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Old 11th July 2004, 03:30 PM   #301
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Quote:
Originally posted by Pjotr
Actually Jan, a good friend of mine is such a painter and was teached by Melle And yes he knows the difference between cobalt blue and titanium blue, but that both are a kind of blue is no discussion.

Cheers
He must be making his own pigments then. I am an amatuer painter
and I can assure you there is no pigment called titanium blue,
at least not for artist use (maybe there is some industrial pigment
called so). I could give you a long list of other blue pigments
that are used, if you wish.
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Old 11th July 2004, 04:01 PM   #302
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Quote:
Originally posted by Christer

I can assure you there is no pigment called titanium blue,
at least not for artist use ...
Absolutely right Christer

Tianium dioxide is a common known white pigment. But there is a commercial available oil paint (don’t know the brand) with a blue colour called “titanium blue” which is a very deep shiny blue. Probably they do not use titanium for the blue pigment but just named the colour “Titanium blue”. That oil paint is rather expensive and they keep the real pigment(s) secret. Hmm… looks like audio

Cheers
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Old 11th July 2004, 05:35 PM   #303
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Quote:
Originally posted by millwood



why don't we just call them "electron feedback"? afterall, whatever happens, the electrons are sent back and they certainly don't know in what forms they are sent back for. Right?


We could do that, but I am sure somebody (me? no!) will immediately point out that electrons move the ooposite way of the current, so the electrons are actually send forward. Huh? Why do we then call it feedback? The confusion....

Jan Didden
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Old 11th July 2004, 07:29 PM   #304
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Quote:
Originally posted by Christer


Of course it can, as I have explained several times. What you
have trouble with is not an engineering terminology problem
but a philosophical problem, but you fail to recognize it as such.

No....on all counts...
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Old 11th July 2004, 07:43 PM   #305
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Default cfb op-amp makers are right

Folks,

There hasn't been, as far as I know, a universal consensus on what constitutes "current feedback", or CFB. In 1988 I acquired a 1964 General Electric transistor catalog/handbook from a colleague who retired. I kept it for historical reference. Another oldie but goodie textbook in my collection is "Pulse, Digital, and Switching Circuits", by Millman and Taub, c. 1965. The 1964 GE manual describes CFB as what I would call "series feedback". The 1965 Pulse & Digital text describes CFB as a feedback signal derived by sensing the output current with a low valued resistor in series with the load. Only written a year apart, these two references don't even agree.

As far as "low impedance VFB" goes, I beg to differ. The reason that the semiconductor industry chose the term "CFB" makes sense to me. With a conventional VFB (high inverting node input impedance), the closed loop gain and the bandwidth are determined by the feedback *voltage*, not the feedback current. In the non-inverting configuration with input resistance Rin equal to feedback resistance Rfb, the closed loop gain is 2. This value of 2 is the reciprocal of "beta", the feedback factor, which is the fraction of the output voltage that is returned to the input, or "fed back". Forgive me for throwing this in, but "beta" also is used as the BJT forward current gain factor. Should we be upset about this double usage? Anyway, if we reduce Rfb to zero, and/or Rin to an open, we now have a follower (closed loop gain = 1). The bandwidth is now about twice what it was with a gain of 2. The beta for a folower is 1. With Rfb = 4*Rin, closed loop gain is 5 since beta is 0.20, and the bandwidth is 0.20 that of a follower. Suppose we changed the values of Rin & Rfb, but kept the ratio the same. The bandwidth, BW, is still proportional to beta, or the ratio of the resistors. The feedback current changes with value changes, but not the voltage. If Rin = 10 kohm = Rfb, Vin = 1.0 volt, then Vout = 2 volts, and the feedback current is 0.10 mA. Now with Rin & Rfb reduced to 1.0 kohm, the feedback current increases from 0.10 to 1.0 mA, but the gain is still 2, and the BW is unchanged. Clearly, the gain-bandwidth product is proportional to feedback *voltage*, NOT the feedback current, hence the semi industry calls these op-amps "VFB", rightly so.

Now looking at the "CFB" variety of op-amp (low inverting node input impedance), let Rin = 10 kohm = Rfb. The gain is 2, with a specific bandwidth BW, and beta = 0.50, with 0.10 mA feedback current. Now, change Rin & Rfb to 1.0 kohm. The gain remains at 2 , but the BW is 10 times greater due to the 10 times lower feedback resistance. What changed, feedback voltage or feedback current? The feedback voltage is identical in both cases. The feedback current increased from 0.1 to 1.0 mA. In the case with lower Rfb the feedback current increased.

