Using BJT Capacitance Equations

This section describes BJT capacitances.

Using Base-Emitter Capacitance Equations

The base-emitter capacitance contains a complex diffusion term with the standard depletion capacitance formula. The diffusion capacitance is modified by model parameters TF, XTF, ITF, and VTF.

Determine the base-emitter capacitance cbe by the following formula:

where cbediff and cbedep are the base-emitter diffusion and depletion capacitances, respectively.

Determining Base-Emitter Diffusion Capacitance

Determine diffusion capacitance as follows:

ibe <= 0

ibe > 0

where:

The forward part of the collector-emitter branch current is determined as follows:

Determining Base-Emitter Depletion Capacitance

There are two different equations for modeling the depletion capacitance. Select the proper equation by specifying option DCAP in an OPTIONS statement.

DCAP=1

The base-emitter depletion capacitance is determined as follows:

vbe < FC · VJE

vbe >= FC · VJE

DCAP=2

The base-emitter depletion capacitance is determined as follows:

vbe < 0

vbe >= 0

DCAP=3

Limits peak depletion capacitance to FC · CJCeff or FC · CJEeff, with proper fall-off when forward bias exceeds PB (FC >= 1).

Determining Base Collector Capacitance

Determine the base collector capacitance cbc as follows:

where cbcdiff and cbcdep are the base-collector diffusion and depletion capacitances, respectively.

Determining Base Collector Diffusion Capacitance

where the internal base-collector current ibc is:

Determining Base Collector Depletion Capacitance

There are two different equations for modeling the depletion capacitance. Select the proper equation by specifying option DCAP in an .OPTIONS statement.

DCAP=1

Specify DCAP=1 to select one of the following equations:

vbc < FC · VJC

vbc >= FC · VJC

DCAP=2

Specify DCAP=2 to select one of the following equations:

vbc < 0

vbc >= 0

External Base -- Internal Collector Junction Capacitance

The base-collector capacitance is modeled as a distributed capacitance when the model parameter XCJC is set. Since the default setting of XCJC is one, the entire base-collector capacitance is on the internal base node cbc.

DCAP=1

Specify DCAP=1 to select one of the following equations:

vbcx < FC · VJC

vbcx >= FC · VJC

DCAP=2

Specify DCAP=2 to select one of the following equations:

vbcx < 0

vbcx >= 0

where vbcx is the voltage between the external base node and the internal collector node.

Using Substrate Capacitance

The function of substrate capacitance is similar to that of the substrate diode. Switch it from the collector to the base by setting the model parameter, SUBS.

Using Substrate Capacitance Equation -- Lateral

Base to Substrate Diode

Reverse Bias vbs < 0

Forward Bias vbs >= 0

Using Substrate Capacitance Equation -- Vertical

Substrate to Collector Diode

Reverse Bias vsc < 0

Forward Bias vsc >= 0

Using Excess Phase Equation

The model parameter, PTF, models excess phase. It is defined as extra degrees of phase delay (introduced by the BJT) at any frequency and is determined by the equation:

where f is in Hertz, and you can set PTF and TF. The excess phase is a delay (linear phase) in the transconductance generator for AC analysis. Use it also in transient analysis.