[quote author="mikep"][quote author="hagtech"]
First lesson is to reduce the forward open loop distortion. [/quote]
Maybe I should have clarified, this is what I am doing. Im trying to get a handle on the sources of *open loop* distortion.
[quote author="hagtech"]
Odd harmonics come from differential circuits - as they tend to cancel out the even ones.
[/quote]
And this is the primary reason I am interested in locating sources of odd harmonics. Is it possible to design a differential circuit that does not have predominantly 3rd harmonic? Seems like the only way to do that would be to locate the mechanisms that are causing the distortion in the first place.
mike p[/quote]
There is a good discussion of distortion in Dennis Feucht's Handbook of Analog Circuit Design. Since it is virtually unobtainable, PM me here with an email addy and I could be persuaded to copy an excerpt, or if you can spring for the coin find his website and get the CD ROM that is the revised version of the book.
Even-order mechanisms are basically due to asymmetry around the quiescent operating point of an instantaneous gain curve. If my volts out for a given volts in curve looks like the right-hand half of a parabola (or more likely the upper of lower half of a y^2 = x parabola), I will see second harmonic in the output of an input sinusoid, along with a d.c. shift. That is, in this situation my instantaneous (i.e., no memory effects, phase lags, etc.) gain is progressively higher on one side of my operating point on the parabolic segment than on the other.
If, instead, my gain curve looks like an S curve and my operating point is at the center of the inflection, my gain is falling off symmetrically either side of the operating point, and I will get at least 3rd harmonic distortion. Jim's comment is correct in that balanced and/or differential arrangements of things with second-order will tend to cancel the second if the sections are matched, and you are right that it is tougher to cancel odd-order.
Some ways besides feedback involve predistortion of the signal well-matched to the nonlinearities of the amplifying devices. An example: Barrie Gilbert's current gain cells using bipolars.
You also might want to check out the thread I started in the Drawing Room discussing a paper by Boyk and Sussman: http://www.groupdiy.com/index.php?topic=13703
By the way, good for you to use your temporary spare time to study this stuff!
EDIT: for typo and grammar