The bandwidth is directly proportional to the feedback current, NOT the feedback voltage. So the semi industry correctly identified this topology as "CFB", which makes perfect sense to me. Holding Rfb at 1.0 kohm, and opening Rin results in a follower (gain of 1). The feedback current, however is still 1.0 mA, and the feedback voltage increased form 0.50 to 1.0 V, but the BW remains unchanged. Again, the BW is proportional to feedback current, not feedback voltage. The gain changes with feedback voltage, by design. Hence, we obtain high BW independent of gain, the whole idea behind CFB.

The CFB nomenclature specifically describes the relation between bandwidth and feedback current. It does not mean that the feedback quantity itself is the actual output current, or a sensed facsimile of the same. I see no reason to argue with or slam the semi industry. I understand what is meant by the phrase "CFB" by examining the context where it is used. Their app notes explain this in great detail, and I find them very informative.

As far as transfer functions go, a resistive feedback network (or R-C) has four possible transfer functions, Iout/Iin, Iout/Vin, Vout/Iin, and Vout/Vin. Two are dimensionless ratios, one is transimpedance, one is transadmittance. A transfer function is a ratio, and cannot be simply a "voltage", or a "current".

Those of you in the minority who insist that the op-amp circuit designers and field application engineers for the semi industry have it all wrong, and that you know better, please forgive me for asking these two rather pointed questions.

Why aren't YOU the ones that are designing the op-amps and writing the app notes???!!! Have the op-amp producers hired the WRONG people???!!!

Best regards to all.
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Old 11th July 2004, 08:35 PM   #306
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Claude...

Send me mail....will attach cherry paper to reply...

cheers.
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Old 11th July 2004, 10:35 PM   #307
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Quote:
Originally posted by mikeks



No....on all counts...
Yes. By your logic I could argue that electrical current cannot
be called current, because the word "current" already has
another meaning synonymous or similar to the word "now".
Surely you wouldn't agree with that conclusion?
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Old 11th July 2004, 10:46 PM   #308
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Quote:
Originally posted by Pjotr


Absolutely right Christer

Tianium dioxide is a common known white pigment. But there is a commercial available oil paint (don’t know the brand) with a blue colour called “titanium blue” which is a very deep shiny blue. Probably they do not use titanium for the blue pigment but just named the colour “Titanium blue”. That oil paint is rather expensive and they keep the real pigment(s) secret. Hmm… looks like audio

Cheers
I assume you are not referring to a paint for artist use. I have
not seen any such thing as titanium blue from any of the major
manufacturers and it is not mentioned in any of my handbooks.
If you actually know of such a paint, I would be curious to know
the brand since it must be some small and obscure brand that
not many artists use. Alternatively, it is some cheap brand
or studio quality series paint. They sometimes make up
non-standard names for such paint. Perhaps it is just a mix
of titanium white and some blue pirgment, like phtalo blue.
For instance, Winsor & Newton even has an artist quality
colour called cadmium green, but it is a made up name and is
really a mix of cadmium yellow and phtalo green.

If it is an industrial pigment, then it is another thing. It might
then be a pigment that is not suitable for artist use for one
reason or another.
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Old 11th July 2004, 10:48 PM   #309
mikeks is offline mikeks  United Kingdom
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Quote:
Originally posted by Christer


Yes. By your logic I could argue that electrical current cannot
be called current, because the word "current" already has
another meaning synonymous or similar to the word "now".
Surely you wouldn't agree with that conclusion?

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Old 11th July 2004, 10:50 PM   #310
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Thumbs up Thanks Claude!

What a detailed but very understandable post explaining the difference between current feedback and voltage feedback amplifiers. Almost all of the data sheets on current feedback op amps outline the optimum the relative independence of bandwidth and voltage gain when compared with voltage feedback amps. The designations make perfect sense when looking the terminology from the input terminals of the amp where the amp feedback signal is sensed. The VFB amp's open loop voltage gain creates a small error voltage between the noninverting input and the inverting input; while the CFB amp amp's current to voltage gain or transimpedance (given in ohms) creates a small error current between the noninverting input and the inverting input. I seems clear the anyone that knows that looking
into the base of a transistor with the emitter grounded, the impedance is very much greater than looking into the emitter of a transistor with the base grounded.

Knowing the inverting input is trying to follow the voltage at the noninverting input through the action of the open loop gain and feedback, the feedback terminal sees a low impedance open loop as well closed loop and the error signal is a current. The same principle applies with tubes and FETs. I am really perplexed that this whole thing became an issue. The only reasons I can think of are a deliberate effort to cause confusion for fun (or ego), or a lack of understanding two principles above. This should not be confusing and I think your explanation is clear to people with a working knowledge of electronics and doesn't required an Electrical Engineering degree to understand. Thank you for your contribution to the forum and I hope it doesn't invite the wrath of those who don't want others to understand the different topologies, and the rational for their naming. Of couse it probably will.

http://www.analog.com/UploadedFiles/...70284AD846.pdf

http://www.analog.com/UploadedFiles/...099AD811_d.pdf
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