Floating (Synthetic) Inductor

[Jason Stout]

I was reading the varicap thread and it reminded me of a circuit I had seen while watching the Bob Pease Show titled "analog computers". I looked it up again and redrew the circuit.
...I know at least one person that would like to see a floating simulated inductor.  

[Mike Burgundy]

Yah, me.
Thanks

[Transmogrifox]

Sedra/Smith Microelectronic circuits has one of those types of circuits.  It actually looks similar to that. I would have to look more closely at it and see if it isn't the same one.  It's actually noted to be a very stable circuit not given to op-amp deficiencies and component variations.

[Jason Stout]

I have an older edition of Sedra/Smith, what's in mine (if I'm not mistaken) is half of the circuit above which results in an inductance to ground.
And like the Grounded Inductor Gyrator, The inductance of the above circuit is:
in series with R1 & R3
In parallel with R2 &R4

[zachary vex]

here's a circuit that requires no power supply to simulate an inductor, which is adjustable by selecting the proper cap for C1... the synthesized inductance is 1 henry per 2 uF capacitance.

of course, it's not floating, being that one end is tied to +V, but it's small and cute!  8^)

[Jason Stout]

Some documentation for Zachary's circuit can be seen here.
Thanks Zach.

[David]

here's a circuit that requires no power supply to simulate an inductor, which is adjustable by selecting the proper cap for C1... the synthesized inductance is 1 henry per 2 uF capacitance.

of course, it's not floating, being that one end is tied to +V, but it's small and cute!  8^)

Very interesting...   :idea:
So to make a 500mH inductor for a wah, you substitute a 1000uF cap for xx.  Cool.  Can you help us visualize how it would patch into the circuit?
Can a 3904 or 5089 be subbed for the BC547?

[zachary vex]

Some documentation for Zachary's circuit can be seen here.
Thanks Zach.

great!   i didn't remember where i ran into it... and that site doesn't look familiar, but i bet the image has been parked all over the place.

[stm]

Man, I see a lot of potential into this circuit!

I want to try sagging power supply, where I need to have a series inductor fed from Vcc and a shunt capacitor.  The load will vary according to audio output, sagging the supply voltage that feeds the circuit.

Code Select Expand


      Ls
Vcc--()()()--+--+----to variable load
            |   \___to circuit supply
            =Cs
            |
           GND


Regards,

STM

[zachary vex]

here's a circuit that requires no power supply to simulate an inductor, which is adjustable by selecting the proper cap for C1... the synthesized inductance is 1 henry per 2 uF capacitance.

of course, it's not floating, being that one end is tied to +V, but it's small and cute!  8^)

Very interesting...   :idea:
So to make a 500mH inductor for a wah, you substitute a 1000uF cap for xx.  Cool.  Can you help us visualize how it would patch into the circuit?
Can a 3904 or 5089 be subbed for the BC547?

oh, no, you'd only use a 1uF cap for 500 mH.  2uF gets you 1000 mH or 1H.  

unfortunately, there's no easy way to use it in a typical wah circuit because one end of the inductor is not connected to + power in that circuit.  from rg keen's site:

[David]

Dang it!  I KNEW there would be a catch!   :cry:  :cry:  :cry:

[puretube]

that`s an example, where you need a floating inductor for...

[R.G.]

The standard wah inductor is indeed floating. However, one end is AC grounded through that big cap to ground. I think it is possible to reformulate the circuit to use a grounded inductor.

On another thread, the variable cap:
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.

[puretube]

next month will feature some linx to tube-gyrators and capacity-multipliers, if I don`t forget.....