"Gyrator" equivalent to capacitors?

[strungout]

Oy.

Just wondering if there was such a circuit that would fill in for a capacitor in a similar way that a gyrator does for an inductor?

Ciao.

[Transmogrifox]

Well...

you need a reactive element to model a reactive element.  The purpose of a gyrator is to simulate an inductive impedance using capacitors, which are cheaper and less susceptible to noise and external interference.

You could simply reverse the gyrator design using an inductor and simulate a capacitor, but that would not be very practical unless you wanted to make a variable capacitance...in which case you should use something similar to the crybaby circuit where the feedback network and pot more or less create a variable capacitor.

You can use diodes as capacitors where the capacitance depends on the magnitude of the reverse voltage potential on the diode...

[R.G.]

There is, but not perhaps like you are thinking.

A gyrator is a device that transforms an impedance into its complex dual impedance. A capacitor may be made to look like an inductor by clever gyrator circuits.

By similar methods, an inductor may be gyrated into its dual - a capacitor. So if you want to simulate a capacitor by gyration, you first have to have an inductor. That's why you've never seen a capacitor-simulating gyrator - capacitors are cheaper and more perfect components than practical inductors, so it's like shoveling sand against the tide to make a capacitor by gyrating an inductor.

You cannot gyrate a resistor into either a capacitor or an inductor. There's a long winded explanation, but it's not possible.

There are capacitor multiplier circuits. These are active circuits that have a capacitor in them that give the appearance of a bigger capacitor at another terminal. This is how the Vox Wah circuit works - see The Techology of Wah Pedals at GEO for how this works. There are also some circuits used in power supplies that are called capacitive multipliers that are used for filtering purposes.

Did any of those answer your question? If not, ask further.

[zachary vex]

here's a gyrator that makes 1 Henry inductance per 2uF capacitance inserted where the cap goes.

[puretube]

if I remember this thread after holidays, I`ll point to, ar draw a variable cap 100p to 100µ with 2 opamps - floating version and "to ground" version - very simple....

now on with vacation  

[Ansil]

if I remember this thread after holidays, I`ll point to, ar draw a variable cap 100p to 100µ with 2 opamps - floating version and "to ground" version - very simple....

now on with vacation  

i would like to see that    also a tube version too..  

[strungout]

Oy. Thnx for the replies.

Well, yeah, actually I was thinking about a variable capacitor. Those I,ve seen at my parts supplier are from something like 5pf to 10pf, which won't make much of a difference if used in the EF of the Dr. Quack, for exemple, to adjust decay, or inthe pulsar to adjust chop. Anyway, my question's been answered I'll have to consider it more seriously sometime in the future.

Ciao.

[puretube]

if I remember this thread after holidays, I`ll point to, ar draw a variable cap 100p to 100µ with 2 opamps - floating version and "to ground" version - very simple....

now on with vacation  

sorry folks, I mixed up a couple of schems - don`t have a floating cap -,
- don`t have a 2opamp cap - (at the moment...)

BUT: here is the 4 transistor VARICAP, variable 1:1000 (ccw to cw),
Cmin= 30pF;

(the source [Elektor] I got it from, claims it can go to "100F",
which I guess is a typo... - me thinks, it`ll go from 100pF to 100nF,
with the parts dimensioned like shown in the schem.)

[lightningfingers]

Wow. Puretube: think that'll work down to +/- 12V? And what will changing "C"(100n) do?

[DDD]

Hi all,
How do you think, is it possible to emulate variable capacitor with the help of DAC but with C-2C set (matrix) instead of traditional R-2R "ladder"?
Of course it will  not be a complete simulation of variable cap, but "switchable" capacitor made of few usual caps?

[puretube]

I`d rather do the switched-capacitor-thingie: 4066/PWM/cap....
(if I`d want to get glitchy);


Lightning: I have never tried that circuit, but think it`ll work @ 12V;
Transistors are universal types like BC108/178;

There was not much theory in that article from the early 70`s....

