| Wavebourn |
Since when I touched a little bit some feedbacks I got an attack on my personality let me touch it deeper snd explain some feedback basics.
Feedback means some part of a signal from output of a device is applied to the input of the same device. Negative feedback means to subtract some part of output signal from the input signal, positive feedback means some part of output signal is added to the input signal.
Negative feedback decreases a total gain of a device, while positive feedback increases it.
Why to decrease a gain?
A first, all known active elements such as vacuum tubes and transistors are not absolutely linear, they distort signals they amplify. A negative feedback makes an amplifier more linear; subtracting distortions made by amplifier itself. The overall linearity of such amplifier is better, and its frequency response is better, which means its gain is more equal on a broader band of frequencies.
We may take for a feedback both output current and output voltage, and apply to both in parallel and in series with input signal. It gives us a great flexibility to increase or decrease output resistance (impedance), to increase and decrease input resistance.
The picture shows several types of a negative feedback (for simplicity purposes I used transistors):
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| bear |
Just curious, by what mechanism is the input resistance changed?
By how much?
_-_-bear |
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| janneman |
| quote: | Originally posted by bear
Just curious, by what mechanism is the input resistance changed?
By how much?
_-_-bear |
Easy. In the last pic, you will see that any rise in input voltage (at the base) will, through the feedback, give an almost equal rise in emitter voltage.
Because the emitter voltage follows the base voltage, there is no change in input current as a result of the voltage rise at the base (the Vb-e remains the same, almost).
According to ohms law, input resistance is (delta) input voltage divided by (delta) input current.
No change in current with change in voltage means infinite resistance.
Jan Didden |
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| Claude Abraham |
The input resistance presented to the input source driving the base is:
Rb + ((beta + 1) * Re).
If no base resistor is present, then Rb becomes rbb', the internal base spreading resistance. As an example, if Rb = 0, rbb' = 10 ohms, beta = 100, and emitter resistance Re = 50 ohms, then Rin = 5,060 ohms.
In order for the circuit to present infinite input resistance, the transistor forward current gain, beta, would have to be infinite, OR, Re would have to be infinite. With real world finite values, the input resistance is always finite.
Have I answered the question(s)? BR. |
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| Giaime |
| quote: | Originally posted by Wavebourn
[BThe picture shows several types of a negative feedback (for simplicity purposes I used transistors):
[/B] |
You shouldn't, we're in the "Tube" forum :smash: |
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| lumanauw |
If the base resistance is 5060ohm (this range), where does the "negative base impedance" coming from?
It is usually cured by base stoppers about 100-220ohm. 220ohm is small compared to 5060ohm?
What is really happening in the case of "negative base impedance"? Why is base/grid stoppers are needed? |
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| Claude Abraham |
| quote: | Originally posted by lumanauw
If the base resistance is 5060ohm (this range), where does the "negative base impedance" coming from?
It is usually cured by base stoppers about 100-220ohm. 220ohm is small compared to 5060ohm?
What is really happening in the case of "negative base impedance"? Why is base/grid stoppers are needed? |
http://www.diyaudio.com/forums/show...4311#post424311
The above thread discussed this issue, and my answer is included. Basically, the negative resistance appears due to the emitter impedance, Ze, being reactive as well as resistive, and the forward current gain, beta or "hfe", being complex, not pure real. The product of two complex numbers, can contain a negative real part (resistance) even if both resistances are positive. I hope this helps. |
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| janneman |
| quote: | Originally posted by Claude Abraham
The input resistance presented to the input source driving the base is:
Rb + ((beta + 1) * Re).
If no base resistor is present, then Rb becomes rbb', the internal base spreading resistance. As an example, if Rb = 0, rbb' = 10 ohms, beta = 100, and emitter resistance Re = 50 ohms, then Rin = 5,060 ohms.
In order for the circuit to present infinite input resistance, the transistor forward current gain, beta, would have to be infinite, OR, Re would have to be infinite. With real world finite values, the input resistance is always finite.
