Hints, Tips and Solutions

 

Volume 12, Number 2, February 2002

Q. Can ATLAS be used to perform large signal sinusoidal analysis ?

A. The Atlas syntax is flexible enough to allow the definition of sinusoidal nodal voltages by defining them on the SOLVE statement line. To illustrate this the structure shown in Figure 1 has been used. Before the sinusoidal pulse is applied a dc operating condition is set with

Vcollector=2V
Vbase=0.7V
Vemitter=0V

 

Figure 1. Two-dimensional structure of a BJT used in this study.

 

The sinusoidal pulse is then specified on the SOLVE statement as a part of a transient analysis run. For instance the syntax

SOLVE TRANS.ANALY FREQUENCY=5.0e8 \ VBASE=0.8 TSTOP=2.0e-8 TSTEP=3.125e-11 \ CYCLES=10

will result in a sinusoidal waveform with a magnitude of (VBASE-Vdc)=(0.8-0.7)=0.1V with a frequency of FREQUENCY and continue on for a maximum of 10 cycles. When applied to the structure in Figure 1 the resultant base voltage vs time characteristic is shown in Figure 2. As shown the curve appears fairly "uneven" which is caused by the internal timestep control producing nonlinear time steps. Although, each solution is correct the user may wish to produce a more smoothly varying response. This may be obtained by defining a smaller tolerance for the time step algorithm, for instance with the syntax

METHOD NEWTON TOL.TIME=1e-6

Figure 2. Simulated base voltage versus transient time from ATLAS. Although the
simulation converges correctly the sinusoidal curve has not been well reproduced.

The resultant curve is shown in Figure 3 where the sinmusoidal curve has become much smoother.

 

Figure 3: Simulated base voltage versus transient time from ATLAS with
an improved tolerance set on transient time stepping algorithm.

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