SPICE circuit simulations

Based on this design a memory/adder model (Fig. 6(c)) using 464 transistors could be constructed and evaluated on grounds of SPICE circuit simulations. Four bits of information were read from four different memory cells, added as two 2-bit words, and the resulting 2-bit was moved through registers (clocked D-latches) to a subsequent computation. It must be noted... 

Since its introduction in 1971, SPICE (Simulation Program with Integrated Circuit Emphasis) has become the most popular analog simulation tool in use today. In the last 15 years, we have seen explosive growth in the use of SPICE, with the addition of Berkeley SPICE 3 enhancements, and support for C code model and mixed-mode simulation using XSPICE (Cox et al. 1992, Kielkowski 1994).We have also seen many new companies emerge as developers of SPICE-based simulation tools, most of which are currently available for the PC platform. 

Still, there are limitations to the capabilities of SPICE and similar circuit simulators. While the sophistication of simulation increases, the hardware breadboard will still remain a necessary step in the design process. This book will aid the engineer in using SPICE simulation as a very powerful tool in the design process. 

The theory of operation of each circuit is discussed, followed by the circuit schematic, the simulation results, and a comparison to laboratory data. Advantages and disadvantages of each circuit are added, along with any tips or hints useful in modeling the circuit accurately. We have attempted to perform each simulation using several versions of SPICE for comparison. Also included are the run times for each circuit simulation. 

Steven M. Sandler is the founder of AEi Systems, LLC, the world leader in SPICE modeling and worst case circuit analysis since 1995. He has developed and taught courses at Motorola University and has published many books and articles on circuit simulation for McGraw-Hill and Power Electronics, PCIM, and PEIN magazines. 

SPICE [32] simulations indicate that the electrochemical transistor can be used favorably to implement a number of useful digital and analogue circuits. Naturally, the slower speed, compared to traditional electronics (by about a factor 108 ), should be taken into consideration. The area of (silicon-based) electronic circuit theory offers numerous solutions to the implementations of circuits. However, depletion-type transistors are less favorable to use in system designs than their enhancement counterparts. For this reason, it is rare to find circuits that use depletion transistors. One has to go back to the electron tube era to find solutions that can be used as templates for circuits suitable for the electrochemical transistor. 

Methods to calculate time dependent current/voltage response for circuits whose parameters depend on voltage or current are well developed in theoretical electronics and free open source tools for numerical calculations are available, such as general electronic circuit simulator SPICE (Hageman [1993]). 

Commercially available circuit simulation programs are useful for evaluating a given drcuit for the result of temperature change. SPICE, for example, will run simulations at any temperature with elaborate models included for all circuit components. 

The main distinguishing feature of semicustom ASICs, compared to full custom ones, is that the basic circuit building blocks, whether analog or digital, are already designed and proven to work. These basic circuits typically reside in libraries within a CAD system. The users simply select from the library the components needed, place them on their circuits, and interconnect them. Circuit simulation is also done at a much higher level than SPICE, and the designer is, therefore, not required to be familiar with either semiconductor or device physics. 

Nonlinear circuit problems such as those discussed in these two examples are usually solved in Electrical Engineering by use of the SPICE circuit analysis program. For complicated circuits, this should certainly be the means of solving such problems. This program has built-in models for all standard electronic devices and is very advanced in approaches to achieve convergence. However, at the heart of the SPICE program is an approach very similar to that of the much simpler nsolv() program used here. SPICE will automatically set up the equation set to be solved, but uses first-order linearization and iteration to solve the nonlinear equations just as employed in nsolvQ. While SPICE is the preferred tool for its domain of application, a tool such an nsolv() can be readily embedded into other computer code for specialized solutions to problems not appropriate for an electronic simulation. 

The beginning of the book concentrates on the basics of computer simulation of electronic circuits. A brief overview of four popular SPICE programs is provided along with their basic differences. 

