Introduction: Symbolic SPICETM is an analog circuit analyzer and approximator. It can be used as a pre-processor to find design formulas either directly or by making some global approximations. Symbolic SPICE can also be used as a post-processor using biasing point information of transistor parameters to approximate design formulas. Symbolic SPICE has a high degree of flexibility so as to be applicable to many different types of analog circuit problems.
Features: The program performs the .AC analysis in SPICE symbolically with optional numeric evaluation and user selected threshold approximation.
The input file format is similar to that of SPICE. Element definitions begin with a key letter and are uniquely named with additional numbers or letters. Symbolic SPICE, however, does allow the user to equate symbols. This is useful in simplifying formulas. Input files are also scanned for proper element format.
There are over 30 elements in Symbolic SPICE. Besides the standard SPICE elements, Symbolic SPICE has low and high frequency models for diodes and transistors. There are subcircuits for an ideal op-amp and transformer. All output is directed to user prompted files.
Basic Analysis: After accepting the input file, Symbolic SPICE inserts the appropriate models for the elements. It forms the nodal equations and lists the unknowns. The user can then make use of an algebraic editor to find desired transfer functions.
The output is sorted by like symbols and by powers of the Laplace variable 's' or a user selected symbol. Common terms in a transfer function are removed.
User Specific Applications: Symbolic SPICE can optionally examine any transfer function to determine if it is second order. It then tries to identify it as a low-pass, high-pass, band-pass, band-reject or all-pass filter. Once found, Symbolic SPICE finds pass-band gains, cutoff or center frequencies and quality factors. In addition for active filters, Symbolic SPICE can determine the effect finite gain-bandwidth-product has on these parameters.