Transistor Circuit (NPN)

All semiconductor devices are specified by using two command lines. The first statement defines the element and the second one is the model statement.

The syntax for the model statement is:

.MODEL MODName Type (parameter values)

MODName is the name of the model for the device.. The Type refers to the type of device and can be any of the following:

NPN: npn bipolar transistor

PNP: pnp bipolar transistor

NMOS: nmos transistor

PMOS: pmos transistor

The parameter values specify the device characteristics as explained below.
Bipolar transistors

Element: Qname C B E BJT_modelName

The bipolar transistor's name has to start with a Q

Model statement: .MODEL BJT_modName NPN (BF=val IS=val VAF=val)

in which BF is the ß, IS is the saturation current and VAF is the Early voltage. If no values are specified, the default values are assumed (ß=100; IS=1e-16A, and VAF=infinite).

MOSFETS

Element: Mname D G S B ModName L= W=

The MOS transistor's name has to start with a M

Model statement: .MODEL ModName NMOS (KP= VT0= lambda= gamma=)

in which KP=uCox. The default values are KP=20uA/V²; and the rest is equal to 0.


Example of a NPN Transistor Amplifier

Consider the circuit of the figure below. We are interested in finding the DC voltages and current, as well as the voltage gain and the input resistance seen by the input source.

*** Example of a NPN transistor

vin 1 0 ac 1

rs 1 2 1

c1 2 3 100uf

rb 5 3 465k

rc 5 4 3k

vcc 5 0 dc 10

q1 4 3 0 npn-trans

.model npn-trans npn (is=2e-15 bf=100 vaf=200)

*calculation of the operating point and small signal parameters

.op

*calculation of the small signal gain

.ac dec 10 100 10k

.plot ac vm(4)

* cacluations of the small signal input conductance (i/v)

.plot ac im(vin)

.end