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Electronic circuit simulation
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Electronic circuit simulation uses mathematical models to replicate the behavior of an actual electronic device or circuit. Simulation software allows for the modeling of circuit operation and is an invaluable analysis tool. Due to its highly accurate modeling capability, many colleges and universities use this type of software for the teaching of electronics technician and electronics engineering programs. Electronics simulation software engages its users by integrating them into the learning experience. These kinds of interactions actively engage learners to analyze, synthesize, organize, and evaluate content and result in learners constructing their own knowledge.[1]
Simulating a circuit’s behavior before actually building it can greatly improve design efficiency by making faulty designs known as such, and providing insight into the behavior of electronic circuit designs. In particular, for integrated circuits, the tooling (photomasks) is expensive, breadboards are impractical, and probing the behavior of internal signals is extremely difficult. Therefore, almost all IC design relies heavily on simulation. The most well known analog simulator is SPICE. Probably the best known digital simulators are those based on Verilog and VHDL.
Some electronics simulators integrate a schematic editor, a simulation engine, and an on-screen waveform display (see Figure 1), allowing designers to rapidly modify a simulated circuit and see what effect the changes have on the output. They also typically contain extensive model and device libraries. These models typically include IC specific transistor models such as BSIM, generic components such as resistors, capacitors, inductors and transformers, user defined models (such as controlled current and voltage sources, or models in Verilog-A or VHDL-AMS). Printed circuit board (PCB) design requires specific models as well, such as transmission lines for the traces and IBIS models for driving and receiving electronics.
Types
[edit]While there are strictly analog[2] electronics circuit simulators, popular simulators often include both analog and event-driven digital simulation[3] capabilities, and are known as mixed-mode or mixed-signal simulators if they can simulate both simultaneously.[4] An entire mixed signal analysis can be driven from one integrated schematic. All the digital models in mixed-mode simulators provide accurate specification of propagation time and rise/fall time delays.
The event-driven algorithm provided by mixed-mode simulators is general-purpose and supports non-digital types of data. For example, elements can use real or integer values to simulate DSP functions or sampled data filters. Because the event-driven algorithm is faster than the standard SPICE matrix solution, simulation time is greatly reduced for circuits that use event-driven models in place of analog models.[5]
Mixed-mode simulation is handled on three levels: with primitive digital elements that use timing models and the built-in 12 or 16 state digital logic simulator, with subcircuit models that use the actual transistor topology of the integrated circuit, and finally, with inline Boolean logic expressions.
Exact representations are used mainly in the analysis of transmission line and signal integrity problems where a close inspection of an IC’s I/O characteristics is needed. Boolean logic expressions are delay-less functions that are used to provide efficient logic signal processing in an analog environment. These two modeling techniques use SPICE to solve a problem while the third method, digital primitives, uses mixed mode capability. Each of these methods has its merits and target applications. In fact, many simulations (particularly those which use A/D technology) call for the combination of all three approaches. No one approach alone is sufficient.
Another type of simulation used mainly for power electronics represent piecewise linear[6] algorithms. These algorithms use an analog (linear) simulation until a power electronic switch changes its state. At this time a new analog model is calculated to be used for the next simulation period. This methodology both enhances simulation speed and stability significantly.[7]
Complexities
[edit]Process variations occur when the design is fabricated and circuit simulators often do not take these variations into account. These variations can be small, but taken together, they can change the output of a chip significantly.
Temperature variation can also be modeled to simulate the circuit's performance through temperature ranges.[8]
Simulation from admittance matrix
[edit]A common method of simulating linear circuits systems is with admittance matrices, or Y matrices. The technique involves modeling the individual linear components as an N port admittance matrix, inserting the component Y matrix into a circuits nodal admittance matrix, installing port terminations at nodes that contain ports, eliminating ports without nodes though Kron reduction, converting the final Y matrix to an S or Z matrix as needed, and extracting desired measurements from the Y, Z, and/or S matrix.
See also
[edit]Concepts:
HDL:
Lists:
- List of electrical engineering software
- List of free electronics circuit simulators
- Comparison of EDA software
Software:
References
[edit]- ^ "Disadvantages and Advantages of Simulations in Online Education". Archived from the original on 2010-12-16. Retrieved 2011-03-11.
