FET phaser Distortion and Harmonics

[markphaser]

The FET distorts because of the
               1.) Time constants?
               2.) Slew rate?
               3.) Bandwidth?

[MR COFFEE]

The FET distorts because of the
               1.) Time constants?
               2.) Slew rate?
               3.) Bandwidth?

None of the above. The FETdistorts because

1.) The FET behaves as a Linear resistance only for small values
     of source-drain voltages Vds in either polarity

Non-linearity above small values of source-drain voltage + higher amplitude of audio signal (which equals larger values of source-drain voltage) = distortion.

AN-129 shows measured curves of the linear and non-linear regions of conduction for JFETs.

[markphaser]

The FETdistorts because why?

[gez]

The FETdistorts because why?

This is my understanding, so no guarantee it's correct!  Think of the hydraulic analogy and think of a FET as a funnel.  Pour water in at a slow rate (low voltage) and what trickles out of the narrow tube does so at the same rate as what trickles in.  But, increase the water pressure (higher voltage across the channel) and the funnel starts to fill up - the rate at which water is going in is greater than the rate it's coming out.  Any further increase in water pressure does little to the stream of water at the other end coming from the tube - it can only come out at so fast a rate. 

FETs operate in a similar manner - the field controls how narrow the funnel's tube is, so to speak.  With small voltages across the channel current flows in a fairly linear fashion, but large increases result in distortion (funnel fills up).

If the above is complete b*llocks then my apologies, but you pays your money (nothing) and takes your choice!