Cvar is being virtually created at pins 1 & 2;
Cmin=30pF (=transistor`s junction-capacity) with wiper set to ccw;

My guess is: a C-multiplier,
where if C=100n, Cvar will go from either
100n to 100µF,
or it works the other way `round: from 100pF to 100n;

[strungout]

Ooo nice. Thnx puretube! Gonna have to try it out.

[stm]

I simulated the circuit posted by puretube and converted it into a "DIY guitar FX" opamp version:



I verified capacitance remained constant while sweeping frequency from 50 Hz to 20 kHz, and stepping the pot in 10% increments, as shown below:



This circuit has good potential also for tone controls and similar stuff.

Regards,

STM.

[puretube]

WOW       THANX!!!

great, stm:
it does indeed look very similar to a gyrator now, with that
"cat-chases-tail" opamp configuration.

Note, that in the "Elektor"-schem, you don`t need that input cap, cause
it`s got bipolar supply with complementary transistors...
(I`m sure you can do the same with opamps...)

Now we need a version with a variable resistor (2 connections),
instead of a rheostat (=pot, 3 connexions)....
so we can use an LDR, FSR, 4066/PWM, and so on...

btw: the headline of that article really was: "Variable Capacity up to 100 F"

[stm]

Hi, to control the circuit with just a LDR (photoresistor) I think you have to replace the pot with:

Code Select Expand


    100k
-----////-----+---to second opamp
             |
             /
             /  LDR: 1k to 400k
             /
             |
            GND


This would allow to vary the C1 value roughly from 1% to 80% C1. (80:1 ratio).

By the way, if the input to the varicap is already referred to Vref (i.e. floating around 4.5V), input cap C2 can be omitted.

Also (though I have not verified it) if you convert the first buffer stage to have 10x gain, I think max capacitance should go to 10x C1. This is untested, and eventually you may have clipping problems!

Regards,

STM.

[Elektrojänis]

Also (though I have not verified it) if you convert the first buffer stage to have 10x gain, I think max capacitance should go to 10x C1. This is untested, and eventually you may have clipping problems!

But it might be cool or at least interesting clipping depending on the rest of the circuit.

[DDD]

Great circuit!
At the same time - is the 2nd Op Amp necessary? It's just a voltage follower, and if the input source has enough impedance, the X5 is not necessary and the 100n capacitor can be connected directly to X3 pot. To get better results X3 can be about 1 to 2 kOhm as well as R2 and and R3.

[lightningfingers]

sweet!

Thnx puretube!

[/b]

[puretube]

Lightning: thanks belong to STM!

DDD: C1 should be driven by a constant, low impedance - which the pot (be it 10k or 2k...) isn`t.
the first opamp, btw., also is "just a follower"...

STM: with gain, there might be a gain/bandwidth- or an offset-problem too (?). This would call for a fast FET-input opamp...


The "opamp-varicap" circuit may be dual-power-supplied, with the virtual cap, and the pot referenced to ground, and the non-inverting inputs via 1M or higher resistors DC-grounded?

for further thoughts see the other thread:
http://www.diystompboxes.com/sboxforum/viewtopic.php?t=25504

[stm]

DDD:

I tried two different aproaches to avoid one opamp:

1) Eliminated X4 and replaced the pot (X3) with 1 Meg. This behaves reasonably well.  Of course the 1 Meg pot appears in parallel with the capacitor, deviating from ideal behavior.  This is mostly noticeable when C1 is multiplied for small values, like 10 nF or less.

In other words, you have to think if your circuit can withstand a 1 Meg resistor in parallel with the cap.

In this case the 1 Meg biasing resistor was removed also, since the pot is now acting as a biasing network.

In summary, this effectively allows saving an opamp.  Also, the pot can be made LOG for better cap control in a 100:1 ratio.  One drawback is that the pot cannot be replaced by a LDR due to the high impedance requirement.

2) Eliminated X5 and replaced the pot (X3) with 1 k. This adds some NEGATIVE resistance (5 k at 100 Hz for maximum capacitance).  Don't know if this would bother or make unstable a circuit at this point, so 1) is preferred in case you just want to save an opamp.

This is the simplified circuit:



Regards,

STM