C: Capacitor

## 3.2 `C`: Capacitor

### 3.2.1 Syntax

`C`xxxxxxx n+ n– value
`C`xxxxxxx n+ n– expression
`C`xxxxxxx n+ n– value {`IC=`initial-voltage}
`C`xxxxxxx n+ n– model {`L=`length} {`W=`width} {`TEMP=`temperature} {`IC=`initial-voltage}
`.CAPacitor` label n+ n– expression

### 3.2.2 Purpose

Capacitor, or general charge storage element.

### 3.2.3 Probes

The following probes (Transient, DC, and OP analysis) are available in addition to those available for all basic elements.

`DT`
  Time step. The internal time step used for this device for numerical integration. It is not necessarily the same as the global time step.
`TIME`
  Time. The time of the most recent calculation of this device. It is not necessarily the same as the global time.
`TIMEOLD`
  The time of the previous calculation of this device. It is not necessarily the same as the global time.
`TIMEFuture`
  The latest recommended time for the next sample, as determined by this device. The actual time will probably be sooner than this.
`CHarge`
  The charge stored in this capacitor.
`Q`
  The same as `Charge`.
`Capacitance`
  The effective capacitance of this device. For a fixed capacitor, it is constant. It will vary if this device is nonlinear.
`DQDT`
  The time derivative of charge. Hopefully this is the same as current, but it is calculated a different way and can be used as an accuracy check.
`DQ`
  The change in charge compared to the previous sample. Its primary use is in debugging models and numerical problems.

### 3.2.4 Comments

N+ and n– are the positive and negative element nodes, respectively. Value is the capacitance in Farads.
The (optional) initial condition is the initial (time = 0) value of the capacitor voltage (in Volts). Note that the initial conditions (if any) apply only if the `UIC` option is specified on the `transient` command.
You may specify the value in one of three forms:

1. A simple value. This is the capacitance in Farads.
2. An expression, as described in the behavioral modeling chapter. The expression can specify the charge as a function of voltage, or the capacitance as a function of time.
3. A model, which calculates the capacitance as a function of length and width, referencing a `.model` statement of type `C`. This is compatible with the Spice-3 “semiconductor capacitor”.

### 3.2.5 Model statement

A model statement may be used,, with model type `C` or `Cap`. The parameters are:

`CJ` = x
  Junction bottom capacitance. (Farads / meter squared). (Default = 0.)
`CJSW` = x
  Junction sidewall capacitance. (Farads / meter). (Default = 0.)
`DEFW` = x
  Default width. (meters). (Default = 1e-6)
`NARROW` = x
  Narrowing due to side etching. (meters). (Default = 0.)
`TC1` = x
  First order temperature coefficient. (Farads / degree C). (Default = 0.) (Not in Spice.)
`TC2` = x
  Second order temperature coefficient. (Farads / degree C squared). (Default = 0.) (Not in Spice.)
`TNOM` = x
  Parameter measurement temperature. (degrees C.). (Default = 27.) (Not in Spice.)

Capacitance is computed by the formula:

    capacitance = CJ * (L - NARROW) * (W - NARROW)
      + 2 * CJSW * (L + L - 2 * NARROW)

After the nominal value is calculated, it is adjusted for temperature by the formula:

    value *= (1 + TC1 * (T-T0) + TC2 * (T-T0)^2)