- ^ Mengue and Vignat, Entry in the University of Marne, at Vallee
- ^ Fishwick, P. "Entry in the University of Florida". Archived from the original on 2000-05-19. Retrieved 2021-07-23.
- ^ Pedro, J; Carvalho, N. "Entry in the Universidade de Aveiro, Portugal" (PDF). Archived from the original (PDF) on 2012-02-07. Retrieved 2007-04-27.
- ^ L. Walken and M. Bruckner, Event-Driven Multimodal Technology Archived 2007-05-05 at the Wayback Machine
- ^ Pejovic, P.; Maksimovic, D. (May 13, 1995). "A new algorithm for simulation of power electronic systems using piecewise-linear device models". IEEE Transactions on Power Electronics. 10 (3): 340–348. Bibcode:1995ITPE...10..340P. doi:10.1109/63.388000.
- ^ Allmeling, J.H.; Hammer, W.P. (July 13, 1999). "PLECS-piece-wise linear electrical circuit simulation for Simulink". Proceedings of the IEEE 1999 International Conference on Power Electronics and Drive Systems. PEDS'99 (Cat. No.99TH8475). Vol. 1. pp. 355–360 vol.1. doi:10.1109/PEDS.1999.794588. ISBN 0-7803-5769-8. S2CID 111196369 – via IEEE Xplore.
- ^ Ohnari, Mikihiko (1998). Simulation engineering. Ohmsha. ISBN 9784274902178. Retrieved October 12, 2022.
External links
[edit]Electronic circuit simulation
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Overview
Definition and Purpose
Electronic circuit simulation is the process of employing mathematical models and software algorithms to predict and analyze the behavior and performance of electronic circuits prior to physical fabrication. This approach allows engineers to replicate circuit responses to various inputs and conditions virtually, mitigating the risks and expenses associated with prototyping.[6] The primary purpose of electronic circuit simulation is to accelerate design iteration cycles, lower development costs, facilitate exploration of hypothetical scenarios, and enable performance optimization across analog, digital, and mixed-signal circuit domains. By simulating circuit responses under diverse operating conditions, it supports rapid validation and refinement, particularly for complex integrated circuits where physical testing would be prohibitively expensive.[6][7] Key benefits include substantial time savings through early detection of design errors, which prevents costly rework in later stages, and seamless integration with printed circuit board (PCB) layout tools to streamline the transition from simulation to manufacturing. These advantages enhance overall design efficiency and reliability, allowing for iterative improvements without hardware dependencies.[7][8] A typical simulation workflow commences with netlist creation, a textual representation of the circuit's components, connections, and parameters, which serves as input to the simulation engine. This is followed by executing analyses using methods like nodal analysis, culminating in the visualization of results such as voltage waveforms, current plots, and performance metrics to inform design decisions.[9][10]Historical Development
The development of electronic circuit simulation originated in the vacuum tube era of the 1940s and 1950s, when engineers relied on manual calculations and analog computers for analyzing linear circuits, often using techniques like nodal analysis for sinusoidal steady-state responses.[3] These methods were labor-intensive and limited to simple networks, prompting the transition to digital computing in the early 1950s with electromechanical relay-based systems at institutions like Bell Labs for solving equilibrium equations.[3] By the 1960s, computer-based simulation emerged, with programs such as ECAP (Electronic Circuit Analysis Program) developed by IBM, enabling automated analysis of linear and some nonlinear circuits on mainframes like the IBM 1620.[3] Concurrently, SCEPTRE was introduced for aerospace applications, focusing on transient analysis using explicit integration methods for complex systems.[3] Bell Labs also advanced early digital tools in the mid-1960s for integrated circuit design, laying groundwork for broader adoption.[3] A pivotal milestone occurred in 1972 with the release of SPICE (Simulation Program with Integrated Circuit Emphasis) by Donald Pederson and Larry Nagel at the University of California, Berkeley, which standardized nodal analysis and became a cornerstone for integrated circuit design through its public-domain availability and support for nonlinear DC, AC, and transient simulations.[11] SPICE's influence spurred rapid evolution; by 1975, SPICE2 enhanced integration algorithms, solidifying its role in academia and industry.[12] The 1980s marked commercial adoption, exemplified by PSpice in 1984 from MicroSim, which adapted SPICE for personal computers, democratizing access for board-level design.[13] In the 1990s, simulation integrated deeply with electronic design automation (EDA) tools, enabling full-chip verification and mixed-signal analysis.