What are the difference between CLASS A and PURE

Depending on the design details, an amp labeled "Class A" may under some extreme circumstances move to Class AB operation. These conditions usually involve load and power demand.

One way to look at it is that in a push-pull power stage the higher the bias (within limits of course) the longer the stage stays in Class A operation before moving into Class AB. Class amps run with very high bias to stay in Class A under most operating conditions.

Pure Class A is the limiting case of this situation. The stage is designed and biased so that it will not move out of Class A operation under any normal operating conditions.

Note that be definition all single ended amp stages are Pure Class A. If the stage move out of Class A it clips.

In practice the difference is mostly marketing.

Think of Pure Class A and Class B as defining the endpoints of a spectrum. The zone between them is Class AB, and AB1 and AB2 are simply defined points on the spectrum.

Consider amplification of a sine wave. The entire output stage on a Pure Class A amp conducts on all 360 degrees of the waveform. The topology may be single ended or push-pull.

A pure Class B amp must be push-pull. Each half of the output stage conducts for exactly 180 degrees of the waveform before the other half takes over for the remaining 180 degrees.

A Class AB amp must be push-pull. Each half of the output stage conducts for more than 180 degrees of the waveform. The limiting case of this is that both halves of the output stage conduct for 360 degrees, or pure Class A operation.

Depending on design parameters, an amp's class of operation may vary with signal level, so that the output stage operates in Class A for relatively small signals but smoothly moves into AB operation at higher signal levels. The stronger the signal and corresponding power output, the further into Class AB the amp operates.