PMA said:
Hi Pavel,
Some of your results and observations are a bit inconsistent with my earlier posts showing simulations of the effect of NFB on a Class-AB output stage.
In my case, I used MJ21193/MJ21194 output transistors with 0.47 ohm RE and over-biased at 152 mA so as to exhibit gm doubling crossover distortion. The load was 8 ohms.
My results are shown below. They show a very well-behaved and predictable reduction in all harmonics as NFB is applied and increased. Note that this was a simulation at 1 kHz and about 10 V p-p into the load.
I wonder if the differences between our results are due to the use of different models or due to my operating into the gm-doubling regime. I did the simulation in the gm doubling regime because I usually prefer to err on the side of over-biasing an output stage (within thermal stability considerations), so gm-doubling distortion is what I am most interested in seeing reduced by NFB.
Cheers,
Bob
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Bob Cordell said:
Hi Bob,
as I have explained earlier today, I found a significant error in the 2SA1302/2SC3281 models that I used in MC8. So please forget the post (image) you are referring at. The 1st valid simulation of crossover residual should be that of 21193/4 with 0.33 Re into 4 ohms:
http://www.diyaudio.com/forums/showthread.php?postid=1288036#post1288036
Regards,
Pavel
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PMA, your last graph demonstrates why I like to use high I(q) in my output stage as well, when I can.
What would be REALLY useful is the LOWEST I(q) that we could use and still have VERY LOW 7th harmonic. ONLY very low 7th harmonic is what is necessary here. Everything else can be compromised somewhat. How could we do that?
What would be REALLY useful is the LOWEST I(q) that we could use and still have VERY LOW 7th harmonic. ONLY very low 7th harmonic is what is necessary here. Everything else can be compromised somewhat. How could we do that?
Bob Cordell said:
I think that many factors besides just the quiescent current of the output stage should be considered when trying to reach a conclusion as to whether or not an amplifier is of good design or not.
There are literally millions of perfectly well engineered and performing BJT-output amplifiers in existence in the 50mA Iq category. A designer may choose to skimp on Iq and use a higher RE for gains in efficiency (resulting in slightly higher crossover distortion) but choose not to roll off their open loop bandwidth for a unity gain cross-over at a meagre 200kHz.
Cheers,
Glen
PMA said:
Hi Glen,
could you, please, mention just one?
Regards,
Pavel
Just one? Try about almost any up-market Japanese amplifier with discrete BJT outputs.
Heck, the last high power (~250Wpc) Marantz I repaired ran significantly less than 50mA Iq for each of the paralleled BJT’s. Very nice, well performing design, though I can’t recall the model # off the top of my head ATM.
Cheers,
Glen
john curl said:
I do not know how we could do it.
Now the graph for 3.5W/4 ohm. Again, only CFP output stage biased at 900mA. It is all quite fine until we get into AB. Then higher order h. start to rise, and only NFB can decrease them. Which is not the best way to do it.
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PMA, the whole point is to be in the class AB transistion. Anybody can do OK class A. It is the 7th harmonic generated during the class AB transition that is the MOST important factor to me. It 'may' be that there is an optimum Re to Iq ratio that is not optimum for lowest overall THD, BUT is best for lowest 7th Harmonic distortion for a given Iq. This is where a computer could be very valuable. For example, I usually recommend 15mV-25mV drop across the resistor. However, maybe 35mV gives lowest 7th harmonic distortion at the transistion. I don't know yet. Or, it might be 9mV gives lowest 7th harmonic distortion. Or maybe there is NO 7th harmonic minimum for a given Iq. Who knows?