[12] The 2000s saw open-source advancements, such as ngspice (initially released in 1993 but significantly developed post-2000), which extended SPICE with modern features like mixed-signal support while maintaining compatibility. Post-2010, cloud-based platforms like EasyEDA and CircuitLab emerged, allowing browser-accessible simulations without local installation, facilitating collaborative design and scalability for large-scale analyses.[14] Simultaneously, AI-assisted methods gained traction, with machine learning optimizing circuit sizing and parameter tuning, as demonstrated in evolutionary algorithms integrated with neural networks for analog IC design since around 2015.[15] These innovations, including foundation models for VLSI, continue to accelerate simulation accuracy and efficiency.[16]Mathematical Foundations
Nodal Analysis
Nodal analysis serves as a foundational technique in electronic circuit simulation, relying on Kirchhoff's Current Law (KCL) to determine the voltages at each node in a circuit by formulating a system of linear equations.[17] Under KCL, the algebraic sum of currents entering a node equals zero, allowing the expression of currents through conductances (admittances) between nodes.[18] This approach treats node voltages as the primary unknowns, with one node designated as the reference (ground) at zero potential to avoid redundancy.[19] The process begins by identifying all nodes in the circuit, excluding the reference node, and labeling the unknown voltages. For each non-reference node, a KCL equation is written by summing the currents leaving the node—expressed as the voltage differences across admittances connected to that node—set equal to any external current sources injected at the node. These equations form a system of linear equations for a circuit with nodes, which can be solved simultaneously for the node voltages.[17] In matrix form, the nodal equations are compactly represented as: where is the admittance matrix, is the vector of node voltages, and is the vector of nodal current sources; for the -th node, this corresponds to .[20] The diagonal elements are the sums of admittances connected to node , while off-diagonal (for ) are the negative admittances between nodes and .[17] This method assumes a lumped-element model, where components are idealized and interconnected at discrete nodes without distributed effects, and incorporates initial conditions for reactive elements like capacitors and inductors to handle time-dependent simulations.[21] Its advantages include a systematic procedure well-suited for large-scale circuits, as the resulting matrix is sparse and can be efficiently solved using numerical techniques, forming the basis for extensions like modified nodal analysis (MNA) in modern simulators.Admittance Matrix Formulation
The admittance matrix, often denoted as Y, is a square matrix in nodal analysis that relates node voltages to nodal currents in a linear circuit. For a circuit with n nodes (excluding the reference ground node), Y is an n × n matrix where the diagonal element Yii represents the sum of all admittances connected to node i, and the off-diagonal element Yij (for i ≠ j) is the negative of the admittance between nodes i and j. This formulation stems from Kirchhoff's current law (KCL), expressed as I = YV, where I is the vector of nodal currents and V is the vector of nodal voltages.[22] The process of constructing the admittance matrix involves the "stamp" method, where each circuit element contributes specific entries to Y based on its topology and parameters. For a resistor with conductance G = 1/R connected between nodes i and j, the stamp adds G to Yii and Yjj, and subtracts G from Yij and Yji; if the resistor connects to ground (node 0), only G is added to the corresponding diagonal Yii. Similar stamps apply to other elements: for a capacitor C between nodes i and j, the contribution is sC (in the Laplace domain) to the diagonals and -sC to the off-diagonals, while inductors and dependent sources follow analogous patterns adjusted for their constitutive relations. This element-by-element assembly enables efficient matrix building in simulation software.[22] To solve YV = I, direct methods like Gaussian elimination with partial pivoting are used for linear circuits to handle potential singularities arising from floating nodes or ideal sources, ensuring numerical stability by selecting the largest pivot element during factorization. For nonlinear circuits, where Y becomes a Jacobian matrix dependent on V, iterative methods such as the Newton-Raphson algorithm linearize the system around an operating point and solve successively until convergence, often incorporating damping or source stepping for robustness. These techniques form the core of solvers in tools like SPICE.[22] In high-frequency applications, the admittance matrix can be converted to scattering parameters (S) for port-based analysis, using the relation S = (Y0 - Y) (Y0 + Y)-1, where Y0 = 1/Z0 I is the diagonal admittance matrix for the reference impedance Z0 (typically 50 Ω), and I is the identity matrix; this transformation facilitates matching and stability assessments in RF circuits.[23] As a numerical example, consider a simple RC circuit with a 1 kΩ resistor between node 1 and ground (node 0), and a 1 μF capacitor between node 1 and ground, analyzed in the s-domain for AC simulation. The admittance matrix Y (1×1, since one internal node) is: where G = 1/1000 S and C = 10-6 F; the nodal current I1 equals Y V1, yielding the transfer function directly. This illustrates the sparse, diagonal-dominant structure typical of such matrices.Simulation Example: Chebyshev Filter
