From: jl@glen--canyon.com   
      
   On Fri, 13 Feb 2026 20:42:27 +0000, piglet    
   wrote:   
      
   >On 13/02/2026 8:06 pm, john larkin wrote:   
   >> On Fri, 13 Feb 2026 19:36:23 +0000, piglet    
   >> wrote:   
   >>   
   >>> On 13/02/2026 3:40 pm, Christopher Howard wrote:   
   >>>> piglet  writes:   
   >>>>   
   >>>>> If high precision is not needed and only LF response is enough then I   
   >>>>> have used PWM techniques:   
   >>>>>   
   >>>>> Circuit A is the basic squaring concept, relies on modern open-drain   
   >>>>> output comparators being pretty good switches to ground. Assumes you   
   >>>>> already have a source of sawtooth or triangle waves with defined zero   
   >>>>> and peak values in the system.   
   >>>>>   
   >>>>   
   >>>> Could you please explain better how the PWM squarer circuit works? It   
   >>>> look like it would be easy to build, but I don't grasp what is going on.   
   >>>>   
   >>>   
   >>> JL has beaten me to it. In full detail: imagine the sawtooth or triangle   
   >>> wave swings between 0V and 1V and is nicely linear, i.e. straight   
   >>> sloped. The comparator in my circuit (A) compares that with the input   
   >>> voltage to be squared Vx. When Vx is 0 the output is always low; when Vx   
   >>> is 1V the output is always high; and when Vx is 0.5V the output is   
   >>> toggling with 50% duty cycle.  This pulse width modulation can then be   
   >>> used to switch the desired output between input Vx and 0V so when Vx is   
   >>> 0V then output is zero, when Vx is 1V the output is Vx and when Vx is   
   >>> 0.5V the output is 50% of the time that 0.5V and 50% of the time zero or   
   >>> in other words an average of 0.25V. The low pass filter averages the   
   >>> pulsating output to a steady level.   
   >>>   
   >>> In circuit (A) the switching isn't done by an explicit analog switch but   
   >>> by shorting the output to ground - very conveniently done by choosing a   
   >>> comparator with open-drain output.   
   >>>   
   >>> I had multiple square and root channels so it made sense to have one   
   >>> common triwave generator serve them all. Instead of 1V full scale unity   
   >>> I scaled to use 5V so ordinary comparators can be used. If you had no   
   >>> other need for a sawtooth/triwave elsewhere one could build a   
   >>> self-oscillating PWM generator and use that to drive the chopper. Let me   
   >>> know if you want that detailed more fully.   
   >>>   
   >>> piglet   
   >>>   
   >>   
   >> You can do a pretty good voltage-to-PWM with a single opamp and a few   
   >> passives.   
   >>   
   >>   
   >> John Larkin   
   >> Highland Tech Glen Canyon Design Center   
   >> Lunatic Fringe Electronics   
   >   
   >Yep, a comparator will do too and avoid opamp slew rate.   
   >   
   >piglet   
      
   Some of the rrio opamps make good comparators.   
      
   Most are compensated by, in effect, Miller caps on the complementary   
   output mosfets. So there's no deep-inside compensation node to wind   
   up.   
      
   My gumdrop opamp is OPA197, which works as a nice rrio  comparator,   
   something like 2 us as I recall.   
      
      
   John Larkin   
   Highland Tech Glen Canyon Design Center   
   Lunatic Fringe Electronics   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)