mikeks said:
I confess this has much more to do with personal preferences than with "rightness".
Michael, look again at your figure , assume for a moment the error is null, then you have as input to your S1, x from the left and Kx from the right, so if K <> 1, there is a net output even for a perfect amplfier.
I find more satisfying to have in this case a null error output from your S1, which requires to scale back the output sample to a level comparable with the input (if K <> 1). The scale factor is precisely the inverse of the nominal ampifier gain K.
Rodolfo
traderbam said:
Good Brian !!! You insist in shoehorning error correction as a particular case of negative feedback.
I (and some others) prefer to view it from a different perspective. This does not imply gain expressions cannot be made to look equivalent, they can. It is simply a matter of starting goal. I guess were it not for the fact we have negative feedback so deeply ingrained in our minds, the error correction concept should look much more natural from the standpoint of design. Think of it, first compute the error, then substract it, that simple. I suspect if Black had at hand the electronic tools available today, he should have invented error correction.
It is equally true the availability of - for all practical purposes - infinite gain with the advent of usable operational amplifiers (741 onwards), cleared the path for applications appart from simple linearization, like transfer function synthesis exploiting negative feedback in this context.
Now back to the point. You know if we assume a first order network for the active part of summing node S1, then the "cancellation" condition leads to an equivalent perfect integrator of the same gain-bandwidth product as the original network. The same result could be accomplished designing this integrator in the first place, whatever the means the result is the same.
I find the inner loop positive feedback a good way of doing it.
Rodolfo
If the result is the same then what are you claiming? I would really like to understand what the advantages are of the implementation you suggest, other than being of academic interest?
For example. Can you make a more linear integrator using PFB? Is there some improved stability of the circuit? Is one less sensitive to summer distortion than the other? Is one cheaper?
Can you show, for example, that a PFB loop circuit can be made more linear than an integrator, in practice? How do you justify having to have 2 summing nodes?
Help me understand what the benefit is. 🙂
mikeks said:
Sorry mikeks but it seems that you consider the VAS stage as part of the output stage.
The SM17 is all emitter follower from the bias adjust to the output node… so no voltage gain greater than unity…
Yes. You are absolutely right!
The SM17 is unity gain from its bias node to its output
BUT it has gain greater than unity from the so-called HDAM module to the bias network, courtesy of the complementary Wilson mirrors.🙄
Note that i have lumped the later together with the unity-gain (nominal) output stage in the discussion to which you've referred. Kapitsch? 😎
The SM17 is unity gain from its bias node to its output
BUT it has gain greater than unity from the so-called HDAM module to the bias network, courtesy of the complementary Wilson mirrors.🙄
Note that i have lumped the later together with the unity-gain (nominal) output stage in the discussion to which you've referred. Kapitsch? 😎
PB2 said:
Hi Pete,
The Hafler lateral MOSFET amps came out well before my paper. I don't know of any connection to the Hafler amps.
Bob
mikeks said:
Mike I think the input to your output stage is in error. Its (Kx-epsilom)/K, NOT (x-epsilon)/K..
Jan Didden
janneman said:
Look carefully:
It is indeed (Kx-epsilon)/K, which, as shown in the figures, amounts to:
(x-epsilon/K)
Viz. Only the ''epsilon'' is divided by the ''K''.
ingrast said:
"In 1921, the major task at the Western Electric Company's old West Street laboratories in New York City was to improve the Bell System's new open-wire telephone system. I discovered that the system's push-pull repeater amplifiers were a major source of trouble. No one knew how to make amplifiers linear or stable enough in those days, and consequently they were subject to an intolerable amount of distortion. Starting from 1921 and during the next two years, I spent many weekends and evenings reading all I could about the unwanted generation of products. At 2 a.m. of March 17, 1923 I restated my own problems as follows: Remove all distortion products from the amplifier output. In doing this, I was accepting an imperfect amplifier and regarding its output as composed of what was wanted plus what was not wanted. I immediately observed that by reducing the output to the same amplitude as the input, and subtracting one from the other, only the distortion would remain. The distortion could then be amplified in a separate amplifier and used to cancel out the distortion in the original amplifier output. This isolation and elimination could be accomplished with two biconjugate networks as a three-winding transformers or....."
