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Old 31st August 2012, 08:30 PM   #21
Elvee is online now Elvee  Belgium
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Quote:
Originally Posted by rcw666 View Post
Your modeled circuits do not have the pole splitting capacitor, so I am assuming that the slew is limited by the transistor characteristics alone, putting the capacitor in the simulation might be informative.
rcw
No, I am investigating the subject mainly from a discrete implementation perspective, where you are at liberty to tailor the compensations, currents, etc for the best perfomance tradeoff in a given situation.

I understand CFB does have an advantage in the >automatic compensation/one size suits all case<, typically a general purpose integrated video amplifier, but what I am interested in is: above and beyond that, does the CFB offer striking advantages over an equivalent (BOM, cost, or otherwise) VFB.
Also, is it always possible to classify one type of circuit into one or the other category, or is it always possible to emulate either of them, for example by adding parallel and series degeneration resistors to a normally VFB or CFB amplifier.
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Old 31st August 2012, 10:43 PM   #22
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The old rule of thumb is that when you reached a frequency of ft/b, then the input and output time constants of the individual devices start to become significant and feedback over more than one stage starts to become a problem.

I have not simulated the circuit bellow but it works very well and is for a project that needs a buffer line driver in the 10-20MHz. region

The overall gain is enough just using the common base stage with local feedback and bootstrapping, thus avoiding the problems with common emitter amplifiers.

Since the input and output are emitter followers with current source loads you can look at it as having 100% voltage feedback in these two stages, and feedback across the common base stage base resistor.
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Old 2nd September 2012, 09:10 PM   #23
wahab is offline wahab  Algeria
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For audio purposes a so called CFB amp not only yield no advantages
over the usal VFB but will more surely bring a lot of drawbacks compared
to the latter.

Some will argue for higher slew rates without explaining how this will
translate in sound quality since theses kind of amps are not more linear
than the more traditionnal VFBs wich seems to be better in respect of
global perfs.
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Old 3rd September 2012, 02:01 AM   #24
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Yes this is so.
The fact is that the 072 and 5534 op amps are all you need for audio. and anybody who says they can hear the inferiority of these is speaking through an orifice lower and to the rear of their mouth.

Much of the symmetrical and local only feedback topologies were originally developed for instrumentation etc. where there is a need for it, and the only need for it in audio is esthetic and marketing hype, not practical.
The circuit I posted for instance uses a CA3083 transistor array and is currently buffering a 10MHz. crystal oscillator and driving a 50 Ohm line with 500mV. rms.

I could have used a current feedback op amp but I wanted to design a discrete circuit for old times sake, and had a CA3083 on hand.
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Old 3rd September 2012, 08:22 AM   #25
Elvee is online now Elvee  Belgium
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Quote:
Originally Posted by wahab View Post
For audio purposes a so called CFB amp not only yield no advantages
over the usal VFB but will more surely bring a lot of drawbacks compared
to the latter.

Some will argue for higher slew rates without explaining how this will
translate in sound quality since theses kind of amps are not more linear
than the more traditionnal VFBs wich seems to be better in respect of
global perfs.
Quote:
Originally Posted by rcw666 View Post
Yes this is so.
The fact is that the 072 and 5534 op amps are all you need for audio. and anybody who says they can hear the inferiority of these is speaking through an orifice lower and to the rear of their mouth.

Much of the symmetrical and local only feedback topologies were originally developed for instrumentation etc. where there is a need for it, and the only need for it in audio is esthetic and marketing hype, not practical.
The circuit I posted for instance uses a CA3083 transistor array and is currently buffering a 10MHz. crystal oscillator and driving a 50 Ohm line with 500mV. rms.

I could have used a current feedback op amp but I wanted to design a discrete circuit for old times sake, and had a CA3083 on hand.
rcw
This confirms my own impressions: an integrated CFB amplifier is a useful general purpose building block, but if you brew your own circuit, you are completely free to tailor the compensations as you wish, and to add internal degenerations if necessary to attain the best possible performance with a VFB circuit.
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Old 3rd September 2012, 09:38 AM   #26
Jay is offline Jay  Indonesia
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Quote:
Originally Posted by rcw666 View Post
The fact is that the 072 and 5534 op amps are all you need for audio. and anybody who says they can hear the inferiority of these is speaking through an orifice lower and to the rear of their mouth.
Interesting opinion. And it would be more interesting to know what amplifier that you can suggest for the top class amplifier that can be built as DIY amplifier. I have no idea what would it be. A Lynx amplifier? Musical Fidelity clone? MBL clone? Audio Analogue Puccini? The only idea that I have is that you have none. But it can't be that simple. The answer must be interesting I suppose.
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Old 3rd September 2012, 10:41 AM   #27
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Quote:
Originally Posted by wahab View Post
For audio purposes a so called CFB amp not only yield no advantages
over the usal VFB but will more surely bring a lot of drawbacks compared
to the latter.

Some will argue for higher slew rates without explaining how this will
translate in sound quality since theses kind of amps are not more linear
than the more traditionnal VFBs wich seems to be better in respect of
global perfs.
The first statement is incorrect, VFB has as many advantages as it does have disadvantages over CFB. Common Wahab you know that.

How does slower slewrate argue for sound quality ??? Until Edmond showed TMC compensation it was impossible for a VFB amp to come close to THD20 performance that can be obtained by CFB. I havent looked but it might just be possible to use TMC in CFB amp. Better THD was obtained by CFB amps and this has been the case ever since CFB made its appearence, only recently performance is much closer.
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Old 3rd September 2012, 12:54 PM   #28
juma is offline juma  Germany
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Quote:
Originally Posted by rcw666 View Post
... The fact is that the 072 and 5534 op amps are all you need for audio. and anybody who says they can hear the inferiority of these is speaking through an orifice lower and to the rear of their mouth....
Such a claim can be made only by a person that instead of brain, for thinking and listening purposes, uses the stuff that is regularly found inside the orifice that you mention.

The difference is easily heard, even by untrained ear. Two conditions are required to accomplish this:
1. Person must have undamaged hearing
2. Person must listen for the difference on, at least, half-decent system that can be bought in an average hi-fi store for 1k$ (source, amp, speakers). The higher the quality of the reproduction chain, the difference becomes more pronounced.
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Old 3rd September 2012, 01:19 PM   #29
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The only thing I can say Juma is that post a documented and verified account of your assertions. otherwise you are making noises out of the orifice that you purport to be your mouth, but is in fact located in another position.

As to hearing that is undamaged, my own was subjected to the usual 60's ansd 70's rock n' roll , plus playing a bass guitar, and also a small stint in a boiler making shop.
If you can actually hear the difference between a 741 op amp and your pet device with a statistically significant figure of more than fifty percent then you might convince me and the bulk of other scientists on the planet, until then your assertions are entertaining but irrelevant.
rce
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Old 3rd September 2012, 02:04 PM   #30
Elvee is online now Elvee  Belgium
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Quote:
Originally Posted by homemodder View Post
The first statement is incorrect, VFB has as many advantages as it does have disadvantages over CFB.
It is in principle always possible to convert any working CFB amplifier configuration into a VFB one having at least the performances of the CFB original (that's in theory at least).

Doing the opposite might also be possible, but it looks more difficult.
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