Circuit simulation

A circuit sim runs the analog side of a schematic straight from its netlist: the operating point every time, and an AC sweep, a transient and a noise analysis when the spec asks for them. No layout is needed, so it answers the questions that decide the schematic: does the bias sit where it should, what is the gain at 1 kHz, where does the output clip, how loud is the hiss.

#Start one

agentee new sim preamp-ac --kind circuit

The preamp example runs a 9 V op-amp preamp through every analysis. This is its AC and noise run, cut down:

name = "preamp-ac"
kind = "circuit"
schematic = "preamp"
record = ["VCC", "VREF", "GAIN_OUT", "FILT_OUT", "OUT"]

[[drives]]
net = "VBAT"
dc = "9V"

[[drives]]
net = "IN"
ac = 1
resistance = "100ohm"

[ac]
from = "1Hz"
to = "1MHz"
points = 20

[noise]
output = "OUT"
input = "IN"
from = "20Hz"
to = "20kHz"

[[checks]]
v = "VREF"
min = 4.1
max = 4.3

[[checks]]
v = "OUT"
at = "1kHz"
min = 18.3
max = 19.3

[[checks]]
noise = "OUT"
at = "1kHz"
max = "500n"

[[blocks]]
ref = "U1"
kind = "opamp"
gain = 1e5
gbw = "0.7MHz"
en = "40n"
vos = "3mV"
headroom_high = "1.5V"
headroom_low = "5mV"

agentee sim preamp-ac
agentee render sim:preamp-ac -o preamp-ac.png

The run writes preamp-ac.result.json beside the spec, with the operating point of every net, the current through every part, each check and its value, and the curves of the recorded nets. The viewer and render plot the gain, phase and noise.

#Drives

Only a drive drives a net, so give every supply one. A drive is a voltage source to ground, or to the net in return, with resistance in series. current = true makes it a current source, which is how the preamp takes the op-amp's quiescent current off its rail. ac marks the input of the AC and noise analyses. For the transient, a drive takes steps (piecewise linear), a pulse or a sine.

Ground is every net named GND, 0V, GROUND or starting with VSS.

#Analyses

• Operating point: always. Newton iteration with junction limiting, then gmin stepping, then source stepping when it will not settle.
• AC: [ac] sweeps from from to to at points per decade, linearised at the operating point. Gains are in dB against the ac drive.
• Transient: [tran] runs to stop with step as the largest step. It also stores the spectrum of each recorded net over its second half.
• Noise: [noise] gives the density at output and, with input, the input referred curve, plus the RMS over the band. Resistors, junction currents, MOSFET channels and op-amp en all contribute.

#Checks

A check reads one number and holds it between min and max. What it reads depends on at:

A check that fails is an error on the sim. Raise the 1 kHz check above to min = 19 and the run fails, and agentee check reports it as an error while that result is current:

error: ./preamp-ac.sim.toml: preamp-ac result: v(OUT) at 1kHz is 18.7870 dB, wanted 19 to 19.3 dB

The result goes stale when the spec or the schematic's netlist changes: check notes it as stale and drops its failures until the sim runs again, so rerun the sim after every change to the circuit. Start a thd or freq span after the circuit settles, or the start-up step reads as distortion.

#Models come from the datasheet

Each part becomes an element by its reference letter: R, C and L by value, D as a diode, Q as a BJT or MOSFET by its pin names. Connectors, test points, holes and crystals are left out, and anything else is an error until it has a block or sits in ignore.

The built-in devices are generic stand-ins: a silicon diode, a small-signal BJT, a logic-level MOSFET. A result that rests on one describes the stand-in, not your part. Give each part that matters a [[blocks]] entry with values from its datasheet:

• Op-amps are a single pole with gain, gbw, rout, vos, en, ib, slew, psrr, and a smooth clamp headroom away from each rail. Every unit of a dual or quad is its own op-amp on shared rails.
• Diodes take is and n. Fit them to two typical VF points a decade of current apart: n = dV / (Vt ln 10), then is = I / exp(V / (n Vt)).
• BJTs and MOSFETs take is, bf, va or vth, k, lambda, and their capacitances.
• LDOs hold vout until the input comes within dropout, then follow it down.
• Potentiometers take a position from 0 to 1, so the wiper loads and is loaded like the real part.

Put the datasheet and the table each value came from in the sim's description. The next person to read the result, often the agent, then knows what it rests on.

#Examples

• examples/preamp: the operating point, gain and noise of a gain stage and Sallen-Key filter on an LM358, the thump at switch-on, clipping, and the reverse battery diode.
• examples/ladder-vcf: a transistor ladder filter whose sims check 1 V/oct tracking at three CVs, the resonance peak, and self-oscillation in a transient.
• examples/lna: lna-supply checks the OR-ing of two power inputs through Schottkys, and lna-biastee how much RF the bias tee lets onto the supply.

Every key is in the circuit reference.