Have I answered the question(s)? BR. |
I think he wanted to know what the feedback did to Zin. That is what I tried to explain in understandable terms. I may or may not have succeeded...
Jan Didden |
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| lumanauw |
Hi, Claude
Thanks for the explenation :D Make things alot clearer for me.
I got 3 questions :
1.Is there any other way to deal with this besides base stoppers? You said Re cannot help.
2.Is this only happens with EF? Why is that? Common emittor doesn't experience this?
3.Why I never see C in the attachment below? Inductor in place of base stoppers resistor?never see C in the attachment below? Inductor in place of base stoppers resistor? |
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| janneman |
| quote: | Originally posted by lumanauw
Hi, Claude
Thanks for the explenation :D Make things alot clearer for me.
I got 3 questions :
1.Is there any other way to deal with this besides base stoppers? You said Re cannot help.
2.Is this only happens with EF? Why is that? Common emittor doesn't experience this?
3.Why I never see C in the attachment below? Inductor in place of base stoppers resistor?never see C in the attachment below? Inductor in place of base stoppers resistor? |
But, David, you see the C often, its the miller comp cap!
And sometimes you see the inductor in the form of a bead.
Jan Didden |
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| Claude Abraham |
| quote: | Originally posted by lumanauw
Hi, Claude
Thanks for the explenation :D Make things alot clearer for me.
I got 3 questions :
1.Is there any other way to deal with this besides base stoppers? You said Re cannot help.
2.Is this only happens with EF? Why is that? Common emittor doesn't experience this?
3.Why I never see C in the attachment below? Inductor in place of base stoppers resistor?never see C in the attachment below? Inductor in place of base stoppers resistor? |
1) Another way is to use a ferrite bead in the base side. The reason I advise against using Re to suppress the disturbance is that of signal attenuation. If the output load resistance in the emitter side is shunted by a large capacitance, adding a large enough series resistance between the emitter terminal and the load, can and will suppress the oscillation. However, this resistance is in series with the load, and the voltage divider action results in attenuation. I prefer a method which does not incur signal loss.
2) With the EF, the output is driven from the emitter. Any stray capacitance due to long wires shunts the load, and is reflected back, at high frequencies, as negative resistance. The resulting oscillation is *local* in nature. The base stopper resistor provides positive resistance value to counter the negative. With the CE, large capacitive loading does incur instability, but it is *global* in nature, and a base stopper resistor doesn't work. The global feedback loop must be taylored instead.
3) The "C" value is any capacitive loading, which could be due to but not limited to, long transmission lines, speaker crossover networks, the input of another stage (FETs have substantial input gate capacitance), etc. It is not part of the EF, but picked up from the outside world. As far as inductors instead of base stoppers goes, I've heard of some people that use them, although I can't say that I have. I would prefer a ferrite bead instead, since the ferrite at high frequencies presents not only inductance, but substantial positive resistance, which is much needed to counter the negative component. Does this help? BR. |
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| rdf |
| quote: | Originally posted by Wavebourn
Feedback means some part of a signal from output of a device is applied to the input of the same device. Negative feedback means to subtract some part of output signal from the input signal, positive feedback means some part of output signal is added to the input signal.
A first, all known active elements such as vacuum tubes and transistors are not absolutely linear, they distort signals they amplify. |
The devil's in the details. Rather than "all known active elements distort signals they amplify", I think it's more correct to say "all known active elements distort signals they pass" to capture the functionality of followers and the like. Yes, they also amplify in a sense but keeping it general avoids confusion when dealing with negative gains such as feedback. Now, there shouldn't be any issue in saying all active elements distort passing signals regardless of the port being fed. Drive a tube at its cathode or grid and distortion components appear on the plate. Typically feedback in a tube circuit returns the correction signal to the cathode, therefore the correction isn't perfect and the result of imperfect feedback manifests itself on the plate as additional, higher harmonic components not present in the no-NFB case, albeit at a much lower level than the low harmonics. Devil is, the ear is much more sensitive to these upper harmonics than the lowers and the distortion weighting to best capture the effect on the ear, the only thing that really counts in sound reproduction, has been argued since the days of Shorter and Crowhurst.
