AW: volt controlled synthetic capacitor?

Haible Juergen Juergen.Haible at
Mon Dec 15 16:26:44 CET 1997

	> All I want is a one chip sin oscillator that goes from say 0.1
Hz to 
	> 20KHz with say 0 to 5 volt control voltage.... 
	>please dont tell me not to use the ICL8038,

I wanted (how did you know (;->) ), but then:

	>BTW, is there a cit\rcuit that 'synthesises' a capacitor the
way a 
	>gyrator synthesises an inductor?

It is possible, I have seen circuits with OTAs that simulate
a voltage controlled capacitor (would have to search
for them - maybe it's a 13700 application, if memory serves it's
easy for grounded caps, and slighly more tricky for floating ones,
and you'd surely have a parasitic resistance across the cap ...),

but the whole thing runs down to a not-single-chip solution
(at least one 8038 + the otas to simulate the cap), so
where is the advantage?
Even if someone would give me these 8038's for free,
I'd probably go a different path: triangle VCO + waveshaping.
(What's inside the 8038, btw ??)

If you want a dirt-cheap *almost* single chip solution, start
with a 13700. Use one ota + buffer as integrator, two
discrete transistors (this was the "almost") for a hsyteretic
switch, the second ota + buffer for tri/sine waveshaping.
Expect 2% distortion, and slight offset voltage problems.

If you want a better solution, I'd suggest 3 chips:
TL074, CA3080, and MC1496. Use the 074 for
buffers and for hysteretic switch, the 3080 as integrator,
and the 1496 for precision tri/sine shaping.


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