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Device Models

Device Models and .model Directives

Some components in PyCircuitKit rely on SPICE device models defined via .model lines in the netlist. This page shows how to add models and wire typical circuits using the high‑level API.

Key points:

  • Use Circuit.add_directive(line) to append raw SPICE directives.
  • Diodes must reference a named model (e.g. D1 → .model D1 D(...)).
  • Voltage/Current‑controlled switches (VSwitch, ISwitch) require a .model <name> SW(...) with thresholds and on/off resistances.
  • Controlled sources F/H use a named voltage source as current sensor (no .model, but a topology note).

Adding a diode model

from cat.core.circuit import Circuit
from cat.core.components import Vdc, Resistor, Diode
from cat.core.net import GND, Net
from cat.analysis import OP

c = Circuit("d_fwd")
n1 = Net("n1")
V = Vdc("1", 1.0)
R = Resistor("1", 1000.0)
D = Diode("1", "D1")  # references model D1

# Add a diode model (Shockley params shown as an example)
c.add_directive(".model D1 D(Is=1e-14 N=1.9 Rs=5 Cjo=2p)")

c.add(V, R, D)
c.connect(V.ports[0], R.ports[0])
c.connect(R.ports[1], n1)
c.connect(D.ports[0], n1)  # anode
c.connect(D.ports[1], GND)  # cathode
c.connect(V.ports[1], GND)

res = OP().run(c)
print("V(n1)=", float(res.traces["v(n1)"].values[-1]))

Typical parameters for the SPICE diode (D) model include Is, N, Rs, Cjo, Bv, etc. Choose values appropriate to your target device.

Voltage‑controlled switch (VSwitch)

from cat.core.circuit import Circuit
from cat.core.components import Vdc, Vpulse, VSwitch, Resistor
from cat.core.net import GND, Net
from cat.analysis import TRAN

c = Circuit("vswitch_demo")
vin = Net("vin")
vout = Net("vout")
en = Net("en")

# Control signal: 0→1 V pulse
EN = Vpulse("en", 0.0, 1.0, 0.0, 1e-6, 1e-6, 1e-3, 2e-3)
V1 = Vdc("1", 5.0)
S1 = VSwitch("1", model="SW1")
RL = Resistor("L", 1000.0)

# Recommended switch model (tune as needed)
c.add_directive(".model SW1 SW(RON=10 ROFF=1e9 Vt=0.5 VH=0.1)")

c.add(EN, V1, S1, RL)

# Source side
c.connect(V1.ports[0], vin)
c.connect(V1.ports[1], GND)

# Switch: p→vin, n→vout, control cp→en, cn→GND (active high)
c.connect(S1.ports[0], vin)   # p
c.connect(S1.ports[1], vout)  # n
c.connect(S1.ports[2], en)    # cp
c.connect(S1.ports[3], GND)   # cn

# Load
c.connect(RL.ports[0], vout)
c.connect(RL.ports[1], GND)

# Control signal wiring
c.connect(EN.ports[0], en)
c.connect(EN.ports[1], GND)

res = TRAN("0.1ms", "5ms").run(c)
print("final V(vout)=", float(res.traces["v(vout)"].values[-1]))

Notes for switches:

  • The switch turns on when (V(cp) − V(cn)) > Vt. VH sets hysteresis.
  • Choose RON and ROFF appropriate to your application.
  • For current‑controlled switches (ISwitch), the control is the current through a named voltage source Vref (current sensor) provided via the ctrl_vsrc argument.

Current‑controlled elements (F/H) and sensors

CCCS (F) and CCVS (H) measure the current through an existing voltage source. A common pattern is to insert a 0 V source as sensor:

from cat.core.components import Vdc, Resistor, CCCS

# Sensor source at the node of interest
VSENS = Vdc("S", 0.0)
F1 = CCCS("1", ctrl_vsrc="S", gain=2.0)  # 2× current mirror

# Wire VSENS in series with the branch you want to sense
# Then connect F1.p/n to the output branch where mirrored current flows.

This pattern avoids modifying the DC operating point (0 V across the sensor) while providing a reference for F/H controlled elements.