I'm not of the camp that considers feedback universally bad and use it locally in moderation. I still have much to learn about its sonic impact. However I've come to believe that "A negative feedback makes an amplifier more linear; subtracting distortions made by amplifier itself. The overall linearity of such amplifier is better, and its frequency response is better, which means its gain is more equal on a broader band of frequencies" can't be considered a universal truism. 'Linearity', prefaced by 'audible', is another one for the devil's dictionary. |
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| Wavebourn |
| rdf, I am absolutely agree with you. Distorted feedback is the next topic. ;) |
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| Wavebourn |
| quote: | Originally posted by bear
Just curious, by what mechanism is the input resistance changed?
By how much?
_-_-bear |
People believe, by electrons and holes, but nobody knows for sure. :xeye:
Nice to see you, Bear! How is your copperless pure oxigen cable? :D |
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| lumanauw |
Hi, Janneman,
Is that miller cap gives the same result as inductive base stoppers?
Hi, Claude,
Thanks for the explenation. It has helped me with one of power amp riddle of mine.
Base stoppers usually placed after VBE multiplier, before predriver.
I saw so many designs that survive without base stoppers at all. I asked "how come"?
The answer is this : It has output inductor!! :D
If one design does not use output inductor, then base stopper is a must, right?
| quote: | | The "C" value is any capacitive loading, which could be due to but not limited to, long transmission lines, speaker crossover networks, the input of another stage (FETs have substantial input gate capacitance), etc. It is not part of the EF, but picked up from the outside world. |
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| janneman |
| quote: | Originally posted by lumanauw
Hi, Janneman,
Is that miller cap gives the same result as inductive base stoppers?
[snip] |
I'm not sure about that one. Claude explained that the (lack of) base stopper causes local instability. Does the miller cap of the driving stage prevent that? Maybe Claude has an opinion on that?
Jan Didden |
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| janneman |
I like to come back to the question why neg fb increases the input impedance, as discussed earlier. Looking at the 4th fig in the start of the thread where the feedback is returned to the emitter of the input transistor; signal input is at the base.
If we assume that the gain of the amp without feedback is very, very high (a normal requirement for feedback to work), that would mean that you only need a very, very small effective input voltage to get the output voltage. The effective input voltage is the signal voltage between B and E, that causes the ib that causes the ic etc,
If that b-e voltage is very, very small, that means that it varies very, very little anyway even if the input signal varies considerably. That in turn means that it take only a very, very small signal current even if the input signal varies considerably.
So, if we look to the circuit as a black box (hi David!), we see an input that takes very, very little current change from a largish voltage change -> high impedance....
Now you also see why that effective input voltage can be so small: the signal V b-e consists of the input signal (at B) *minus* de feedback voltage (at E). The feedback, by making the effective input voltage very small, causes the output to be relatively small even if the amp gain itself is very large.
Jan Didden |
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| Johan Potgieter |
Just a few things.
Wavebourn, you have scooped me! I was still going to react to our discussion elsewhere, but am a little busy right now. (Will follow that up.)
1. I think there might have been a misunderstanding looking at the drawing of Lumanauw (post #9)? I took it he meant his sketch (C), displaying inductors, - not C as a capacitor. But that was sort of picked up.
2. Just a technicality: Application (3) in Wavebourn's neat sketches actually lowers the input Z - but that also by the way.
3. Rdf, you have touched on a very important point; to me this is where the major misunderstanding arises. Perhaps it is just semantics, but if a correction is not perfect, it does not mean that some useful correction does not take place! Nobody claims that D = 0 with feedback. In fact, it can never be perfect (could one also say sufficient) since it controls itself. One easy way to look at it is to determine whether any non-linear active device is more linear after NFB, and that is comparatively easy to determine to prove. I could say: Case closed!