A representative example of electronic circuit simulation using the admittance matrix formulation is the analysis of a 3rd-order low-pass Chebyshev type I filter, normalized for a cutoff frequency of 1 rad/s and a characteristic impedance of 1 Ω, with 0.5 dB passband ripple.[24] The ladder topology consists of a shunt capacitor C1 connected to the input port, followed by a series inductor L2, and a shunt capacitor C3 connected to the output port. The component values, derived from standard prototype tables, are C1 = 1.0316 F, L2 = 1.1474 H, and C3 = 1.0316 F, with the network doubly terminated by source resistance Rs = 1 Ω and load resistance Rl = 1 Ω.[25] The filter is modeled as a reciprocal two-port network, where the port currents and voltages are related by the admittance matrix : with for the capacitors and for the inductor. This matrix is obtained by applying the standard admittance stamping rules to the circuit topology: shunt elements contribute to the diagonal entries, while series elements add to both diagonal and off-diagonal terms with opposite signs. For larger circuits with internal nodes, subcircuit Y-parameters are combined into a global matrix by block partitioning, where internal node voltages are eliminated via the Schur complement reduction: the reduced port admittance is , ensuring efficient handling of sparse structures without explicit inversion of the full matrix. At a specific frequency, such as rad/s, the numerical entries are computed as follows: S and S, yielding S and S. To incorporate the terminations for the full simulation, the system equations become and , which is solved for the voltages and (where ) at each frequency. Post-processing involves converting the unterminated to scattering parameters for standardized RF analysis, using the formulas followed by , , and symmetrically for and , assuming Ω.[23] A frequency sweep from 0.1 to 10 rad/s produces the magnitude response, where displays equiripple behavior in the passband with three peaks, attenuating monotonically beyond rad/s at approximately -60 dB/decade roll-off. Special cases in the simulation include handling unterminated ports by setting the port current to zero (e.g., for open-circuit output impedance computation, yielding ). For voltage sources with zero internal resistance, a small parallel conductance (e.g., S) is added to the corresponding admittance entry to prevent matrix singularity. The transfer function can be evaluated numerically via the solved system at or symbolically for this low order by forming the 2×2 coefficient matrix and computing its inverse or determinant. Verification against the analytical response confirms accuracy, as the simulated matches , where is the 3rd-order Chebyshev polynomial of the first kind and corresponds to 0.5 dB ripple (i.e., or -0.5 dB).[24] At DC, both yield (0 dB), and the passband ripple aligns within numerical precision of 0.01 dB.Types of Analysis
DC Analysis
DC analysis in electronic circuit simulation computes the quiescent operating points, or Q-points, of nonlinear circuits, such as amplifiers and bias networks, by determining the steady-state voltages and currents under constant DC excitation. This process is essential for establishing the bias conditions that serve as the foundation for subsequent analyses like transient or small-signal simulations. In SPICE-based simulators, inductors are treated as short circuits and capacitors as open circuits during this phase to focus solely on DC behavior.[26] The method involves solving a system of nonlinear equations derived from the circuit's topology, typically formulated using modified nodal analysis. For a nonlinear circuit, the governing equation is expressed as , where is the vector of node voltages, is the voltage-dependent admittance matrix, and represents the independent current sources. This nonlinear system is solved iteratively using the Newton-Raphson method, which linearizes the equations around an initial guess and refines the solution through successive approximations. The iteration proceeds as: where is the Jacobian matrix, containing the partial derivatives . Convergence is achieved when the residual falls below a specified tolerance, typically on the order of relative error. In the linear case, this simplifies to a direct solution of the nodal admittance matrix equations.