Computer Aided Design
Conf.dr.ing. Ovidiu Pop
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Course structure
Analysis:
•DC analysis
•AC analysis
•Transient analysis
•Parametric analysis
•Statistic analysis
Modeling of semiconductor devices:
•Diodes
•BJT
•MOS
•Operational amplifiers
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Note:
Laboratory:
•Half semester exam: L1
•Final exam: L2 (Project)
•Laboratory activity L3
N=0.2*L3+0.4*L1+0.4*L2 ; L1,L2,L3>4
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Minimum Knowledge:
•Editing of electrical circuits
•Primary and Secondary Analysis Categories
•Analysis Settings
•Simulation of circuits
•Interpretation of results
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INTRODUCTION ON CIRCUIT SIMULATION TECHNIQUES
1.
Motivation
Why simulate circuits?
– Design technique of the 50s and 60s:
 Circuits built-up of discrete elements
 Experiments by breadboarding, measuring, tuning
– Later: ICs with growing density of integration
 Breadboarding with discrete elements not useful
 Only chip pads accessible for measurements
 Time-to-market pressure
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What do we gain by simulation?
– test new ideas (‘dry lab’)
– fast changes possible
– considering parasitic effects ( with suitable models)
– simulation of ‘ ideal’ elements (isolation of parasitic components)
– run circuit under conditions which are:
 difficult to realize (loading, noise, etc)
 dangerous ( destruction of the circuit)
 not customary ( needs special equipment)
– fast turnaround
– reduction of costs
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Computer Aided Design Course
CAD
Glossarry
CAD =
Computer Aided Drafting , Computer Aided Design
ECAD= Electronic Computer Aided Design
CAE=
Computer Aided Engineering
SPICE= Simulation Program with Integrated Circuit Emphasis
PSPICE=Personal Computer SPICE
CAI=
Computer Aided Instruction
CAM=
Computer Aided Manufacturing
CAP=
Computer Aided Planning
CAQ=
Computer Aided Quality Control
VLSI IC= Very Large Scale Integration , Integrated Circuit
PLD=
Programable Logic Device
ASIC=
Application Specific Integrated Circuits
SoC=
System On Chip
EDAE= Electronic Design Automation Environment
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Computer Aided Design Course
CAD
Reference
1. “ECAD Course in Distance Education” Elena Skoikova, Ana RusuCDROM, 1999
2. SPICE –User Manual
3. “The SPICE Book “ Andrei Vladimirescu, J. Willey & Sons, 1994
4. “A Guide to Circuit Simulation & Analysis Using PSPICE” P.W.
Tuinenga
5. “Semiconductor Device Modeling with SPICE” P. Antognetti, G.
Massobrio, McGraw Hill, 1996
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3.
History
50s: development of fundamental knowledge
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Network description
Graph theory
State equations
60s: development of first solvers/models/integration algorithms
experience:
-“The first generation simulation programs” (ECAP I in 1965, to
IBM) could only solve piecewise linear networks and their
scope was fairly limited.
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Loop analysis has disadvantages of network contains capacitances and high
resistive sources
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State equations are not optimal for numerical calculations
70s: application of Nodal Analysis (NA), Modified Nodal Analysis (MNA),
Sparse Tableau Analysis (STA);
algorithmic improvements (integration formulas), development of
semiconductor models
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3.
History
1971: CANCER, SPICE1, SPICE2 (free distribution)- University of Berkeley
“The second generation simulation programs”- Advances in numerical techniques led
in the late 1960s to development of nonlinear analysis programs, such as SPICE1, initially named
CANCER (in 1971) to University of Berkeley, California.
80s: SPICE becomes standard simulation tool
development of PC versions (1983:PSPICE)
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commercial versions with better user interface, model support, better stability and
accuraccy
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integration into CAE design environment
- development of “third generation techniques” for simulation of large-scale
circuits. The efforts of three decades ago have crystallized in the two circuit simulators
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now most often used, SPICE2 and ASTAP, currently called ASX
90s: - lot of commercial versions based on SPICE2/ SPICE3
- development of algorithms for very large circuits
-versions for multiprocessor systems/supercomputers
- development of mixed-signal simulators
2000s: lot of integrated CAD/CAE tools for VLSI submicron circuits
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4.