.... but you touch on a further most important point: The generation of higher order harmonics.
(As an aside, yes, this was mentioned as early as I believe 1957 by Shorter and Crowhurst, but with valves it was not of major consequence, since it never really took place to a disturbing degree. But with semiconductors there was a large wake-up call. If my memory serves me this was appreciated especially by Peter Walker when to the astonishment of many his very good first transistor amplifier with very low t.h.d. caused more listener fatigue in a black-box blind test than his Quad II! So we are certainly NOT oblivious to this fact, on the contrary!)
I do not want to over-extend this post and would summarise by saying that it is not true to jump from the statement that NFB is not "perfect", to the statement that there must as a consequence exist AUDIBLE higher harmonic products. This is my very meaning when in the past I repeatedly spoke of the wrong application of NFB (usually meaning too much)..... and since you are talking about mathematical things I hope you will allow that I call them measurable, and that we leave subjectivism there for the moment. (We have spectrum analysers that can do -140 dB - way below what will ever cause audible effects in whatever combination.)
It can be shown very nicely experimentally how increasing NFB will lower all harmonics, until at some factor the high orders progressively stop that tendency, then with further NFB they become dominant (that is in a weighted capacity). It is this latter case that occurs all too often. I will include illustrative figures of my own experiments somewhat later, to motivate - I am not at home with my records now.
Still very important, you are correct in stating that applying NFB applied ("mixed") in a grid-cathode interface is non-linear (let us stay with tubes - though semiconductors have been used in this discussion that case is quite different and a discussion on its own). Although you did not point it out as such, it is important that non-linearity in the adding (mathematical) interface is NOT cancelled by NFB. This is avoided by "mixing" passively, such as series feedback in the case of the well-known inverting op-amp.
Because of the nature of the tube transfer characteristics we kind of got away with this; spectrum analysis shows that all is still well except for too much NFB (back to that point again). But that is why one of the saving techniques especially with semiconductor amps, is to "mix" passively, i.e. in an inverter fashion (this can also be illustrated visibly by spectrum analysis). This is in fact what I do in my amps. (Yes, one can argue about balanced input stages, but again let us keep it basic for now.)
Though lengthy this is still a summary, but will hopefully contribute.
Thanks for patience. |
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| Wavebourn |
| quote: | Originally posted by Johan Potgieter
Just a few things.
Because of the nature of the tube transfer characteristics we kind of got away with this; spectrum analysis shows that all is still well except for too much NFB (back to that point again). But that is why one of the saving techniques especially with semiconductor amps, is to "mix" passively, i.e. in an inverter fashion (this can also be illustrated visibly by spectrum analysis). This is in fact what I do in my amps. (Yes, one can argue about balanced input stages, but again let us keep it basic for now.)
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I can argue about the device that drives your input. Lovering input impedance you demand more power from the source... Probably increasing its distortions... ;) |
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| Johan Potgieter |
Exactly so; this is the penalty, which has to weighed against the advantage.
E.g. in my own transistor case the input requirement was 0,6Vrms into 5K1, which I supplied from a unity-gain pair of Zout of 130E at well below audible distortion. But it is naturally included in the whole equation - almost part of the power amp. |
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| Brian Beck |
| quote: | Originally posted by Johan Potgieter
.... but you touch on a further most important point: The generation of higher order harmonics ... I do not want to over-extend this post and would summarise by saying that it is not true to jump from the statement that NFB is not "perfect", to the statement that there must as a consequence exist AUDIBLE higher harmonic products. This is my very meaning when in the past I repeatedly spoke of the wrong application of NFB (usually meaning too much)..... and since you are talking about mathematical things I hope you will allow that I call them measurable, and that we leave subjectivism there for the moment. (We have spectrum analysers that can do -140 dB - way below what will ever cause audible effects in whatever combination.)