[27] To ensure reliable convergence, especially in circuits with sharp nonlinearities like diodes or transistors, techniques such as damping factors are employed. These include gmin stepping, which adds a small conductance (e.g., S) from each node to ground—increasing the diagonal elements of the admittance matrix—to improve numerical stability, gradually reducing it over multiple steps, and source stepping, which ramps independent sources from zero to their final values in increments (e.g., 10 steps). Initial guesses are often obtained from a linear DC solution or user-specified node sets via directives like .NODESET, preventing divergence in multistable circuits.[28] The primary outputs of DC analysis include node voltages, branch currents through elements, and device-specific parameters, such as collector current and base-emitter voltage for transistors. These results provide critical insights into operating conditions, power dissipation, and potential issues like saturation or cutoff, enabling engineers to verify circuit functionality before more complex simulations.[26]AC Analysis
AC analysis in electronic circuit simulation performs frequency-domain evaluation of linearized circuits to assess their response to sinusoidal steady-state inputs. It builds on the DC operating point by applying small-signal models, which approximate nonlinear elements as linear equivalents around the bias condition, enabling the computation of gain, phase shift, and bandwidth. This analysis is essential for understanding how circuits behave across a range of frequencies, such as identifying the 3 dB bandwidth where the gain drops to 70.8% of its maximum value.[29] The method involves representing circuit variables as phasors in the complex frequency domain, transforming time-varying sinusoids into steady-state complex amplitudes. Passive and active elements are modeled with frequency-dependent impedances or admittances; for instance, a capacitor's admittance is , where , is the angular frequency, and is the capacitance. The entire circuit is formulated as a system of linear equations using the complex admittance matrix , which relates nodal voltages to input currents via A small-magnitude AC stimulus (typically 1 V or 1 A) is applied at each frequency in a user-specified sweep (e.g., logarithmic from 1 Hz to 1 GHz), and the matrix is solved using techniques like Gaussian elimination to yield phasor solutions for voltages and currents.[29][30] Key outputs from AC analysis include Bode plots, which graph the magnitude response in decibels and phase in degrees against the logarithm of frequency, providing insights into flat-band gain, corner frequencies, and roll-off rates. Transfer functions, expressed as , capture the overall input-output relationship, while impedance and admittance spectra detail port behaviors. Noise figure computation extends this by integrating stochastic models of device noise (e.g., thermal and shot noise) to quantify signal degradation, often plotted as equivalent input noise over frequency.[29] Common techniques enhance interpretation of these outputs. The decibel scale for magnitude is given by , compressing wide dynamic ranges (e.g., gains from 1 to 1000 span 0 to 60 dB) and highlighting factors like 6 dB/octave roll-off in single-pole systems. Pole-zero analysis identifies the complex roots of the transfer function's denominator (poles) and numerator (zeros), revealing stability margins—such as phase margin from the distance to -180° phase—and predicting resonant peaks or notches.[29][31] A representative application is the frequency response of a common-emitter amplifier, where AC analysis plots the voltage gain versus frequency to determine the midband gain (e.g., 40 dB), low-frequency cutoff due to coupling capacitors, and high-frequency roll-off from transistor parasitics, ensuring the design meets bandwidth specifications. In filter design, such as a low-pass Butterworth circuit, it measures attenuation in the stopband (e.g., >40 dB beyond the cutoff) relative to the passband ripple, verifying selectivity without transient simulation.[30]Transient Analysis
Transient analysis in electronic circuit simulation examines the time-domain behavior of circuits in response to time-varying inputs, such as step functions or pulses, to capture dynamic phenomena including switching transients, settling times, and transient responses in both linear and nonlinear systems. This type of analysis is essential for evaluating how circuits evolve from initial conditions to steady-state operation, particularly in applications involving rapid changes like digital logic transitions or power supply startup. Unlike steady-state analyses, transient simulation provides a complete temporal profile, enabling engineers to assess performance metrics critical for reliability and timing in integrated circuits and systems.