We have made a reasonable effort to make apples to apples comparisons between the simulation speeds of the software in this book by using commonly available Berkeley SPICE 2 OPTIONS. The reader will notice that it is not predictable which software package will run the fastest on any given circuit. The real purpose of including the run times is to provide the user with an estimate as to how long the circuit will take to simulate on his or her own computer, nothing more. That being said, the simulation times noted after the simulations are reasonably accurate. 

The reader will also note that in some circumstances, one or more of the simulation software results did not match the hardware results. We have attempted to explain the reasons why this might have occurred. Bear in mind that SPICE is one of those labors in life where you get out of it what you put into it. If you put very little effort into understanding what the models and circuit are doing, chances are your simulation accuracy will be poor. 

Unfortunately, the lab used in the creation of the circuits in this book closely resembles the lab of other engineering companies around the world. We use 5% tolerance resistors and 10% tolerance capacitors that are either soldered to a vector board or plugged into a solderless breadboard. This introduces various parasitics and inaccuracies in the results. In order to be more precise in showing the accuracy of SPICE simulation software, we frequently run the simulations with the stated values of the resistors or capacitors used in our lab breadboards. The measured values for each resistor and capacitor used in our breadboard configuration which may be different, are shown in Fig. 3.3. 

The schematic in Fig. 3.44 of the Chebyshev band pass filter utilized the predicted values from the MathCAD file, where lab resources allowed. Close approximations were used, to which the circuit performance was extremely sensitive. Any deviations from the values predicted in the MathCAD file resulted in gain in the pass band. Using SPICE to test possible circuit realizations greatly reduces the time to implement hardware. SPICE will predict if a given circuit realization will perform as desired with available parts, before actual hardware measurements are made. This is helpful because Chebyshev circuit realization can be difficult small changes in the circuit elements can result in undesired performance. The simulated AC results from IsSpice, PSpice, and Micro-Cap are shown in Figs. 3.45, 3.46, and 3.47, respectively. The measured breadboard AC response of the filter is shown... 

The SPICE equivalent circuit schematic is shown in Fig. 4.14. Note that the DCR (DC Resistance) of the inductor LI has been added (R DCR) to the circuit. Also added to the circuit is a voltage source between the inductor and the output in order to measure inductor current. The input voltage is pulsed from V to 20 V in order to help get the simulation started. 

SPICE tip SPICE models for the LM78S40 were not provided in the Micro-Cap software package. This circuit was simulated using IsSpice and PSpice only. 

These results show that all three of the SPICE simulators were not equipped to handle simulations of this particular zener diode in its soft region. However, zener diodes are not well defined or tested in this region of operation, so it may just be that the circuit tested had a worse than average zener diode in it. To further explore this problem, the experiment was modified to put more current than the specified test current in the zener diode. The circuit with these modifications made is shown in Fig. 6.64. The measurements across the zener diode are shown in Table 6.6, and the the resistance-to-voltage response is contained in Fig. 6.65. 

The resulting output of the breadboard is shown in Fig. 8.47. The outputs of the SPICE simulations are shown in Figs. 8.48a and 8.48b. The output of the circuit is a clean, low-distortion 500 Hz sine wave. 

In order to get this circuit to start, the UIC statement must be included in the. TRAN simulation. Flip-flops inside the model need the UIC directive in order to initialize properly. The SPICE engine has a difficult time determining the steady-state operating point of bistable circuit like the flip-flop without the UIC. Also, the ABSTOL OPTIONS parameter for current absolute current error tolerance has been changed from lp to 1/x to aid convergence. 

In order to assist convergence in this circuit, the UIC statement was included in the. TRAN simulation. This statement helps in circuits where a steady-state operating point may not exist, multiple stable operating points exist, or it is difficult for SPICE to determine the correct operating point. 

Thanks to Ron Rohrer, Larry Nagel, and all the students at the University of California, Berkeley, who worked hard in 1969 and 1970 to develop the first computer simulation software, Cancer (Computer Analysis of Non-Linear Circuits Excluding Radiation). This effort would result in the release of SPICE into the public domain in 1971. 

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