CFA vs. VFA: A Short Primer For the Uninitiated
You can download the article here: CFA vs classic Lin VFA topology
CFA topology amplifiers have been around in the IC industry for 30 years. Following a patent claim by inventor David Nelson, the earliest commercial offering was a module from Comlinear in 1982 and a few years later, IC’s from both Comlinear and Elantec. Prior to this, they were also described and analyzed in a number of papers. With regard to discrete based audio amplifiers, the topology has been used by a few esoteric brands in audio, with Accuphase, a Japanese company based in Yokohama, being a notable exponent. Cyrus, a small UK company, has also marketed CFA based power amplifiers. There are examples of Pioneer amplifiers from the early 80’s that used CFA, which apparently even pre-date the IC offerings and Mark Alexander published a design as an ADI application note in the 1980’s. CFA topology audio amplifiers continue to be somewhat upstaged by their more widely understood and deployed VFA counterparts – a situation not helped by the fact that neither Cordell nor Self touched the subject in their otherwise wide ranging audio design books. A CFA’s operation is not as intuitive as a VFA and there are some subtleties in regard to whether a transimpedance (TIS) or transadmitance (TAS) second stage is used and compnensation design in general, so designers preferred to go with something that is generally more widely documented and traditional – i.e. VFA. Although CFA audio power amplifiers have been available commercially for over 35 years (as of 2014), there is still a lot of misinformation out in the audio industry and DIY community about CFA’s, with some notable commentators dismissing them altogether. This is a pity, since they do bring very specific properties to the table that are of benefit in audio power amplifiers.