Feedback basics - Page 4

Johan Potgieter · 8th October 2006, 12:23 PM

Following:

Johan Potgieter · 8th October 2006, 12:34 PM

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.

smoking-amp · 8th October 2006, 05:57 PM

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

Johan Potgieter · 8th October 2006, 09:50 PM

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!

8th October 2006, 05:57 PM	#33
smoking-amp diyAudio Member Join Date: Dec 2001 Location: Hickory, NC	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 __________________ Ohms Law V = I R

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8th October 2006, 12:34 PM	#32
Johan Potgieter diyAudio Member Join Date: May 2005 Location: Pretoria, South Africa	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.

8th October 2006, 09:50 PM	#34
Johan Potgieter diyAudio Member Join Date: May 2005 Location: Pretoria, South Africa	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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