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From: Christopher Howard <christopher@librehacker.com>
Newsgroups: sci.electronics.design
Subject: Re: cheap analog square function?
Date: Mon, 16 Feb 2026 09:36:05 -0900
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> 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.

I do have access to a good benchtop signal generator, which I could use
for generating the triangle wave. But having a separate triangle wave
generator built in to the project also has some appeal.

-- 
Christopher Howard