This may seem like a strange question, for a feedback amplifier it is only the closed-loop distortion which ever appears at the output. What prompted this piece was having seen numerous statements to the effect that starting with a highly nonlinear open-loop response and 'straightening it out' with lots of feedback was a really bad approach. Having designed an amplifier, the MJR6, with more than 10% open-loop distortion and 66dB feedback at 20kHz I feel I should comment on this.
This idea that high open-loop distortion is a problem is often associated with a series of articles by Peter Baxandall in Wireless World in 1978, but possibly began even earlier with an article I mentioned on another page:
"In 1961 M.G.Scroggie published what at first sight appears to be an analysis of almost exactly the same circuit arrangement used by Peter Baxandall. This was reprinted and updated in Wireless World, Oct 1978, p.47-50, the author writing under the name 'Cathode Ray'. A square law device has overall feedback applied, but with 40dB feedback there is now over 7% third harmonic, 3% 4th, and so on."
These figures are far worse than Baxandall's results, but they are based on about 8dB higher open-loop distortion, yet the Baxandall figures with 40dB feedback are far better, e.g. 3rd harmonic is about 60dB lower, so what is really going on?
The factor which is frequently overlooked is that the Scroggie article is really about clipping, and the signal level used was such that the open-loop gain actually fell to zero at one point on the wave. The '40dB feedback' only applied to zero input voltage, and was higher for one polarity, and lower for the opposite polarity, eventually falling to zero. This is not therefore a failure of feedback to reduce distortion, it is really a failure to apply any feedback at all at some output level. The feedback loop gain actually varies from zero to over 40dB with this signal.
My MJR6 has over 10% open-loop distortion measured at 6V output, well below clipping. Over this peak to peak output voltage range the feedback loop gain probably varies from about 65dB to 67dB, so there is still high feedback over the entire output wave. The result is extremely low closed-loop distortion, with third harmonic 0.006% at 20kHz. At 1kHz loop gain is around 80dB and closed-loop third harmonic is -120dB.
A common idea is that open-loop distortion is not really eliminated by feedback, it is merely transformed into another form, for example the dreaded 'high order harmonics'. As I have mentioned elsewhere, there are transfer functions for which the addition of feedback makes no difference to the relative levels of the harmonics, and the square-law example used by Baxandall is just an extreme example where harmonics not originally present are produced when feedback is added. Even then using high levels of feedback the harmonics will fall in level as the order increases.
To summarise, a given percentage open-loop distortion such as 10% can involve incremental gain variations which reduce feedback to zero, e.g at clipping, or at zero-crossing with an unbiased class-B output stage, and in this case trying to 'straighten out' the transfer function with feedback is doomed to failure because when the open-loop gain is zero there can be no feedback. The MJR6 has none of the problems associated with high feedback or high open-loop distortion, and is an excellent counter-example. High feedback in this case really does 'straighten out' a relatively mediocre open loop response. With the addition of one more transistor to improve the open-loop linearity the MJR7 has even lower closed-loop distortion, but really the MJR6 was already excellent.
