Hi Friends,
I took some couple of month to develop no overall feedback amplifier, but it is so hard to do it.
Hardly to specify the voltage gain and gain matching between L and R channel.
I saw Moon Evolution series power amplifiers with claimed that no overall feedback with shorter signal path. Simaudio MOON W-8 Dual-Mono Power Amplifier
Now I have done a current feedback amplifier design, but not so sure how it sounding.
Can you please share what the advantage of current feedback circuit?
My new integrate amplifier is like below.
Buffer Amp-->Low impedance volume control-->Voltage gain-->Current based amplifier.
I took some couple of month to develop no overall feedback amplifier, but it is so hard to do it.
Hardly to specify the voltage gain and gain matching between L and R channel.
I saw Moon Evolution series power amplifiers with claimed that no overall feedback with shorter signal path. Simaudio MOON W-8 Dual-Mono Power Amplifier
Now I have done a current feedback amplifier design, but not so sure how it sounding.
Can you please share what the advantage of current feedback circuit?
My new integrate amplifier is like below.
Buffer Amp-->Low impedance volume control-->Voltage gain-->Current based amplifier.
Some of the older chipamps are current feedback and they're reputed to sound better than the more modern VFB ones. Thinking here specifically of TDA2003A and TDA2009. I tend to prefer the sound of CFB opamps to VFB ones. LM6172 though is VFB and normally sounds fine - internally though its CFB architecture.
Maybe worth following up these Members thoughts.
http://www.diyaudio.com/forums/soli...s-self-wants-your-opinions-2.html#post3409004
and
http://www.diyaudio.com/forums/soli...s-self-wants-your-opinions-2.html#post3409670
http://www.diyaudio.com/forums/soli...s-self-wants-your-opinions-2.html#post3409004
and
http://www.diyaudio.com/forums/soli...s-self-wants-your-opinions-2.html#post3409670
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Very simple. BJTs are current amplifiers and so to speak by their very nature
( that is the physics of semiconductors) these feel good if they just do current amplification that is current in current out. So it is just "natural" to employ current feedback because there are two "current to voltage transformations" less involved in the loop.
Everything is not so simple!
extract from wikipedia:
In analog circuit design, the current-control view is sometimes used because it is approximately linear. That is, the collector current is approximately times the base current. Some basic circuits can be designed by assuming that the emitter–base voltage is approximately constant, and that collector current is beta times the base current. However, to accurately and reliably design production BJT circuits, the voltage-control (for example, Ebers–Moll) model is required.[1] The voltage-control model requires an exponential function to be taken into account, but when it is linearized such that the transistor can be modelled as a transconductance, as in the Ebers–Moll model, design for circuits such as differential amplifiers again becomes a mostly linear problem, so the voltage-control view is often preferred. For translinear circuits, in which the exponential I–V curve is key to the operation, the transistors are usually modelled as voltage controlled with transconductance proportional to collector current. In general, transistor level circuit design is performed using SPICE or a comparable analog circuit simulator, so model complexity is usually not of much concern to the designer.
So do Gilbert, Wurcer, Self, Horowtiz and Hill and many other audio designers who all certainly agree with the Wikipedia extract ?
You must have a different datasheet, because the one I checked http://www.datasheetcatalog.org/datasheet/SGSThomsonMicroelectronics/mXutuqt.pdf
says nothing of the kind in page 2.
The actual "official" application circuit (which by the way is the one followed by 99.999% of users) is the one on page 6, labelled as "Figure 16. Typical application circuit" and it clearly shows
composed of a 220 ohms resistor and a 2.2 ohms one.
Of course you may use it *without* NFB , a.k.a. "open loop" for some unfathomable reason which is up to you.
That's not what Bruno Murari, the original TDA2002 designer wanted, by the way, and he went to great lengths to avoid it, that's why the low impedance *voltage* feedback network was implemented.
Perhaps the amp's operation mode is opaque to you? If you care to examine the schematic, the negative feedback is provided to Q4's emitter. That's by nature a low impedance node, hence the feedback is by means of current. For a voltage feedback amplifier, the -ve input is high impedance, normally the base of an LTP.
There might be a semantic issue here. Traditionally, the name 'current feedback' has been given to the kind of feedback that returns a sample of the output current. This could be for instance the voltage across a small resistor in series with the load. Current feedback thus makes the output impedance very high.
With the new so-called 'CFA' opamps that appears to have taken on a new meaning (blame it on the marketeers) namely: feedback to a low impedance node. Personally I think this is an ambiguous way to name it because, what's a low impedance node? The emitter impedance of a starved BJT can be pretty high, while 'voltage feedback' to an invering input fed from a low-impedance source may be pretty low impedance.