State of the Art
General-Purpose Circuit Simulators like
SPICE family: SPICE2G.6, HSPICE, PSPICE9 ( from OrCAD, now Cadence),
IsSPICE(IntoSoft), AIM-Spice3.2,
MicroCapV,
ElectronicWorkBench,…
ELDO (mixed-mode) (Anacad, now Mentor)
SABER ( mixed-level) (Analogy)
Spectre (Cadence)
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Simulators for Special Purpose
RF and Microwave (Spectre RF – Cadence, MDS- HP EESoft, Serenade-AnSoft)
Steady State (Oscillator Design)
Symbolic Analysis
Mixed-Domain Simulators ( Electro-Thermal, Electro-Mechanical,…)
Interconnection Analysis
Signal Integrity
Experimental Simulators ( Univesity, Research Institute) –some may became
products
Educational Software
RSPICE, OPTIMA
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INTRODUCTION TO ELECTRICAL COMPUTER SIMULATION
1.
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Purpose of Computer Simulation of Electrical Circuits
Knowledge of the behavior of electrical circuits requires the
simultaneous solution of a number of equations.
The easiest problem is that of finding the DC operating point
of a linear circuit, which requires one to solve a set of
equations derived from Kirchhoff’s voltage law and the branch
constitutive equations (BCEs).
For a small circuit with linear elements, described by linear
branch voltage-current dependencies, the exact DC solution is
readily available through hand calculations.
For larger linear circuits the DC solution and especially the
frequency-domain or time-domain solutions are very complex.
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INTRODUCTION TO ELECTRICAL COMPUTER SIMULATION
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The analysis of circuits that contain elements described by a
nonlinear relation between current and voltage adds another
level of complexity, requiring the solution of the nonlinear
branch equations simultaneously with the equations based on
Kirchhoff ‘s law.
Only small circuits can be solved by hand calculations, which
yield only approximate results. Engineers learn in electronics
courses to make certain approximations in order to predict the
DC operation of small circuits by hand.
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INTRODUCTION TO ELECTRICAL COMPUTER SIMULATION
Another level of complexity is added when one has to
predict the behavior in time or frequency of an
electrical circuit.
The nonlinear equations become integro-differential
equations, which can be solved by hand only under
such approximations as small-signal approximation or
other limiting restrictions.
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Since the electrical design engineer did not have the
luxury of a trial-end-error approach in silicon to verify
the correctness of the design, a virtual breadboard
was needed.
This could be produced on a digital computer by
means of an electrical-analysis or simulation program.
Programs intended for the electrical analysis of
networks without taking any shortcuts in the solution
of the KCL, KVL, and BCE equations are called circuit
simulators.
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Another important factor contributing to the
development of computer programs for analysis of
electrical circuits was the rapidly growth in computers
manufacturing.
This two technological factors, ICs and powerful
computers, defined both the need and the tool for
automating the design process of electronic circuits.
A number of researchers started studying the best
techniques and algorithms for automating the
prediction of the behavior of electric circuits.
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2. What is SPICE?
SPICE is a general-purpose circuit-simulation program for
nonlinear DC, nonlinear transient and linear AC analysis. As
outlined above, it solves the network equations for the
node voltages. The program is equally suited to solves
linear as well as nonlinear electrical circuits.
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2. What is SPICE?
Circuits can contain:
resistors, capacitors, inductors, mutual inductors,
independent voltage and current sources,
dependent voltage and current sources,
transmission lines,
the most common semiconductor devices:
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diodes,
bipolar junction transistors (BJTs),
junction-field effect transistors (JETs),
metal-oxide-semiconductor field effect transistors (MOSFETs),
metal-semiconductor FETs GaAs (MESFETs);
special devices and integrated circuits.
Any general-purpose circuit simulation program must to give the following
three basic solutions: bias point (OP), frequency response (AC) and
transient response.
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The DC analysis part of the program computes the bias point of the
circuit with capacitors disconnected and inductors short-circuited. SPICE
uses iterations to solve the nonlinear network equations; nonlinearities are
due mainly to the nonlinear current-voltage (I-V) characteristics of
semiconductor devices.

The AC analysis mode computes the complex values of the node voltages
of a linear circuit as a function of the frequency of a sinusoidal signal
applied at the input. For nonlinear circuits, such as transistor circuits, this
type of analysis requires the small-signal assumption; that is, the
amplitude of the excitation sources are assumed to be small compared to
the thermal voltage for BJTs (Vin << Vth= 25mV, for small distortions).
Only under this assumption can the nonlinear circuit be replaced by its
linearized equivalent around the DC bias point.
