XPost: sci.electronics.design   
   From: pcdhSpamMeSenseless@electrooptical.net   
      
   chuck  wrote:   
   On 2024-11-18 3:14 p.m., Phil Hobbs wrote:   
   >   
   >>   
   >> Any oscillator with a nonlinear or bilinear gain control element that   
   >> has to respond during a cycle has to deal with the distortion caused by   
   >> that element.  OTAs, JFET variable resistors, PIN diode attenuators,   
   >> Vactrols, light bulbs, and so on, all have that problem.  Tail current   
   >> sources can avoid it, because you can make them as stiff as you like by   
   >> cascoding, and filter the control voltage as well as you like.  (I often   
   >> use two- or three-pole capacitance multipliers on the supply rails of   
   >> discrete circuitry, which is a similar idea.)   
   >   
   >>   
   >> Cheers   
   >>   
   >> Phil Hobbs   
   >>   
   >   
      
   > The FET regulator is a filtered servo with fast cutoff and very slow   
   > build up over thousands of cycles. The same is done with other   
   > non-linear limiters using exponentially orders of magnitude higher   
   > resistance when regulating so the decay rate is slow and low distortion.   
   >   
   > Phil I think you had different assumptions for your au contraire opinion   
   > on tail currents vs quasi-linear low decay currents when regulated   
   > effectively.  I don't see the difference except a non-linear device   
   > starts up faster.   
      
   You’re apparently missing the distinction between a control element that   
   has to respond within a cycle, such as a JFET variable resistor used as a   
   feedback element,   and one that just sits at a very slowly varying   
   operating point, such as a cascoded BJT tail current source with a big   
   emitter resistor and lots of bypassing on the base.   
      
   The first kind gets run through its (inevitably somewhat nonlinear) I-V   
   curve on every half cycle, regardless of the bandwidth of the control loop.   
   This contributes an amount of distortion that isn’t improved by narrowing   
   the loop BW.  The second kind’s distortion can be reduced to any desired   
   degree by careful design.   
      
   The active element’s nonlinearity is of the first kind, of course, but   
   that’s just amplifier design.   
      
   Cheers   
      
   Phil Hobbs   
      
   --   
   Dr Philip C D Hobbs  Principal Consultant  ElectroOptical Innovations LLC /   
   Hobbs ElectroOptics  Optics, Electro-optics, Photonics, Analog Electronics   
      
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