It can be shown very nicely experimentally how increasing NFB will lower all harmonics, until at some factor the high orders progressively stop that tendency, then with further NFB they become dominant (that is in a weighted capacity). It is this latter case that occurs all too often. I will include illustrative figures of my own experiments somewhat later, to motivate - I am not at home with my records now.
Still very important, you are correct in stating that applying NFB applied ("mixed") in a grid-cathode interface is non-linear (let us stay with tubes - though semiconductors have been used in this discussion that case is quite different and a discussion on its own). Although you did not point it out as such, it is important that non-linearity in the adding (mathematical) interface is NOT cancelled by NFB. This is avoided by "mixing" passively, such as series feedback in the case of the well-known inverting op-amp.
Because of the nature of the tube transfer characteristics we kind of got away with this; spectrum analysis shows that all is still well except for too much NFB (back to that point again). But that is why one of the saving techniques especially with semiconductor amps, is to "mix" passively, i.e. in an inverter fashion (this can also be illustrated visibly by spectrum analysis). This is in fact what I do in my amps. (Yes, one can argue about balanced input stages, but again let us keep it basic for now.)
Though lengthy this is still a summary, but will hopefully contribute.
Thanks for patience. |
Nicely explained Johan! This topic deserves better understanding. |
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| Wavebourn |
| quote: | Originally posted by Johan Potgieter
Exactly so; this is the penalty, which has to weighed against the advantage.
E.g. in my own transistor case the input requirement was 0,6Vrms into 5K1, which I supplied from a unity-gain pair of Zout of 130E at well below audible distortion. But it is naturally included in the whole equation - almost part of the power amp. |
Right;
each particular case is different. No optimal system may be built optimizing each component of it. The sound track represents equation with multiple variables, and the art is to find the optimal combination of component that results in the optimal solution of the whole system. To simplify combinations and re-combinations there were standards developed for studio equipment that specify impedances and signal levels, but in case of home audio where we have more flexibility they are not optimal. |
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| smoking-amp |
Some late night cogitating:
Several of the commonly used input subtraction techniques for feedback affect the input impedance seen by the signal source. These usually cause the input Z to increase due to the tracking voltage difference across the differencing device(s) as mentioned above. This is fine as long as it is consistant.
But the actual distortion components do not track the input signal and so will modulate the input Z and thereby require error currents from the input signal. These will corrupt the reference signal if not derived from a much lower impedance.
The output difference of a high loop gain amplifier's input stage is almost exclusively the error difference signal. At least high loop gain reduces the difference error down to a very small amount. But low loop gain, or low global feedback, designs would be susceptible to this distortion mechanism.
This has probably been written about in journals under the heading "interface distortion", but I haven't heard much about it mentioned on the input side. Mainly just regarding the speaker interface. If anyone has some references on this, please let us know.
Using a Fet or Vacuum tube differential input design in non-inverting mode to get high input Z doesn't fix this. The comparison is still being made between the first source/cathode follower output Z and the feedback Fet source or cathode follower Z, so impedances are comparable at the tail differencing point. So still allows error differences to mistrack the Zs and corrupt the reference. (Of course thats how the diff. stage actually works to get current differencing at its output. So not much can be done I guess. If the impedance changes were linear for both elements, then there wouldn't be the odd harmonic distortion generally seen. )
Even the inverting mode differencing approach with resistors is susceptible, since the - input is still moving around with the error signal. Only the linear parts cancel to a (near) null at the - input.
Of course, more loop gain reduces the error magnitude to give a better voltage null.
Maybe what is needed is a hi Z current drive from the input signal side, then error voltages won't matter.