But whatever you fancy, you should at least be clear which definition you want to use in this particular discussion.
As to pros and cons: like always in engineering, you can make very good or very bad amps with either technology.....
jan
With the new so-called 'CFA' opamps that appears to have taken on a new meaning (blame it on the marketeers) namely: feedback to a low impedance node. Personally I think this is an ambiguous way to name it because, what's a low impedance node? The emitter impedance of a starved BJT can be pretty high, while 'voltage feedback' to an invering input fed from a low-impedance source may be pretty low impedance.
But whatever you fancy, you should at least be clear which definition you want to use in this particular discussion.
As to pros and cons: like always in engineering, you can make very good or very bad amps with either technology.....
jan
A lot of earlier (and nice-sounding) amps used second collector to first emitter feedback. Output followers of course don't count. They didn't need a well-matched and balanced diff pair to see all the benefit of the feedback applied. I'd imagine that the thermal feedback on a TDA20x0 is pretty severe and relying on a diff pair staying balanced was pretty iffy when that device was designed. Doesn't surprise me the choice of input toploogy.
No, nothing to do with marketing. It's an illustration of the four types of feedback circuits. Each of these types has it's own formalism, in either h, g, y or m parameters. These are chosen "naturally", based on the impedances, "low" means current, "high" means voltage (at the input) and the other way around for the output. Actually, these formalisms are not bringing anything special, you can analytically evaluate any of the four topologies using e.g. only voltages and impedances (but, you or the computer, be prepared to handle some stupid numbers).
Your simulator doesn't give a rat's *** if the curcuit is voltage or current feedback, it just determines the transfer function you need (U/U, I/U, U/I, I/I).
There are only circuits with inputs and outputs. Some of them can be conveniently analyzed using the feedback concept. For humans, when analyzing (or designing) a new circuit, it is easier to go through an euristic phase and perhaps identify if the circuit functionlity could be understood using some pre-fabricated methodology (like feedback theory).
My point was that the 'traditional' terms were named after what sample you returned, iow voltage feedback meant you took (a sample of) the output voltage and returned that as feedback, while if you sampled the output current you would call that current feedback. It had nothing to do with the impedance of the node to where you send the feedback.
Your interpretation is the newer interpretation that came into swing after the 'CFA' came onto the market.
Neither is intrinsically right or wrong - you agree to the definition and that's it. It's just that there are two types of definitions afoot and I just wanted to flag that.
jan
Hi, Guys:
The distinction between voltage feedback (VFB), and current feedback (CFB) seems interesting to ponder. We could say that a distinction is that CFB samples the output current back to the input, while VFB does not. Except that, VFB using a long-tailed input pair constructed of bipolar transistors also samples the output current - since bipolar transistors operate on current conducted through their base electrodes.
Alternately, we could argue that VFB samples the output voltage, while CFB does not. Except that, one can measure a small, but present, signal voltage presented to the non-zero ohm feedback input electrode. The impedances of the respective feedback networks of VFB and CFB reflect the relative impedances of their respective feedback input electrodes. So, I don't feel that distinction get's us at the root of the distinction either.
To me, perhaps, the most relevant operational distinction is that in VFB, the feedback input electrode typically drives the base/gate of a grounded emitter/source stage. In CFB, the feedback input electrode typically drives the emitter/source of a grounded base/gate stage. So, with CFB, we get all of the expected benefits of grounded base/gate operation, such as wide bandwidth and better open loop linearity. However, we also get the expected difficulties, such as low input impedance. We are all describing this core distinction, I think, but just from varying perspectives.
The distinction between voltage feedback (VFB), and current feedback (CFB) seems interesting to ponder. We could say that a distinction is that CFB samples the output current back to the input, while VFB does not. Except that, VFB using a long-tailed input pair constructed of bipolar transistors also samples the output current - since bipolar transistors operate on current conducted through their base electrodes.
Alternately, we could argue that VFB samples the output voltage, while CFB does not. Except that, one can measure a small, but present, signal voltage presented to the non-zero ohm feedback input electrode. The impedances of the respective feedback networks of VFB and CFB reflect the relative impedances of their respective feedback input electrodes. So, I don't feel that distinction get's us at the root of the distinction either.
To me, perhaps, the most relevant operational distinction is that in VFB, the feedback input electrode typically drives the base/gate of a grounded emitter/source stage. In CFB, the feedback input electrode typically drives the emitter/source of a grounded base/gate stage. So, with CFB, we get all of the expected benefits of grounded base/gate operation, such as wide bandwidth and better open loop linearity. However, we also get the expected difficulties, such as low input impedance. We are all describing this core distinction, I think, but just from varying perspectives.
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