CFA vs. VFA: A short primer for the uninitiated – Extensively Updated January 2014

There are many explanations about IC CFA topologies like this or  this. Some plunge into math, loop gain equations and so forth, leaving the reader none the wiser, while this one (equations 1~4 and associated gain plots) from Hans Palouda is altogether easier to understand, as is ADI’s here. For VFA, Bruno Putzeys’ explanation is by far the most succinct, even though the main thrust of his article is to dispel some enduring myths about negative feedback. Which  brings me to the reason for this short document: ‘CFA vs. classic VFA– a short primer for the uninitiated’ in which I will try to explain the differences between the two topologies, dispel the myths and hopefully encourage more audio power amplifier designers to experiment with this technique.

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.