Don |
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| Wavebourn |
| I don't remember who in the middle of the previous sentury first raised the question of distorted feedback, there was a belief that small amount of feedback does not help, instead the entire feedback signal is getting distorted adding more distortions of distortions, so instead of just the 2'nd harmonic higher order harmonics are produced. I believe it was wrong interpretation of other facts such as phase shifts that prevented cancellation of nonlinearities (lot of reactive elements), and additional nonlinearities in the feedback loop that differs from nonlinearities in the main signal path (cathode VS grid, for example). |
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| janneman |
On the two last posts:
Yes, the input source impedance has an impact on the distortion. There have been some articles (can't remember exactly from whom, I thought Walt Jung among others) explaining it. Some modern new opamps have such a low distortion that this effect becomes dominant, and their spec sheets say that you should keep the source impedance below 600 ohms to get the spec'ed performance.
Peter Baxandall and other british authors have documented the relation between feedback factor and distortion. What they found was that generally speaking the distortion initially increases if you go from zero to low feedback, and that with increasing feedback the distortion RMS level decreases but you get more harmonics. In fact, to decrease higher harmonics needs more and more feedback the higher the harmonic.
One other thing to remember is that the total amp may have a gain largely determined by the feedback, the amplifying element itself will ALWAYS work open loop! That is the reason for the small effective input voltage. And now it also is clear that if the open loop gain is linear, the effective input voltage is more linear and the effects of input Z are less.
Jan Didden |
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| Wavebourn |
| quote: | Originally posted by janneman
On the two last posts:
Peter Baxandall and other british authors have documented the relation between feedback factor and distortion. What they found was that generally speaking the distortion initially increases if you go from zero to low feedback, and that with increasing feedback the distortion RMS level decreases but you get more harmonics. In fact, to decrease higher harmonics needs more and more feedback the higher the harmonic.
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It would be interesting to find, in which case. No doubt, feedback was distorted. Phase shifts and/or different transfer characteristics for input and feedback points.
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| Wavebourn |
| quote: | Originally posted by janneman
One other thing to remember is that the total amp may have a gain largely determined by the feedback, the amplifying element itself will ALWAYS work open loop! That is the reason for the small effective input voltage. And now it also is clear that if the open loop gain is linear, the effective input voltage is more linear and the effects of input Z are less.
Jan Didden |
Ok, now let's review couple of voltage amplifyer stages.
Suppose, we want to add a local feedback to the second stage.
To minimize effective voltage swing on it's grid let's add a parallel feedback by voltage like it was suggested in one post (it was called "partial feedback"). It means as well, that the first stage will see lower resistance. If it is a triode that may be roughly viewed as a voltage controlled resistor, such trick forces it to produce higher second harmonic. Also, its gain will be less so more input swing needs on its grid to get the same output swing, that means more distortions. Trying to linearize the 2'nd cascade we increased overall distortions.
Now, let's add a series feedback by current to the same 2'nd cascade instead of a parallel one by voltage, increasing resistance in it's cathode tail. As the result, the 1'st triode will see higher resistance load, so it's distortions will be less. Also, it's gain will be higher, so less input swing is needed for the same voltage output of overall amplifier. However, it will have higher output resistance as the result of the feedback by current, but it is the different story. |
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| Johan Potgieter |
| quote: | Originally posted by smoking-amp
But the actual distortion components do not track the input signal and so will modulate the input Z and thereby require error currents from the input signal. These will corrupt the reference signal if not derived from a much lower impedance.
The output difference of a high loop gain amplifier's input stage is almost exclusively the error difference signal. At least high loop gain reduces the difference error down to a very small amount. But low loop gain, or low global feedback, designs would be susceptible to this distortion mechanism.
This has probably been written about in journals under the heading "interface distortion", but I haven't heard much about it mentioned on the input side. Mainly just regarding the speaker interface. If anyone has some references on this, please let us know. |
Very important, Don.
I do not have references but believe that the priciples of interface distortion can be applied here in as much as this input impedance forms the load for a preceeding stage just as a loudspeaker forms it for a power output stage.
A first approximation can be that everything that feeds the input can be characterised as a distortionless generator followed by a resistance (the source impedance). It is this impedance that gets "modulated", thus it is important how this (hopefully low) impedance is arrived at. The problem arises if this is not simply the internal resistance of the feeding stage, but an impedance arrived at from global feedback. Then interface distortion is right back in the picture.
