CFA", whereas this is not the case in a VFA.

However, this doesn't change the fact that so-called "CFAs" are really compromised VFAs.

Read the section entitled "over compensation" in the figure you posted.
I have the whole book... do you?

I developed a Current Feedback topology before it was an IC and published it. I got under .001% with it at all audio freqs and it had a bandwidth of >20Mhz at 22v p-p -- that was several decades ago. In fact, the designers at Elantec called me as they were moving in a similar direction and had filed a patent and were pretty upset when i published a similar design/topology.

Since then the current-mode feedback as it has become named has been refined. They are super good, now. But they leave the vfb behind as you move higher and higher up in frequency. They are now a staple in all analog IC mfr'ers portfolio as their strengths overlap with vfb designs. Its above or below the over-lapping areas (grey) where one shines brighter than another.

Its important to know when to use one or the other and why.

Thx-RNMarsh
  Reply With Quote
Old 24th March 2013, 07:13 PM   #513
Banned
 
Join Date: Oct 2010
Location: Chigwell, Essex
Quote:
Originally Posted by Waly View Post
Lowering the crossover frequency, combined with higher reverse feedback transfer leads to bandwidth independent closed loop gain.
There is no such thing as " bandwidth independent closed loop gain" with so-called "CFAs".
  Reply With Quote
Old 24th March 2013, 07:16 PM   #514
Banned
 
Join Date: Oct 2010
Location: Chigwell, Essex
Quote:
Originally Posted by RNMarsh View Post
Since then the current-mode feedback as it has become named has been refined.
What a profoundly unfortunate and inaccurate name indeed!

Do you think so called "current mode" amplifiers have any valid applications at audio frequencies?

Last edited by michaelkiwanuka; 24th March 2013 at 07:19 PM.
  Reply With Quote
Old 24th March 2013, 07:23 PM   #515
Banned
 
Join Date: Oct 2010
Location: Chigwell, Essex
Quote:
Originally Posted by RNMarsh View Post
I developed a Current Feedback topology before it was an IC and published it.
Can you post the schematic?
  Reply With Quote
Old 24th March 2013, 07:25 PM   #516
Banned
 
Join Date: Oct 2010
Location: Chigwell, Essex
Quote:
Originally Posted by Waly View Post
If Michael doesn't like CFA's for audio (for still unknown reasons), this is his own problem.

What reasons would you advance for using so-called "CFAs" at audio frequencies?
  Reply With Quote
Old 24th March 2013, 07:36 PM   #517
Banned
 
Join Date: Oct 2010
Location: Chigwell, Essex
Quote:
Originally Posted by Waly View Post
If Michael doesn't like CFA's for audio (for still unknown reasons), this is his own problem.
Waly, you obviously haven't read my post here:


Current feedback - Voltage feedback, how do I see the difference?
  Reply With Quote
Old 24th March 2013, 08:39 PM   #518
godfrey is online now godfrey  South Africa
diyAudio Member
 
Join Date: Nov 2009
Location: Cape Town
Quote:
Originally Posted by michaelkiwanuka View Post
So, attempting to increase negative feedback (loop gain) by reducing the amplifier's closed loop gain accomplishes virtually nothing in terms of overall linearity because the amplifier's first stage gain is simultaneously reduced.
Not necessarily.

In the text Richard uploaded, the discussion of equation 4 is perhaps a bit misleading. While the second factor is independent of gain, it is dependent on RF, the feedback resistor. The statement "This shows that the bandwidth becomes independent of the set gain G" assumes that G is set by changing RG.

If instead, we choose to alter G by changing RF, then this no longer holds true, and the graphs of open loop and closed loop gain vs frequency become the same as for a VFB amp.

In a nutshell, when G>>1 then, to a first approximation:
  • Open loop gain is roughly proportional to 1/RG
  • Closed loop gain is roughly proportional to RF/RG
Thus they can clearly be set independently of each other.
Attached Images
File Type: gif cfb-txt.GIF (25.2 KB, 79 views)
  Reply With Quote
Old 24th March 2013, 08:45 PM   #519
RNMarsh is offline RNMarsh  United States
diyAudio Member
 
RNMarsh's Avatar
 
Join Date: May 2012
Location: 2457 Cascade Trail; Cool, CA. 95614
perhaps you havent been using the best model for current-mode feedback?

Need better models.jpg

1. lower noise potential because the devices are in parallel with each other (push-pull);
2. More consistent sound quality character because (?) the distortion's harmonic structure is the same at all frequencies -> at low-mid-high.
3. Very low distortion with only moderate levels of gnfb.
4. Very fast-high speed and wide bandwidth.

THx-RNMarsh

Last edited by RNMarsh; 24th March 2013 at 09:04 PM. Reason: Pro features -
  Reply With Quote
Old 24th March 2013, 08:54 PM   #520
Waly is offline Waly  United Kingdom
diyAudio Member
 
Waly's Avatar
 
Join Date: Jan 2011
Location: London
Quote:
Originally Posted by michaelkiwanuka View Post
What reasons would you advance for using so-called "CFAs" at audio frequencies?
Michael, you obviously haven't read my post here: Pros and Cons of current feedback amplifier.

I'll save you a click:

Quote:
Originally Posted by Waly View Post
I would concede there's no good reason to claim that CFAs are anywhere superior to VFAs for audio, but to state that CFAs are inferior that's, to me, a biased stretch.
Are you sure you are not trolling? 'Cause I've had enough of this.
  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



New To Site? Need Help?

All times are GMT. The time now is 06:55 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