WHOA! Negative Capacitor in LowPass

Whoa! Again! Fails transient analysis, blows up... dreams of zero-phase dashed :-( ...Jim Thompson

Should of just tried a lowpass with C=-1nF without the active elements... that blows up as well.. which should be obvious if one just stops and thinks for one moment:

I = -C*dV/dt Kaboom! ...Jim Thompson

If you're just interested in network response, and not practical implememtation, LT Spice is happy to let you use negative r/l/c component values.

Yep. PSpice also. But I got so wrapped up in my amusement with the active-element negative capacitor that I failed to check the simple C=-1nF approach... until the active-element approach produced giga-volts when submitted to transient analysis.

(But it'll pass a dotAC analysis... which leads you down the garden path :-) ...Jim Thompson

Amateur. My negative-c circuits make teravolts.

Sno-o-o-o-ort ;-)

I have default gmin set at 1n which limits the "kick". ...Jim Thompson

Yup, those pesky lower half plane poles. ;)

Cheers

Phil Hobbs

Yes ;-) ...Jim Thompson

There's a schematic floating around of a two op-amp circuit that synthesizes a floating negative inductor; so now one can make an LC ladder filter using all negative elements.

See what it's like to live in the antimatter universe...

What was it that Phil Hobbs was saying about kids these days? "SPICE Monkeys", I think the phrase is...

Your basic lowpass RC filter:

RC H(s) = -------- s + RC

Make R = 1 ohm and C = -1F, just to make the 'rithmatic easy.

Then

-1 H(s) = ------ s + -1

Which you could do _in your head_ if you remembered the basics.

Gawd. MIT grads and their computers...

Right half plane?

Fourier vs two-sided (i.e. bastardized) Laplace (EE version).

Physicists use the opposite Fourier transform sign convention from EEs, but I was using the EE version as a courtesy, since this is SED. ;)

Cheers

Phil Hobbs

Which is what I discovered while doodling in the waiting room of my wife's back-injury therapist... which evoked my simulating the phase going the other way.

THEN... I did a transient analysis... kaboom! ...Jim Thompson

I should be able to make a shock line in Spice, from Ls and -Cs. Gotta try that.

If you want it to be stable with a negative capacitance, use a negative resistance, too. That'll fix things up.

Might get cold if you run much power through it, though.

Leftists would call that "free" energy ?? ...Jim Thompson