Design Simulation Systems Ltd
All of the modelling tools described here produce two things. Firstly, a schematic symbol appropriate to the device and, second, a topological drawing, viewable in GEX and capable of modification by the user. For some devices, like the operational amplifier, a SPICE SUBCKT netlist is also generated.
Vmodel is our general-purpose component modelling tool. It makes SPICE models of diodes, bipolar transistors, jfets and mosfets from parameters found on manufacturers' data sheets. Such models are representative of the particular type of semiconductor but, obviously, data sheet parameters do not completely characterise a device, so SPICE default values are used for any missing values. Bearing in mind the tolerances on semiconductor devices (+/- 300% on some parameters) the performance of the models will correspond to a real device, somewhere within the production spread. For accurate modelling of actual device samples, Vmodel2 is recommended but, of course, the device being modelled should be understood to be, itself, only a sample taken from within the spread of production tolerances.
Vmodel also makes models of operational amplifiers, using the macro-model originally proposed by Solomon & Boyle as a template. The model is recognised throughout the industry as a de facto standard, and the parameters available on a manufacturer's data sheet are perfectly adequate to completely characterise the device.
For applications where it is necessary to exactly model the characteristics of an actual device, perhaps when doing worst-case analysis, with a known-bad semiconductor, we supply Vmodel2. This tool will model bipolar transistors, diodes, JFETs and MOSFETs, but using a totally different method. Several parameters are entered, either from the data sheet, from measurements, or by guesswork, and these are used as starting points in an iterative curve-fitting process. The internal equations used for the curve-fitting are those used by SPICE itself, when simulating. Thus, each model will perform in exactly the same way in simulation as its real life counterpart. Diagnostic outputs are available, so that the characteristic curves can be viewed and compared.
After the few data sheet parameters are entered, one of the 'FIT xx/yy' buttons is pressed, to access the datapoint entry screen. Up to 20 points can be entered from measured or published characteristic curves, and these are used in the curve-fitting process. At the end of the fitting process, the extracted SPICE parameters are shown in the text fields at the top of the screen. These may be modified by the user, and used as starting points for a new set of iterations. A facility is also provided to enable data points to be entered from a file. This is more convenient in the case where the data is derived from instrument measurements, or where it may be necessary to repetitively re-enter the input data.
Xmodel is a variant of Vmodel, with the operational amplifier modelling replaced by Silicon Controlled Rectifier modelling. The model created is loosely based on the J3 SCR model proposed by Avant & Lee, but with some modifications to improve convergence. It can be fully characterised by data sheet parameters.