[32][33] The method involves discretizing the continuous-time differential-algebraic equations (DAEs) that model the circuit, typically formulated as , where is the capacitance matrix, is the admittance matrix, is the node voltage vector, and represents time-dependent sources. At each time step, these nonlinear DAEs are solved iteratively using numerical integration techniques, such as the trapezoidal rule or Gear's method, which approximate the derivatives and ensure stability for stiff systems common in electronics. The backward Euler method, a first-order implicit scheme widely adopted for its unconditional stability, linearizes the system at time to yield the key equation: where is the time step, allowing efficient computation via Newton-Raphson iteration for nonlinear elements. These integration methods were pioneered in early simulators like SPICE, balancing accuracy and computational cost for practical circuit analysis.[33][34] To optimize simulation efficiency and accuracy, variable time-stepping schemes are employed, adaptively adjusting based on local truncation error estimates or convergence behavior, which is particularly useful for handling discontinuities in digital circuits or events like switch closures. For instance, smaller steps are taken during fast transients to maintain precision, while larger steps accelerate simulation in quiescent periods, reducing overall runtime without sacrificing fidelity. This adaptive approach, often integrated with error control mechanisms, enhances the method's robustness for complex, mixed-signal designs.[35][34] The primary outputs of transient analysis are time-domain waveforms plotting voltages and currents versus time, from which key parameters such as rise time, fall time, overshoot, and settling time are extracted to quantify circuit performance. These visualizations and metrics aid in verifying design specifications, debugging issues like ringing or delays, and ensuring compliance with operational requirements in real-world applications.[36][32]Component Modeling
Passive Elements
In electronic circuit simulation, resistors are modeled as linear elements contributing to the nodal admittance matrix through their conductance , where is the resistance value.[37] For a resistor connected between nodes and , the stamping procedure adds to the diagonal elements and , and to the off-diagonal elements and .[37] Temperature dependence is incorporated using the temperature coefficient of resistance (TCR), often parameterized in simulators like SPICE with a linear or quadratic model such as , where is the temperature deviation from nominal.[38] Capacitors are represented in the s-domain with admittance , where is the capacitance and is the complex frequency, corresponding to an impedance of .[39] Initial voltage conditions across the capacitor are specified at the start of transient simulations to account for stored charge, typically via an initial condition (IC) parameter that sets the voltage at time zero.[40] Parasitic effects, such as equivalent series resistance (ESR), are modeled by adding a series resistor to the ideal capacitor, which introduces losses and affects high-frequency behavior.[41] The stamping for a capacitor between nodes and involves adding to and , and to and .[37] In time-domain simulations, companion models linearize the capacitor using integration methods like trapezoidal rule, resulting in a Norton equivalent circuit with a parallel conductance and history-dependent current source.[42] Inductors are modeled with s-domain admittance , where is the inductance, equivalent to an impedance of .[39] Initial current through the inductor is set at the simulation start to capture stored magnetic energy, often using an IC parameter for transient analysis.[43] Mutual coupling between inductors is handled by modified nodal analysis stamps that introduce off-diagonal terms proportional to the mutual inductance , typically where is the coupling coefficient, affecting the admittance matrix for coupled pairs.[44] For a single inductor between nodes and , the stamp adds to and , and to and .[37] Time-domain companion models for inductors also employ Norton equivalents, with a parallel resistance and history current source based on the chosen integrator.[42] Nonlinear extensions of passive elements include varactors, which model voltage-dependent capacitance for applications like tuning circuits, and saturable inductors, where inductance varies with current due to core saturation, often using lookup tables or polynomial fits in simulators.[45] These are briefly incorporated as parameterized models, with detailed nonlinear behaviors addressed in specialized analyses.Active Devices
