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FET Distortion

Discussion in 'Effects [BG]' started by boomertech, Apr 28, 2015.


  1. boomertech

    boomertech Frank Appleton Supporting Member Commercial User

    Apr 8, 2009
    Syracuse, NY
    Designer/Owner of FEA Labs
    I was working on an FET preamplifier circuit recently and thought some may be interested in seeing a very common FET distortion that is heard in many discrete FET effect pedals.

    The difference between the two circuits is just a very small modification of resistance values around the FET. You can easily see where one image has a lot more asymmetrical distortion (compressed tops) that creates even ordered distortion harmonics. A small amount of this distortion has the affect of a richer/thicker tone.

    The input signal is the red trace and the FET output signal is the blue trace. The input signal for both images is the same and it’s a composite of 200Hz and 400Hz sine wave tones of equal amplitudes.

    FET CLEAN. FET DIST.

    -Frank
     
  2. BryanB

    BryanB Moderator Staff Member Supporting Member

    I took me a while to grasp what you are saying. I believe that you are showing how two sine waves, one an octave above another, can appear closer in amplitude due to asymmetric clipping. Is that correct?
     
  3. boomertech

    boomertech Frank Appleton Supporting Member Commercial User

    Apr 8, 2009
    Syracuse, NY
    Designer/Owner of FEA Labs
    There really is no clipping (in the traditional sense) in this circuit with these examples. It is more of an asymmetrical amplification effect that occurs on the FET conduction curve.

    I used the two tone signals (one octave apart) for a more realistic looking audio source. The red trace is only in the image for an un-distorted reference and it has a different voltage scale than the FET output trace (blue). So, just compare the blue traces from the top and the bottom images. The larger positive going portions of the signal are quite ‘compressed’ in the bottom image. You will also see that the smaller positive portions of the signal (in the bottom image) have much less ‘compression’ distortion than the larger portions of the signal. So the signal distortion in this circuit, is mainly on the positive side of the center reference and is non-linear with regards to signal amplitude.

    I have been reading several posts for FET booster pedals lately, and since I was working on this circuit, I thought that some may find this common FET distortion interesting. Not every FET circuit has this much signal distortion… actually, the top image has less than +/- 0.5% linearity error for input signals up to 8V. FETs are used in many extremely linear circuit topologies, but the few popular FET booster pedals that I have looked into have varying degrees of this FET signal distortion.

    As with most everything else in audio electronics…. you can have a FET circuit that is very colored (distorted) all the way down to an FET circuit that has extremely low levels of distortion for the exact same input signal.

    I find FET signal distortions interesting.:thumbsup:

    -Frank
     

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