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
The transient analysis mode computes the voltage waveforms at
each node of the circuit as a function of time. This is a large-signal
analysis: no restriction is put on the amplitude of the input signal. Thus
the nonlinear characteristics of semiconductor devices are taken into
account.
More types of analysis, associated with the above three basic
simulation modes, are available in (P)SPICE.
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WHY USE SPICE?
SPICE is a great tool for learning electronics. You can increase
your understanding of circuits as you play and tinker with
them.
Experiment! Modify the circuit and see what happens! How
long does it take? Change a resistor value and see the effect on
a circuit in seconds.
Ideally, we would actually build and test actual circuits to
understand all of its behaviors. However, you would need
breadboards, components and time to wire the circuit.
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WHY USE SPICE?
Actual circuits also require expensive equipment like power
supplies, signal generators and oscilloscopes. It may be difficult
to physically breadboard every circuit you encounter.
You can spend hours building an actual circuit and only get a
simple concept from it, whereas, SPICE provides the insight in
minutes. SPICE can be your “virtual” breadboard.
Even if you have a short time to spare, you can cover several
circuit principles and applications.
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3. Overview of Simulation Algorithms
The solution process implemented in SPICE for the time-domain
solutions is shown in Fig. 1.
Generally the program first solves for a stable DC operating
point. The solution starts with an initial guess of the operating
point, which is followed by iterations for solving the DC
nonlinear equations.
The iterative process is represented by the inner loop in Fig. 1.
The solution the iterative process converges to represents either
the small-signal bias solution or the initial transient solution.
This is the time zero solution.
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3. Overview of Simulation Algorithms
The iterative process is repeated for every time point at which
the circuit equations are solved in transient analysis.
The time-domain solution uses numerical integration to
transform the set of ordinary differential equations (ODEs) into
a set of nonlinear algebraic equations.
The time-domain analysis is replaced by a sequence of quasistatic solutions.
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3. Overview of Simulation Algorithms
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3. Overview of Simulation Algorithms
A circuit simulator is defined by the following sequence of
specific algorithms: (a) an implicit numerical integration method
that transforms
the nonlinear differential equations into
nonlinear algebraic equations; (b) linearization of these through
a modified Newton-Raphson iterative algorithm, and finally, (c)
Gaussian elimination and sparse matrix techniques that solve
the linear equations.
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4. PSPICE
PSPICE is a mixed analog/digital electrical circuit simulator meaning
PSpice can calculate the behavior of analog-only, mixed
analog/digital and digital-only circuits with speed and accuracy.
PSpice analog and digital algorithms are build into the same program.
Hence, mixed analog/digital circuits can be simulated with tightlycoupled feedback loops between the analog and digital sections,
without any performance degradation.
PSpice calculates “voltage” and “currents” of the analog components and
nodes, and “states” of the nodes connected to digital components. The
results are formulated into meaningful graphical traces, tables and plots
for further analysis.
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4. PSPICE
4.1. Numeric Value and Expression
Literal numeric values are written in standard floating point notation.
PSpice assumes default units for numbers described component values and
electrical quatities. However, values can be scaled by following the number by
the appropriate scale suffix as shown in Table 1.
Numeric values can also be indirectly represented by parameters (PARAM)
Numeric values and parameters can be used together to form arithmetic
expressions. PSpice expressions may incorporate the intrinsic functions (ABS,
SQRT, EXP, LOG, LOG10, PWR, SIN, COS, TAN, ATAN, TABLE, LIMIT)
and user-functions (FUNC).
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Table 1. Scale suffixes for numeric values
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2.Elements, Models and Nodes
Elements and models/macromodels description represents the core of
the circuit description.
An element statement (in netlist file) contains connectivity information
(circuit topology from schematics) and either explicity or by reference
to a model/subcircuit name (from a models library), the value of the
defined element.
Model (MODEL)/macromodel (SUBCKT) statements are necessary for
defining the parameters of complex elements: all semiconductor
devices and many ICs.
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4.3. Conventions
The following conventions must be observed in the circuit
definitions:

A circuit must always contain a ground node, which must
always be number 0.
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Every node in the circuit must have at least two elements
connected to it; the only exceptions are the nodes of
unterminated transmission lines.
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Every node in the circuit must have a DC path to ground. In
DC, capacitors represent open circuits and inductors represent
shorts. This requirement prevents the occurrence of floating
nodes, for which the program cannot find a bias point.