(Many will realise that this is one of the major reasons why a low impedance reached by NFB is to be carefully handled. The often quoted high damping factor of power amplifiers, when realised mainly by NFB, can be a time bomb source of distortion. I am sure the defining article years ago by Otala and Lamasniemi on interface distotion is known.)
regards. |
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| Johan Potgieter |
I am going to venture posting some spectrum diagrams here (following post), although they do not show distortion/NFB magnitude as promised earlier. Rather they show the often neglected influence of certain basic circuit elements on distortion.
Fig. 1 shows a basic power amplifier circuit simply to illustrate the three resistances (impedances) under discussion here. R1 and R2 are the usual sub-ohm compensation resistors in the power emitters, while Rdrive is the output impedance of T3. (T3 should be seen as a stage with adjustable output impedance, not necessarily a single transistor.)
The amplifier used is also not this circuit. It was a 70W amplifier using 27 dB of NFB operated at about 50W. The rise between 20KHz and 25KHz should be ignored - it was an artifact of the analyser programme. The input signal was 1 KHz, and that as well as the 2nd and 3rd harmonic products are off scale to facilitate presenting the higher order products; they were not exorbitant.
Figures are numerated rather backwards! Fig. 5 shows the spectrum of the typical circuit often encountered. The row of odd order harmonics would be noted - they extend beyond 40KHz.
Figs. 2 and 4 show the separate influence of zero R1, R2, and lowering R drive respectively (note the different vertical scale). Fig. 3 shows the reult of these measures combined. This is noticably better than fig. 5, especially concerning higher order harmonics.
Making R1, R2 zero could be regarded as theoretical. Alternative temperature compensation can however be effected by thermal feedback between power transistors and the T3 current source.
If the programme obliges the graphics should appear in the next post.
Regards |
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| Johan Potgieter |
| Following: |
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| Johan Potgieter |
Sorry; I should perhaps mention:
The results were not influenced by a significant change in loop gain as a result of changes in the T3-stage to give the different Rdrives. That was adjusted. The global NFB of about 27dB was maintained. |
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| smoking-amp |
Hello Johan,
Interesting graphs. Are these from a real speaker load or a fixed resistor load?
On the idea of displaying increasing higher harmonic distortion (although lower THD) with increasing NFB, it has been commented somewhere that as the device characteristic curvature is straightened out by increasing NFB, it is described by a power law math. model with power coefficient dropping toward unity but never quite getting there.
A coefficient of say 1.1 being the same as the fraction 11/10, this being equivalent to the 10th root of the 11th power, so no real surprise that higher harmonics are generated.
I recall an article in EW+WW a few years back where it was proposed to build an amplifier using square law Mosfets which would only generate 2nd harmonics, then they cancelled the 2nd harmonics with a push-pull design. Of course, real Mosfets aren't quite perfect square law unfortunately.
regards,
Don |
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| Johan Potgieter |
Don,
Resistor load - these are actually Spice graphs, because I could load the graphics here more easily. But I did check with a simulated loudspeaker load - no mentionable difference. The load was not severe - 8 ohm system going to 16 ohm main resonance and down to 6 ohm; phase shift staying between +/- 15 degrees (equalised). Actual amplifier tests with spectrum analyser into resistive load showed confirmation, but I could not photograph those.
Your further explanation makes sense. Mathematics (a series) confirm that NFB increases high order harmonic products, e.g 3rd generated from 3rd, etc. I did not do such analysis, but it would seem that such products remain very low in amplitude, mostly in noise floor. It would appear to be harmonic multiplication but at very low amplitude. Products can go on over 100KHz, but at negligible amplitude, and so will any intermodulation products also be. This is of course only the case if the open-loop design itself is relatively clean to begin with - old principle.
.... and yes, unfortunately real life products are never that obliging! |
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