Is the CFB topology superior, and why? - Page 6 - diyAudio
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Old 5th September 2012, 01:56 PM   #51
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by homemodder View Post
Could you post a schematic, I dont believe this can happen, at least not at same operating conditions as I mentioned in previous post.
Here is conceptually how you go about it:

On the left is a normal CFB amplifier having a gain of 20dB.
The yellow trace is the output voltage, and the red one the input current: it is clearly a genuine current mode amplifier.

On the right is the same circuit, converted to VFB. The green trace has voluntarily been shifted by 0.5V, otherwise they are perfectly overlaid and undistinguishable.
To make the C to V conversion, the voltage on the (-) input has been isolated, buffered and applied to the (-) input via a resistor equal to the Thevenin resistance of the feedback network.

The circuit therefore operates perfectly identically to the first one, but there is obviously no current into point A: it has become a VFB.

Of course, in this case the operation is useless, but it was done on purpose to show the equivalence.
Morever, with an integrated amplifier like the LT1206, it makes little sense anyway, but in a discrete circuit, this gives you one more degree of freedom: you can free yourself if you want of the effect of the input current.

Some remarks:
This is conceptual, made with spice components. A real buffer would add some phase shift and some capacitance on the feedback node, but these are merely practical problems, and they are perfectly manageable.
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Old 5th September 2012, 04:41 PM   #52
RJM1 is offline RJM1  United States
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You might want to look at this if you haven't seen it already, from TI.
http://www.ti.com/lit/an/slva051/slva051.pdf

Last edited by RJM1; 5th September 2012 at 04:43 PM.
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Old 6th September 2012, 06:07 PM   #53
wahab is offline wahab  Algeria
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Quote:
Originally Posted by homemodder View Post

Name the advantages and disadvantages of VFB, Ill add the disadvantages youre not thinking of.

CFB outperformed all generations of VFB, even now the latest outperform the VFB if we look at opamps. Please name me one VFB opamp that could equal even the old 1980s comlinear opamps. Two gain stages ???, could you demonstrate.

Traditional VFB topology cant and never will reach the speed that can be attained by CFB, theory dictates. VFB can have larger slewrate at the cost of very high currents in the LTP, CFB on the other hand can probably reach 10 times that high if the same current is used.

Apart from slew rate a CFB is no better than a VFB and usualy
not as good in matter of linearity contrary to your sayings
wich was the point that i didnt agree with.

Indeed , the best opamps in this matter are VFBs.

As for high slew rate, it is pointeless if linearity is worse ,
wich is often the case with CFBs.

Last edited by wahab; 6th September 2012 at 06:10 PM.
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Old 6th September 2012, 06:47 PM   #54
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voltage primary current secondary does not matter where voltage is taken from directly from an output or from a sense resistor

audio amps without global feedback sounds much much much better to my ears

the reason is feedback has no clue if it is timbre of instrument or a power supply noise that NFB is currently suppressing
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Old 6th September 2012, 06:58 PM   #55
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by suntechnik View Post

the reason is feedback has no clue if it is timbre of instrument or a power supply noise that NFB is currently suppressing
Actually, feedback does have a clue, which is why it works.
If it had no clue, the amplifier would be dead silent for noise, ripple, distortion ... and music.
Effective, but maybe a little too radical.
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Old 6th September 2012, 09:32 PM   #56
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If you do hear a timberal difference with a no global feedback amplifier it is then you are hearing distortion added to the signal.

That the evil global feedback masks things that are magically revealed by its removal is nonsense, it removes things that amplifiers being inherently imperfect devices add.