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
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Because standard SPICE2G.6 uses modified nodal analysis to solve
for both node voltages and currents of voltage-defined elements,
such as voltage sources and inductors, two restrictions must be
observed:
the circuit cannot contain a loop of voltage sources or
inductors
it cannot contain a cut-set of current sources or capacitors.
The former is disallowed due to Kirchhoff’s voltage law and the
latter due to Kirchhoff’s current law.
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Any violation of the above restrictions results in an error message
and termination of the SPICE program. The possible error
messages and corrective actions are:
1. “Floating Nodes”
During read-in, PSpice checks the topology of the circuit. One of the
checks done is to make sure that there are no floating nodes. If there are
floating nodes, PSpice will indicate a read-in error on the screen and the
output file will contain a message similar to the following:
ERROR: Node X is floating.

This means that there is no DC path to ground from node X. A DC path is one
that draws current, for instance a path through resistors, inductors and
transistors. There are several paths that do not draw current and that do not
count:
The two ends of a transmission line do not have a DC connection between
them: in the following example node 5 has a connection to node 0 (ground):
T1 5 0 4 8 Z0=75 td=20ns
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
Voltage-controlled sources do not have a DC connection to their
controlling nodes, so these sources do not draw current from their
controlling nodes. In the following examples, node 5 has a connection to
ground:
EGAIN 5 0 4 8 100
GA
5 0 4 8 0.8
The two sides of a capacitor have no DC connection between them. In
the following example, node 5 has no DC path to ground:
C5 5 0 0.1u
In all these cases the solution is straightforward: connect the
floating circuitry to ground with a resistor (usually of large
value, 1MEGohm).
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2. “Voltage Source/Inductor Loop”
Another topology check that is done on each circuit is to make sure that
there are no loops with zero resistance. If there are, PSpice will
indicate a read-in error on the screen and the output file will contain a
message similar to the following:
ERROR: Voltage loop involving Vx
This means that the circuit has a loop of zero resistance components, one
of which is Vx.
The zero resistance components in PSpice are: independent voltage
sources (V), inductors (L), voltage-controlled voltage sources (E), and
current-controlled voltage source (H). Examples of such loops are:
a.) Vin 3 0 10V
Vs 3 0 5V
b.) V1 3 5 15V
L1 3 5 10u
E1 3 5 2 7 10
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Note that it makes no difference whether the values of the voltage
sources are 0 or not. Having a zero resistance loop means that the
program will need to divide 10V ( or any value of voltage) by 0, which is
impossible.
In all these cases the solution is straighforward: add a series
resistance to at least one component in the loop. Choose the
resistor’s value to be small enough so that it does not disturb
the operation of the circuit. However, to avoid exceeding the
dynamic range of the double-precision arithmetic used in
PSpice, it is recommended not going bellow 1micro-ohm. To be
more accurate, choose a value that approximates the actual
parasitic resistance of the component.
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3.“Voltage-Controlled Sources”
During the read-in and checking part of a run you may get an
error which prints the message in the output file:
ERROR: Less than 2 connections at node X
The inputs to voltage-controlled sources are not considered
connections during this check. This is because these inputs are
ideal inputs and draw no current (they have infinite impedance).
If this occurs, the solution is: connect a very large resistor
(a Gohms, say) from the source’s input to ground. This
will satisfy the topology check and, if the resistor is large
enough it will not affect the circuit’s behavior.
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4.4. Convergence Problems
The DC sweep, bias point calculation and transient analysis all use
iterative algorithms.
These algorithms start with a set of node voltages and for each iteration
they calculate a new set of node voltages which hopefully are closer to a
solution of Kirchhoff’s voltage and current laws. In other words, an initial
guess is used and successive iterations are supposed to converge to the
solution.
If the iterations do not converge onto a solution, then the analysis fails.
The DC sweep skips the remaining points in the sweep. Failure of the
bias-point calculation prevents other analyses (AC, sensitivity, etc) which
depend on it from being done. The transient analysis skips the remaining
time.
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4.4. Convergence Problems
The DC sweep skips the remaining points in the sweep.
 Failure of the bias-point calculation prevents other analyses
(AC, sensitivity, etc) which depend on it from being done.
 The transient analysis skips the remaining time.
PSpice is successful in analyzing most circuits. Considerable effort
has gone into eliminating problems which impeded the
progression of circuit analysis. However, there are no guarantees.
If an analysis fails for our circuit, the company give us few
suggestions for: DC Sweep analysis, Bias Point, Transient
Analysis.
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