"Gyrator" equivalent to capacitors?

[StephenGiles]

STM - you're at it again!!! That's a very clever circuit indeed. Could this be made to work for tiny capacitors of say 22p up to maybe 150p. I'm thinking variable flanger clock timing capacitors here!!!
Stephen

[stm]

Stephen, I think it will depend on the particularities of your clock circuit, and the quality of the opamp in use.

If you are talking about a clock that will have sharp edges and may reach near 1 MHz maybe you'll need a fast opamp so the circuit behaves indeed like a capacitor. I would try the circuit with a TL072 or equivalent opamp (13 V/us slew rate).  

Also, small capacitors may have a large impedance at low frequencies, but at clock frequencies they may not be large, so I guess the circuit will tolerate well the 1 Meg pot.

Just give it a try!

STM.

[puretube]

the simplified circuit now looks very much like a tube-circuit I`ve seen in an inductorless tube-equalizer.....

[puretube]

well,similar???

[stm]

Very nice.  That circuit must sound sweet, especially as a headphone equalizer!

I remember when I was a kid hearing to a Gründig tube stereo equipment with tube power amp and electrostatic speakers.  Listening to classical music played on a vinyl disc sounded awesome, with very deep bass.  

Later I found out that electrostatic speakers don't have that much bass, but due to the moderate 2nd order harmonic introduced by the valves (and thanks to psychoacoustics of human hearing), you perceive reinforced bass. In fact, I've heard of a (german?) patent applied on old audio amplifiers that exploit the 2nd harmonic generation of a transformer in the audio path for this purpose.

On another line of thought, if you manage to move the frequencies to 100 Hz, 700 Hz and 5 kHz and you replace the valves by FETs you could make a nice guitar equalizer.

Regards,

STM

[R.G.]

From the inductor thread: if you look at it properly, the variable cap simulation works in a way similar to one version of my explanation of the Vox Wah. A cap is fed a variable amount of an AC signal (in the wah from the wah pot, in the emulator the variable signal pot) then that variable amount of signal is fed through a voltage buffer and through the cap being "varied".  The variation in AC signal currents fed through the actual cap makes for a variable amount of signal current phase shifted through the real cap, so Ohm's law as generalized for impedances says that the capacitance seen at the external node looks like C = V/Ic where V is the signal voltage and Ic is the current that gets through the capacitance being varied. Since we varied the amount of AC signal getting through and that varies the Ic, it looks like a variable cap for AC signals, and the voltage buffer driving it looks like "ground" to the cap.

The variable capacitor circuit should work with emitter followers, and *might* work as a capacitor multiplier by putting some gain on the second opamp. The first buffer just isolates the variable volume pot from loading down the emulated capacitor node, and could be ignored for large pot values compared to the emulated and real capacitors connected there.

[DDD]

stm,
I have to say your method of saving opamp looks more elegant.
At the same time it seems to me that if you're trading quality for simplicity of the circuit, you'll get non-ideal device in any case. Also I think that there is no need to base guitar gadgets on ideal building blocks since guitar electronics is not an extremely accurate measuring or control equipment ;-)
And, last but not least: since opamps are loaded with considerable capacitance, they become very non-ideal and maybe unstable. Such a designs need to be very thoroughly arranged and checked up.

2 StephenGiles:
Why do you want to emulate such a tiny caps with opamps? As far as I know there are real varicaps available - some kind of special Zener diodes with their capacity being dependent on biasing voltage - up to 1000 pF with tuning rande at least 10:1 or so. They are widely used in RF applications to control frequency.

[puretube]

B*ss, K*rg, Ib*nez, got nice transistorbased vari-clock BBD circuits...

[StephenGiles]

DDD - because we English like to know these things!!! Welcome to the site I don't recall seeing you here before.
Stephen

[stm]

DDD:

Yes, there are varicaps that allow to do so, but for them to behave properly you must comply with several things:

1) Bias them with a DC voltage

2) The AC signal must be blocked from getting into the DC control path, usually via an inductor (ouch!)

3) The AC signal must be relatively small so its actual value doesn't change significantly the varicap capacitance. Usually voltages as low as 0.5 or even 0.1 volt are used on this circuits, which are not adequate for an inverter-based RC oscillator.

Anyway, you can use to a classic RF oscillator topology like a Hartley or Colpitts oscillator (not sure which of them is easiest to tune with a varicap), but I think it can be more hassle.

Regards,

STM

[puretube]

B*ss, K*rg, Ib*nez, got nice transistorbased vari-clock BBD circuits...

see an example here:

http://www.geocities.com/CollegePark/Library/1355/Bf2.gif

around Q5, in a B*ss BF-2 Flanger...

or here: http://home.hetnet.nl/~chrisdus/download/ce2.gif

[DDD]

Hi guys.
Stephen, thank you for your welcome. Pleased to meet you, too. I'm a newcomer here on the Forum.
As far as I understood you were discussing "digital" CLOCK frequency about 1 MHz or so. Maybe I'm not right? I'm native Russian and my English is not good sometimes.
In any case:
- certainly "Zener" varicap is to be biased with DC as well as any DC-controlled oscillator
- since DC bias on the Zener varicap  is usially below its junction breakdown, it's biased via high-Ohm resistor that is enough for blockung AC voltage without any inductors
- if we're speaking of "digital" clock oscillations, linearity of control element is not important at all
- "digital" clock oscillator can be based on CMOS Schmitt trigger with RC feedback; in that case the easiest way to control frequency is to have variable resistor (FET, LDR and even usual BJT) instead of variable cap
- also, CA3080 or any other OTA is an excellent building block for building DC-controlled high frequency (up to 1 MHz) oscillators.

[puretube]

SOT: I prefer 4046 for BBD variclock,
gyrator for audio.
(if coils are "impractical")

[D45T]

Hello there. Did anyone by chance save the op amp version of the variable capacitor circuit schematics? The original images are dead.