APPLICATION NOTE
4
Figure 4 - Frequency controllable Wien Bridge oscillator output
Modeling Voltage-Variable Capacitors
Some time designers need to model a voltage variable capacitor. The following example circuit file describes a
test circuit that contains a voltage-variable capacitor. This capacitor is constructed by way of a TABLE function
embedded in the VALUE extension to the G (voltage-controlled current source) device. This model is a better
representation of a varicap device than the commonly used YX device. The Probe plot in Figure 6 shows
capacitance versus controlling voltage for a voltage-variable capacitor similar to 1N4155.
From the D device capacitance equations
CJ = CJO * (1 + Vr/Vj)**-M
where
CJO = zero-bias junction capacitance
CJO = p-n potential
M = p-n grading coefficient
Cj is junction capacitance for reverse voltage Vr.
This can be modelled using a voltage-controlled current source with voltage controlled by table-based voltage-
controlled current source.
.subckt tablecap 1 2 PARAMS: C4 = 1pf, M = 0.5, VJ = 1.0
.Ecopy 3 6 1 2 1.0
.Vsense 0 6 0v
