From: jl@glen--canyon.com
On Wed, 18 Feb 2026 00:03:47 +1100, Bill Sloman
wrote:
>On 17/02/2026 5:36 am, Christopher Howard wrote:
>>> 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.
>>>
>>
>> Okay, thank you. I think I grasp the core idea now.
>>
>> My THAT comes with a few comparators, but already less than I need for
>> other things, so I don't want to use those. But I've got a lot of 741 op
>> amps to spare.
>
>The 741 isn't a particularly good op amp.
And it's a really bad comparator.
John Larkin
Highland Tech Glen Canyon Design Center
Lunatic Fringe Electronics
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