H.S. Black. Inventing the negative feedback amplifier. IEEE Spectrum, pages 55-60, December 1977
[quote stolen from: http://digilander.libero.it/paeng/thd__cancellation_techniques_general.htm
I don't really agree with much else on the page though]
traderbam said:
I believe that Rodolfo has lumped the linear, and non-linear transfer function of the output stage into A as I do not see error injection in his model. I understand from the diagram that 1/A' is a correction, or approximate correction for the linear gain and perhaps transfer function, and the feedback loop is the EC to cancel the non-linear part.
Your diagram, traderbam, requires the synthesis of a non-linear network that corrects for the non-linearity of the output stage. This is difficult, if not impossible, as shown in your diagram, whereas in Rodolfo, Bob's, and others using EC it is simply done through subtraction. Simple, predistortion could be used if synthesis of such a non-linear network was straightforward, it is not, and this is why EC is so elegant.
Pete B.
Hi PB2,
I think the Hafler Mosfet amplifiers with dual differential input stages were designed by Erno Boberly.
I think the Hafler Mosfet amplifiers with dual differential input stages were designed by Erno Boberly.
Re: Give SPICE a try (& HF Ingress)
Everyone might like knowing that LT-Spice is now able to use .WAV files, as both inputs and outputs. So maybe now you really CAN hear your amplifier with a simulator!
The details are in the message-archive of the LT-Spice discussion group, at http://www.yahoogroups.com .
You can do lots of other very-interesting things, with it, too. It's a free and reasonably-sized download, from http://www.linear.com . LT-Spice can usually use other Spice flavors' device models, MANY (many!) of which are findable through http://homepages.which.net/~paul.hills/Circuits/Spice/ModelIndex.html .
LT-Spice is great for tweaking the details of a circuit, or just playing "what if", especially if you also model a lot of the parasitics. It is arguably the best Spice available, for any price. And there is astoundingly-good support and help available in the discussion group, mentioned above. The author of the LT-Spice software is always there, too.
Sorry about the delay in posting my comments. I just recently registered, here (and am greatly-appreciating some of the enlightening discussions), having read most of this thread.
[For the record, since this is my first post, here: I DO have a BSEE (Purdue, 1978), but, after several years of doing aerospace control systems, I was working mostly outside of actual EE for 15 years and seemingly "lost" a lot of it. Lately, I've been attempting to design mostly analog test/measurement systems (for the last two+ years or so), and am still trying to learn and re-learn. So, "thanks", for the great discussions!]
--- Regarding "HF Ingress", which has also been discussed in this thread:
In addition to the intermodulation effects that have been discussed here already, one very common problem that can occur when RF gets into a solid-state electronic system (and it often/usually WILL get in, with up to tens of mV) is that, especially in places like BJT opamp inputs, BJT differential pairs and BJTs in general, et al (FETs' susceptibility is about 1/1500th BJTs'), and especially if they are low power, or biased near their "knee", a semiconductor junction can RECTIFY the RF, often resulting in the creation of a DC OFFSET or DC ERROR voltage, which may appear at some output, or cause other problems. This DC offset or error can often appear at the output of an opamp, for example, and can be quite significant, depending on the circuit or application.
There is a good discussion about all of that, in a book that was recently written by some of the people from Analog Devices, "Op Amp Applications Handbook", with Walt Jung, Editor Emeritus. The book's chapters were/are also available on http://www.analog.com , each in a pdf file. But I don't know if they are always accessible publicly. Chapter Seven has a very good practical discussion of these issues.
--- Re: Some earlier "HF Ingress" discussions/questions in this thread (Sorry. I don't have the links to the relevant messages.):
Note that the book that I mentioned does indicate that amplifier OUTPUTS _are_ also vulnerable to EMI/RFI, especially if they drive lengths of cable (e.g. speaker wires), "...which act as antennas. RF signals [sic] received on an output line can couple back into the amplifier input where it is rectified, and appears again on the output as an offset shift."
For amplifier outputs, they recommend adding series resistance or ferrite beads, or an RCR "T" filter.
They recommend low-pass filters on opamp inputs. For inverting opamps, they suggest a symmetric T filter (RCR), so the C isn't connected directly to the inverting input, which would cause instability.
However, they state that the filter bandwidth should be chosen to be at least 100 (one hundred!) times the signal bandwidth, to minimize signal loss. Obviously, I can't quite do that, if I want my amplifier's bandwidth to be over 100 kHz, but also don't want to rectify the signal from the local AM radio station (say, 1000 kHz). So, there are also very good sections about layout, grounding, and shielding, etc. They also mention choosing devices that are not faster than necessary for the application (think "edge rate").
