Augmented Feedback Error Correction (AFEC)
AFEC is a simple technique that augments the feedback of a low OLG CFA topology amplifier to dramatically improve the Large Signal Non-linearity (LSN) distortion performance by up to 20 dB across the audio band. Additionally, AFEC acts to remove any DC offsets and also improve PSRR significantly.
Augmenting the feedback in a manner similar to that described in this document has been tried previously – the first example is from the early 1980’s at Hitachi. However, the technique never entered the mainstream – one can only postulate that lack of very high performance opamps (bandwidth, SR and distortion performance), along with the difficultly of being able to characterize such a system for stability modeling purposes (as we can do now on LTspice for example) were contributing factors to its early demise. However, as of 2014, these problems are now solved, so perhaps we shall now see the return of AFEC as a viable distortion reduction technique.
One of the advantages of CFA power amplifiers is their very high slew rates (200+ V/us is not uncommon), but loop gains tend not to be as high as VFA topology designs. This is fine at low to medium signal levels, where the very good front end linearity and wide loop gain bandwidths of CFAs’ manifest as low distortion; however, at higher power output levels where the output stage and TAS/TIS LSN starts make itself felt, CFA distortion is generally worse, and this is as a direct result of the lower loop gains – note, I am talking here about minimalist CFA topology designs. Cross over is the major type of distortion in class AB amplifiers and there have been various schemes invented to deal with this over the years – most relying on feed forward techniques (see Peter Walker’s Quad Current Dumping for example, or Michael Renardson’s MJR7 design). AFEC does a good job of reducing cross over distortion artifacts, and in particular, those arising from OPS bias current shifts, but it will not compensate for an under biased OPS where there is a clear discontinuity in the crossover region. CFA amplifier PSRR is lower than VFA. This usually necessitates a front end stage regulator – but even then, the best designs are still usually 20~30 dB below that of VFA exemplars. It will be shown that AFEC can improve CFA PSRR such that it matches or exceeds VFA PSRR, whilst at the same time removing any output offsets as a result of the servo action of the AFEC control amplifier.