As I have pointed out local only feedback is used when the frequencies to be amplified become such that the in and output parameters of the individual active devices start to become significant, and they don't even approach this condition for audio signals.
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Old 6th September 2012, 11:23 PM   #57
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Well I am just sharing my own preferences (I am more of audiophile than an engineer when it is related to music and do trust my ears always since it is just a hobby. Owned Carvers, Nakamichis, Accuphases and KRELL FBP behemonsters coupled to full sized Magneplanars before switching to no global NFB topology & back loaded horns. Studied Heaviside and Laplace transform at institute but many-many years ago so no mathematical proofs just like vivid or distorted sound without NFB a bit more. Had sacrificed my beloved Magnepans for that purpose. Some amps have switch NFB on-off BTW.)

On local country forum read that current sense resistor for NFB topology works very nice with full ranger speakers. It was somehow related to speaker crossovers that CFB might interfere with otherwise. Not sure if it is true or not just a possible aspect to consider maybe.
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Old 7th September 2012, 12:03 AM   #58
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Quote:
Originally Posted by Bonsai View Post
One option here is to take a wide band closed loop amplifier (a good fast opamp will do) and feed this through a low pass filter. It's very easy ten to set the RC time constant and hence slew rate independent of the amplier and to test the results both subjectively and quantitively.
No. The question is whether otherwise identical amplifiers with different slew rates produce greater or lesser HF distortion when the global NFB loop is closed. It's fairly obvious that a LP filter following the amplifier might have no effect on a 20kHz fundamental but might still affect harmonics (regardless of whether inside or outside the GNFB loop) particularly if it has a steep rolloff, contributing to an excessively optimistic evaluation of performance.

It certainly seems intuitively that a higher slew rate indicates a higher BW and hence provides correction at higher frequencies when a feedback loop is closed around the amplifier. Many things in electronics are counter-intuitive, however.

It's common to observe that THD @ 20kHz can be expected to greater than that @ 1kHz because gain in a conventionally compensated amplifier is rolled of at 6dB per octave by Cdom and there is consequently less feedback to provide correction.

Slew rate and bandwidth, however are only indirectly related. Slew rate impacts directly on FULL-POWER bandwidth, but harmonics are typically 10's of dB down on the fundamental, and their rise-times are consequently considerably less.

Later in Bob Cordell's book we find:

Recommended Amplifier Slew Rate

The maximum slew rate from a CD source is limited by the very steep anti-alias filtering required by the Red Book standard for audio CDs. A square wave recorded on a CD will have a slew rate of about twice that of a 20-kHz sine wave of the same peak amplitude, or about 0.25 V/ms/Vpk. Newer recoding standards, like SACD and high-rate PCM, increase this maximum, at least in principle. Many amplifier input stages begin to exhibit nonlinearity well before slew rate limiting occurs. For these reasons, it is wise to have an amplifier slew rate that is about 10 times that of a full-amplitude 20-kHz sine wave. For a 100-W amplifier this corresponds to 50 V/ms. The minimum recommended slew rate for a 400-W amplifier is 100 V/ms. These numbers are not difficult to achieve in practice, given a sufficiently fast output stage.

This doesn't contradict the principle expressed by rcw666, but suggests that a more generous allowance might be advisable in practice.

Last edited by counter culture; 7th September 2012 at 12:07 AM.
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Old 7th September 2012, 12:42 AM   #59
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It is true that current feedback is used to increase the output impedance of audio power amplifiers that are used with low Qt drivers, this increases the bass output, and is much used in guitar amplifiers and in bi-amped systems to drive low Qt compression drivers.

Many in diy audio look aghast at this because it removes the, "benefits" of high damping factor.

The use of current feedback is in this case one of semantics because the quantity fed back is a voltage that is proportional to the current in the load, and in what is known as the current feedback op-amp, there is a current fed back that is proportional to the voltage across the load.

The first type is then more correctly called a transconductance amplifier, or vccs, and the second sort a transresistance amplifier or a ccvs.
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Old 7th September 2012, 12:45 AM   #60
Bonsai is offline Bonsai  Taiwan
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I was proposing a quick method of comparing a slew limited signal with a non slew limited one.

If you take a look at my website, there's a write iPods on an amp I finished a few months ago that talks about slew limiting etc. see section 7.
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