WaltJ said:
Hi Walt,
Thanks for bringing that article to the front again. The concept of 'pre-distorting' the input to complement the input stage non-linearity is quite elegant.
I am aware of only one opamp that actually has this on-chip, the LM13700 (and it's sister, the 13600), which could be seen as the archetypical 'current feedback' amp. The diodes are on the chip and a pin is provided to set the diode bias. I have used this chip open-loop and know that the technique is quite effective.
Jan Didden
MikeB said:
There's two major ways to reduce TIM for a voltage feedback configuration. One is to increase the gain-bandwidth product (provided this does not degrade input stage linearity). The other is to linearize the input stage. Or you can do both. Increasing the open-loop bandwidth while holding the gain-bandwidth product constant does nothing to improve TIM whatsoever. It only increases the low-frequency distortion. This post http://www.diyaudio.com/forums/showthread.php?postid=482046#post482046 talks a little more about that, and was one of the posts that started this thread.
Yes, Jan, thnx for pointing that out. These are not true op amps tho, as they have a gm type output stage. But, it is certainly true that their distortion is low, so low in fact that they can be readily used as gain controls.. I think I did an AES paper on this, way back when. Real way back, in fact (1974!)
Walt Jung
PS: Hey, I just thought of a neat experiment. Rig up one of these rascals in a feedback setup, with the linearization diodes switchable in/out! PIM OFF/PIM ON ...
I used them as analog multipliers to have a real-time indication of output transistor dissipation in an amp protection system, modeling - real time - the SOA. Ahh, those, as you say, were the days...
(Audio Amateur 5/83, 'New power amp alternatives: a 200W version', part II, Jan Didden).
Jan Didden
(Audio Amateur 5/83, 'New power amp alternatives: a 200W version', part II, Jan Didden).
Jan Didden
" But one can, optionally, usefully compensate for a non-linear bipolar front end. An example was shown in: Jung, W., "Optimizing Op Amp Transfer Linearity," within Boak, J.: ‘The Headphone Connection’, The Audio Amateur, Issue 3/1982. "
Any suggestions for applying this technique (parts, etc.) to the LM4780, as I have BrianGT's kit on preorder from chipamp.com and was thinking about looking back into that article for help to see if the technique could make a better (in my opinion) chipamp power amplifier...
Thanks!
Any suggestions for applying this technique (parts, etc.) to the LM4780, as I have BrianGT's kit on preorder from chipamp.com and was thinking about looking back into that article for help to see if the technique could make a better (in my opinion) chipamp power amplifier...
Thanks!
mikeks said:
I suspect you'll find TIM is male, and PIM is the alpha-female...
Once there was a nice lady by the name of 1KHz test tone.
She happened to meet a hairy guy, OpAmp he was known as.
Not being the stright and linear type, it was bound to happen. THD was born soon enough.
But the worst was yet to come. For OpAmp managed to have bussiness with a wicked chicken a few hundred hHz away.
So not at all unexpectedly, IM came to see light.
PIM and TIM twins followed suit, though people has a hard time to tell one from the other.
OpAmp is under trial now, the jury is out.
Rodolfo
IP3 for audio
Andy_C (and everybody else),
what do you think about using the IP3 point as a figure of merit for an audio system? I am actually talking loudspeakers rather than amplifiers here.
But for say a prosound system it could possibly be kind of convenient to state say IP3 = xxx dB at xx meters. That would give you a ball park figure as how clean output one could expect and if a proposed system is up to the task.
/M
Andy_C (and everybody else),
what do you think about using the IP3 point as a figure of merit for an audio system? I am actually talking loudspeakers rather than amplifiers here.
But for say a prosound system it could possibly be kind of convenient to state say IP3 = xxx dB at xx meters. That would give you a ball park figure as how clean output one could expect and if a proposed system is up to the task.
/M
For Walt Jung:
Look at this article (maybe you already have) which looks like a single-ended version of the previously discussed 'pre-distortion'. I wonder if this could be used in a LTP. Looks neat, no?
http://www.reed-electronics.com/ednmag/contents/images/91604di.pdf
(Scroll down to : "Diode compensates distortion in amplifier stage")
Jan Didden
Look at this article (maybe you already have) which looks like a single-ended version of the previously discussed 'pre-distortion'. I wonder if this could be used in a LTP. Looks neat, no?
http://www.reed-electronics.com/ednmag/contents/images/91604di.pdf
(Scroll down to : "Diode compensates distortion in amplifier stage")
Jan Didden
Re: IP3 for audio
Interesting thought. I've never looked at distortion spectra of speakers before, so I can't say. Soon I'll be getting a calibrated microphone with the ETF software, which has a distortion measurement capability. It will be interesting to see the results. I'll have to be careful to do the measurements when my neighbors aren't at home though!
Swedish Chef said:
Interesting thought. I've never looked at distortion spectra of speakers before, so I can't say. Soon I'll be getting a calibrated microphone with the ETF software, which has a distortion measurement capability. It will be interesting to see the results. I'll have to be careful to do the measurements when my neighbors aren't at home though!
Yes, I did see that. But not applicable as presented to diff pairs. If you go all the way back to Gilbert's 1968 (I think) IEEE paper on the gain cell, it is clear what is needed to compensate for the diff pair tanh function.
But, for whatever the reasons, no such correction (to my knowledge) has ever been used in a general purpose op amp.
There is another scheme that I have used, which was published in a gain control circuit, for ED magazine. I should dig it out and post it. It used a diff. pair wrapped within a feedback loop of one op amp, and the drive to this diff pair then being taken to a second diff. pair, which serves as a current-controlled gain stage. This was copied by Craig Todd in the Signetics 570 series of gain control ICs.
Walt Jung
Jocko Homo said:
But what about the AD846? I remember an earlier posting of yours about a circuit you had designed (DAC buffer maybe?) that used the 846 and people had an unfavorable subjective reaction to it. I don't understand this at all, having had intimate experience with that op-amp in a radar application. The electrical properties of that op-amp that we measured are outstanding, both in the time domain (settling time and rise time under all kinds of difficult conditions) and the frequency domain (low distortion). I don't have a spec sheet handy, but I suspect differential gain and phase are better than most audio op-amps. Wyn Palmer did a simply superb job on it in IMO. So what does this mean? Are electrical measurements not to be trusted, or are people not to be trusted? I suspect the truth is somewhere in between. Measurements need to be improved, and people's views should not be accepted at face value.
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