Sorry if too much of this was off-topic, or too long. I hope that it might be helpful.
Regards,
Tom Gootee
http://www.fullnet.com/u/tomg
Bob Cordell said:
Everyone might like knowing that LT-Spice is now able to use .WAV files, as both inputs and outputs. So maybe now you really CAN hear your amplifier with a simulator!
The details are in the message-archive of the LT-Spice discussion group, at http://www.yahoogroups.com .
You can do lots of other very-interesting things, with it, too. It's a free and reasonably-sized download, from http://www.linear.com . LT-Spice can usually use other Spice flavors' device models, MANY (many!) of which are findable through http://homepages.which.net/~paul.hills/Circuits/Spice/ModelIndex.html .
LT-Spice is great for tweaking the details of a circuit, or just playing "what if", especially if you also model a lot of the parasitics. It is arguably the best Spice available, for any price. And there is astoundingly-good support and help available in the discussion group, mentioned above. The author of the LT-Spice software is always there, too.
Sorry about the delay in posting my comments. I just recently registered, here (and am greatly-appreciating some of the enlightening discussions), having read most of this thread.
[For the record, since this is my first post, here: I DO have a BSEE (Purdue, 1978), but, after several years of doing aerospace control systems, I was working mostly outside of actual EE for 15 years and seemingly "lost" a lot of it. Lately, I've been attempting to design mostly analog test/measurement systems (for the last two+ years or so), and am still trying to learn and re-learn. So, "thanks", for the great discussions!]
--- Regarding "HF Ingress", which has also been discussed in this thread:
In addition to the intermodulation effects that have been discussed here already, one very common problem that can occur when RF gets into a solid-state electronic system (and it often/usually WILL get in, with up to tens of mV) is that, especially in places like BJT opamp inputs, BJT differential pairs and BJTs in general, et al (FETs' susceptibility is about 1/1500th BJTs'), and especially if they are low power, or biased near their "knee", a semiconductor junction can RECTIFY the RF, often resulting in the creation of a DC OFFSET or DC ERROR voltage, which may appear at some output, or cause other problems. This DC offset or error can often appear at the output of an opamp, for example, and can be quite significant, depending on the circuit or application.
There is a good discussion about all of that, in a book that was recently written by some of the people from Analog Devices, "Op Amp Applications Handbook", with Walt Jung, Editor Emeritus. The book's chapters were/are also available on http://www.analog.com , each in a pdf file. But I don't know if they are always accessible publicly. Chapter Seven has a very good practical discussion of these issues.
--- Re: Some earlier "HF Ingress" discussions/questions in this thread (Sorry. I don't have the links to the relevant messages.):
Note that the book that I mentioned does indicate that amplifier OUTPUTS _are_ also vulnerable to EMI/RFI, especially if they drive lengths of cable (e.g. speaker wires), "...which act as antennas. RF signals [sic] received on an output line can couple back into the amplifier input where it is rectified, and appears again on the output as an offset shift."
For amplifier outputs, they recommend adding series resistance or ferrite beads, or an RCR "T" filter.
They recommend low-pass filters on opamp inputs. For inverting opamps, they suggest a symmetric T filter (RCR), so the C isn't connected directly to the inverting input, which would cause instability.
However, they state that the filter bandwidth should be chosen to be at least 100 (one hundred!) times the signal bandwidth, to minimize signal loss. Obviously, I can't quite do that, if I want my amplifier's bandwidth to be over 100 kHz, but also don't want to rectify the signal from the local AM radio station (say, 1000 kHz). So, there are also very good sections about layout, grounding, and shielding, etc. They also mention choosing devices that are not faster than necessary for the application (think "edge rate").
Sorry if too much of this was off-topic, or too long. I hope that it might be helpful.
Regards,
Tom Gootee
http://www.fullnet.com/u/tomg
Practical EC schematic
Does anyone has other practical electronic schematic other than Hawksford/Cordell or NP-PMA schematic?
Is it EC schematic is only limited to this Hawksford EC?
Does anyone has other practical electronic schematic other than Hawksford/Cordell or NP-PMA schematic?
Is it EC schematic is only limited to this Hawksford EC?
jcx said:
Some years before (!) he get the patent for negative feedback he got one for error correction!
But in contrast to the here discussed "feedback error correction"
this works feed forward.
This is in principle better then negative feedback or feedback error correction, because it don´t suffer under the timelag from the stage which should be corrected.
The big technical (not paper 😉 ) problem is there the "power summer"
Regards
Heinz!