Question about circuits with feedback loops

[hemmer]

Preamble

I'm testing out the following simple circuit, which is just two buffered RC filters with a fixed feedback loop. It's from Moritz Klein's mki series: https://www.ericasynths.lv/media/VCF_MANUAL_v2.pdf

I've actually built it in person, but also checked on falstad and get the following expected behaviour
falstad link: https://tinyurl.com/25r2l538

Julia circuit
I've tried to build the circuit with Julia (actually as part of verifying an independent project where i work through the maths of the paper!), and I just find that input/output are exactly the same. I'm sure it's a silly error but I've been staring at it for a while and can't work it out! Is there a limitation of the method to model feedback loops?

using ACME
using Plots

circ = @circuit begin
    V0 = voltagesource()
    R1 = resistor(33000)
    C1 = capacitor(1e-09)
    OAmp = opamp()
    R2 = resistor(33000)
    C2 = capacitor(1e-09)
    OAmp_1 = opamp()
    J = voltageprobe()

    J[-] ⟷ C2[2] ⟷ V0[-] ⟷ gnd
    V0[+] ⟷ R1[1] ⟷ "1"
    OAmp["in+"] ⟷ C1[1] ⟷ R1[2] ⟷ "2"
    R2[1] ⟷ OAmp["out+"] ⟷ OAmp["in-"]  ⟷ "3"
    OAmp_1["in+"] ⟷ C2[1] ⟷ R2[2]  ⟷ "4"
    J[+] ⟷ OAmp_1["out+"] ⟷ OAmp_1["in-"] ⟷ C1[2]   ⟷ "5"

end
        
model = DiscreteModel(circ, 1/44100.0, SimpleSolver)
# square wave
u = [1.5 * (isodd(fld(200 * n, 44100 ÷ 2)) ? 1 : -1) for c in 1:1, n in 0:499]
y = run!(model, u)

for i in 1:size(y,2)
    println(u[1, i], " ", y[1,i])
end

plot(u[1, :], label="Input", xlabel="Sample", ylabel="Amplitude")
plot!(y[1, :], label="Output")

[martinholters]

Perhaps this should be made more prominent:

ACME.jl/src/elements.jl

Lines 503 to 505 in edad483

	Note that the opamp has two output pins, where the negative one will typically
	be connected to a ground node and has to provide the current sourced on the
	positive one.

Indeed, adding OAmp["out-"] ⟷ OAmp_1["out-"] ⟷ gnd to the circuit description seems to give the results you're expecting, right?

If you have any good ideas what would have kept you from falling into this trap, I'm all ears. Otherwise, I'm gonna close this, as it's actually working correctly, unless I'm missing something.

[hemmer]

Yes sorry, I don't know why I didn't try that. I had read that documentation but interpreted that as "out-" will be automatically grounded, but reading back I think it's relatively clear that you need to ground it yourself. "typically" != "automatically"!