Active devices in electronic circuit simulation require nonlinear models to capture their semiconductor physics, enabling accurate prediction of circuit behavior under varying bias conditions. These models contribute essential equations to the system's nonlinear algebraic framework, solved iteratively during DC and transient analyses. Diodes are typically modeled using the Shockley equation, which describes the current-voltage relationship as , where is the saturation current, is the ideality factor, is the junction voltage, and is the thermal voltage. For more comprehensive simulation, particularly in integrated circuits, the Ebers-Moll model extends this by incorporating forward and reverse currents through coupled diodes, accounting for transport effects in p-n junctions.[46] Bipolar junction transistors (BJTs) employ the Gummel-Poon model for large-signal simulation, which refines the Ebers-Moll approach by including base-width modulation and high-injection effects through parameters such as the forward current gain and forward transit time .[47] In small-signal analysis, the hybrid-pi model linearizes the device around an operating point, with its parameters stamped into the admittance matrix as conductances and capacitances representing transconductance , input resistance , and output resistance .[48] MOSFETs are simulated using progressively sophisticated models, starting from the Level 1 square-law model, which expresses drain current in saturation as , where is the carrier mobility, is the oxide capacitance per unit area, is the aspect ratio, is the gate-source voltage, and is the threshold voltage. Advanced simulations utilize BSIM models (e.g., BSIM4), which incorporate short-channel effects, with the body effect modeled via threshold voltage shifts in subcircuits that adjust based on source-body bias.[49] Operational amplifiers (op-amps) are often represented by behavioral models treating them as an ideal voltage-controlled voltage source (VCVS) with high open-loop gain , where the output voltage is , assuming infinite input impedance and zero output impedance. For frequency-dependent simulations, macromodels extend this by adding poles and zeros to capture bandwidth limitations, such as a dominant pole for unity-gain frequency roll-off.[50] The nonlinear equations from these active device models are integrated into the circuit's nodal analysis framework, where partial derivatives form the Jacobian matrix for Newton-Raphson iterations, ensuring convergence to the solution of the system through updates .[51]Sources and Parasitics
In electronic circuit simulation, excitation sources provide the input signals that drive the analysis, while parasitic elements represent unintended physical effects arising from the circuit's layout and fabrication. Voltage sources are fundamental components that enforce a specified voltage difference between two nodes. An ideal voltage source in modified nodal analysis (MNA) is incorporated by adding an auxiliary equation that constrains the node voltage difference to , effectively stamping coefficients of +1 and -1 in the rows corresponding to the source's branch current variable within the system matrix, with added to the right-hand side (RHS) vector.[52] This approach, introduced in early SPICE implementations, extends standard nodal analysis to handle voltage constraints without singularity issues from infinite admittance.[10] In practice, ideal voltage sources can lead to numerical ill-conditioning, so simulators often model them with a small series resistance (e.g., on the order of Ω) to regularize the matrix while approximating zero impedance.[53] Current sources, in contrast, inject a specified current into the circuit without constraining voltages directly. An ideal current source connected between two nodes contributes to the RHS entry of one node and to the other, with no admittance stamps in the system matrix, as it presents infinite impedance.[52] This formulation aligns with Kirchhoff's current law enforcement in nodal analysis and is standard in SPICE-like simulators.[21] For parallel configurations, multiple current sources simply sum their contributions to the RHS vector at shared nodes. Time-varying sources extend these models to dynamic analyses. In transient simulations, piecewise linear (PWL) sources approximate arbitrary waveforms by linearly interpolating between user-specified time-voltage or time-current pairs, enabling accurate representation of pulses or custom signals without analytical complexity.[54] Sinusoidal sources, defined by amplitude, frequency, and phase, are used for both transient (time-domain sine waves) and AC analyses, where the simulator linearizes the circuit around the DC operating point and computes phasor responses assuming steady-state sinusoidal excitation at a single frequency.[29] In AC analysis, the source's AC amplitude (often set to unity for transfer functions) drives the frequency sweep, while transient modes integrate the full nonlinear behavior over time.[55] Parasitic elements account for non-ideal effects from physical implementation, particularly in integrated circuits where layout geometry introduces unintended capacitances and resistances. Parasitic capacitance arises primarily from overlapping conductors separated by dielectrics, approximated by the parallel-plate formula , where is the permittivity of the insulator, is the overlapping area, and is the separation distance; this models fringing fields between metal layers or to substrate in VLSI designs.[56] More accurate extraction uses field solvers to solve Laplace's equation over 3D geometry, yielding a capacitance matrix for multi-conductor systems. Parasitic resistance in interconnect wires stems from material resistivity and geometry, calculated as for a rectangular conductor with length , width , thickness , and resistivity ; skin effect adjustments apply at high frequencies.[56] These values are extracted from layout geometry using tools that employ pattern matching for speed or boundary element methods for precision, converting GDSII files into equivalent RC networks for post-layout simulation. Special cases in source handling ensure simulation stability. Zero-impedance voltage sources risk matrix singularity, mitigated by internally adding a tiny conductance (e.g., S) across the terminals during solution, removable post-convergence if needed.[57] Floating nodes, lacking a DC path to the reference (ground), can cause indeterminate voltages; simulators detect them by checking connectivity and either issue warnings or implicitly add high-value resistances (e.g., 1 GΩ) to ground for numerical reference, though explicit grounding is recommended to avoid artifacts.[57]Software Tools
Open-Source Simulators
Open-source electronic circuit simulators provide freely available tools for analyzing analog, digital, and mixed-signal circuits, often building on the SPICE framework while offering community-driven enhancements for accessibility and extensibility. These simulators are particularly valued in academic, research, and hobbyist settings due to their lack of licensing costs and support for custom integrations, enabling users to perform DC, AC, transient, and noise analyses without proprietary restrictions.[58][59] Ngspice, a fork of the original Berkeley SPICE3, serves as a robust command-line simulator for electric and electronic circuits, supporting components such as JFETs, bipolar and MOS transistors, and passive elements. It enables mixed-signal simulation through integration with XSPICE code models and, since version 42, incorporates Verilog for efficient co-simulation of digital blocks within analog netlists. Additionally, ngspice facilitates Python integration via libraries like PySpice, allowing scripted automation and data analysis in Python environments for advanced workflows.[58][60][61] Qucs-S, an evolution of the Quite Universal Circuit Simulator (Qucs), provides a graphical user interface (GUI) with schematic capture for intuitive circuit design and simulation, leveraging multiple back-end engines including its own Qucsator, ngspice, and Xyce. It excels in RF and system-level simulations, offering features like harmonic balance analysis, S-parameter extraction for multi-port networks, and support for microwave components such as transmission lines and attenuators. This makes it suitable for high-frequency applications, where users can visualize results through integrated plotting tools without manual netlist editing.[59][62][63] Xyce, developed by Sandia National Laboratories, is a SPICE-compatible simulator optimized for parallel processing on high-performance computing platforms, targeting large-scale circuits with over 100,000 devices. It supports standard analyses including DC operating point, transient, AC frequency-domain, and small-signal noise, with advanced parallel linear solvers to accelerate transient simulations and handle device scaling in integrated circuits. Unlike traditional sequential SPICE tools, Xyce's message-passing interface enables efficient distribution across multiple processors, improving scalability for complex models.[64][65]| Feature | Ngspice | Qucs-S | Xyce |
|---|---|---|---|
| Primary Interface | Command-line with scripting (control language) | GUI with schematic entry | Command-line, supports parallel MPI execution |
| Netlist Format | Standard SPICE .cir | SPICE-compatible via back-ends; schematic-to-netlist conversion | SPICE .cir with extensions for parallel runs |
| Key Analyses | DC, AC, transient, noise; mixed-signal with Verilog-A and Verilog | DC, AC, transient, harmonic balance, S-parameters for RF | DC, AC, transient, noise; optimized for large-scale transient |
| Scripting/Integration | Python via PySpice; custom models in C/XSPICE | GUI scripting; integrates ngspice/Xyce kernels | C++ extensions; parallel scripting via MPI |
| Strengths | Versatile mixed-signal; lightweight for education/research | User-friendly for RF/system design; visual plotting | High-performance for massive circuits; parallel scaling |
| Limitations | No native GUI; requires external tools for schematics | Dependent on back-end kernels for accuracy; less optimized for ultra-large nets | Steeper setup for parallel; limited interactive features |