What is Circuit Simulation?

Circuit simulation is the process of using computer software to model, analyze, and verify the behavior of an electronic circuit or system. By creating a virtual model of a circuit, engineers and hobbyists can test designs, optimize performance, and catch issues before ever building a physical prototype.

Circuit simulators use mathematical models to replicate the electrical characteristics of components like resistors, capacitors, inductors, transistors, and integrated circuits. They can perform DC, AC, and transient analysis to determine voltages, currents, impedances, and other key parameters at any point in a circuit.

Some key benefits of circuit simulation include:

• Rapid iteration of designs without physical assembly
• Testing circuits under a wide range of operating conditions
• Identifying problems early to avoid costly redesigns later
• Gaining deeper insight into circuit behavior and performance
• Learning electronics principles through virtual experiments

Types of Circuit Simulation

There are several different types of analyses that circuit simulators can perform:

Analysis Type	Description
DC Sweep	Calculates the DC operating point of a circuit as one parameter, such as input voltage or component value, is swept through a range of values. Useful for plotting I-V curves and transfer functions.
AC Sweep	Determines the frequency response of a circuit by calculating the AC voltages and currents over a range of frequencies at a given operating point. Generates Bode plots of gain and phase.
Transient	Simulates the time-domain response of a circuit to an input signal like a pulse or sine wave. Calculates instantaneous voltages and currents at specified time steps. Used to test digital logic, switching power supplies, etc.
Noise	Computes the thermal, shot, and flicker noise generated by components to determine the noise figure and dynamic range of circuits like amplifiers and filters.
Monte Carlo	Performs multiple simulation runs with statistical variations in component values to model the effect of real-world manufacturing tolerances on circuit performance and yield.

Popular Free and Open Source Circuit Simulators

While commercial circuit simulators can cost thousands of dollars per license, there are also many powerful free and open source alternatives available. Here are some of the most popular:

SPICE

SPICE (Simulation Program with Integrated Circuit Emphasis) is the granddaddy of all circuit simulators. Originally developed at UC Berkeley in the 1970s, it has spawned an entire ecosystem of derivatives and remains the gold standard for accurate analog simulation.

There are many free and open source SPICE-based simulators available, including:

• ngspice – Open source mixed-level/mixed-signal circuit simulator based on Berkeley SPICE 3f5. Supports all standard SPICE analyses and device models plus some newer ones like BSIM 3/4.
• Xyce – High-performance parallel SPICE simulator from Sandia National Labs. Can handle very large circuits. Adds device models for power electronics.
• SPICE OPUS – SPICE 3f5 plus extensions like VBIC bipolar model and BSIM3v3 MOSFET model. Graphical postprocessor for plotting results.

Falstad

Falstad Circuit Simulator is a free web-based sim with a simple schematic-capture interface and fast real-time 2D visualization of voltages and currents. Great for quickly prototyping and experimenting with basic circuits. Includes oscilloscope and Bode plotter. Large library of example circuits.

Qucs

Quite Universal Circuit Simulator (Qucs) is a free cross-platform circuit simulator with GUI schematic capture, SPICE netlist import, and graphical postprocessing. Supports many standard SPICE analyses and device models plus Verilog-A behavioral models, VHDL files, and RF layouts.

Ngspice

Ngspice is an open source mixed-level/mixed-signal circuit simulator based on Berkeley SPICE 3f5. Supports all standard SPICE analyses and device models plus some newer ones like BSIM 3/4. Can be used standalone with SPICE netlists or integrated with schematic capture front-ends like gEDA and KiCad.

GNUcap

GNUcap is the GNU circuit analysis package. It aims for SPICE-like performance and device models but deviates from the SPICE netlist format. Performs DC, AC, and transient analysis. Good for interactive simulation via command-line interface.

Other Open Source EDA Tools

In addition to pure play circuit simulators, there are also many open source electronic design automation (EDA) tools that bundle simulation capability with schematic capture, PCB layout, etc. Some examples:

• KiCad – Popular cross-platform open source EDA suite for schematic capture, PCB layout, SPICE simulation (via ngspice and Pspice), etc.

• gEDA – Linux-based open source EDA tools including schematic editor gschem, netlister gnetlist, and ngspice integration.

• GEDA-PCB – Linux-based open source PCB layout tool that integrates with the gEDA schematic flow.

• Fritzing – Easy to use open source EDA software for schematic capture and basic PCB layout. Oriented towards hobbyists and students. Includes breadboard view.

Getting Started with Free Circuit Simulation

To get started using a free circuit simulator, follow these general steps:

1. Select a simulator based on your needs and platform. Consider factors like ease of use, active development community, performance, and feature set.

2. Download and install the simulator, along with any dependencies. Make sure to read the documentation for platform-specific instructions.

3. Explore the user interface and familiarize yourself with basic operations like creating a new design, adding components from the library, drawing wires, setting up voltage/current probes, and running simulations.

4. Start with simple example circuits to learn the fundamentals. Most simulators come with built-in designs for common building blocks like amplifiers, power supplies, filters, logic gates, etc.

5. Consult tutorials and user guides to dive deeper into the capabilities of the simulator. Many have active online communities for troubleshooting and design reviews.

6. Validate your simulation results by comparing to hand calculations, measurements on a breadboard, or other simulators. Use caution with very high frequency designs as simulator device models and parasitics may not be accurate.

7. Contribute back to the community by reporting bugs, submitting patches, writing documentation, answering forum questions, etc. Open source thrives when users become contributors.

SPICE Modeling Fundamentals

At the heart of most circuit simulators is SPICE, which uses a netlist format to describe circuits as a set of components connected between named nodes. Each component is defined by a one-letter element type (e.g. R for resistor), a unique reference designator, the nodes it connects, and a list of parameter values.

Here is an example SPICE netlist for a simple common-emitter amplifier:

* Common-emitter amplifier 
VCC 1 0 DC 12
VIN 2 0 AC 1 SIN(0 1m 1k)  
RB 1 3 100k
RC 1 4 3.6k
RE 5 0 1k
Q1 4 3 5 MJD31
.model MJD31 NPN(Is=1E-12 Bf=200 Vaf=100)
.control
* Bias point analysis
OP 
* AC analysis 10 Hz to 10 MHz
AC DEC 20 10 10Meg  
* Transient analysis for 1 ms
TRAN 1u 1m
.endc
.end

This netlist defines a 12 V DC power supply VCC, a 1 mV 1 kHz sinusoidal input source VIN, three resistors RB, RC, and RE, and an NPN BJT Q1. It also specifies .model parameters for the transistor, .control statements for the analyses to run and plotting commands, and a .end statement.

To simulate this circuit, the SPICE engine would:

1. Read in the netlist and build a matrix of equations describing the component values and connectivity
2. Solve the nonlinear DC equations to find the bias point
3. Linearize the equations around the bias point for small-signal AC analysis
4. Perform an AC sweep over the specified frequency range to determine gain and phase
5. Numerically integrate the nonlinear equations over time for transient analysis

The user can then view the results of the operating point, AC, and transient analyses as tables or plots.

SPICE provides many different element types and model parameters to describe components:

Element Type	Description	Key Parameters
R	Resistor	Resistance, tolerance, temp. coeff.
C	Capacitor	Capacitance, ESR, voltage rating
L	Inductor	Inductance, series resistance, Q factor
D	Diode	Is, Rs, N, Cjo, Tt
Q	BJT	Is, Bf, Br, Cje, Cjc, Tf, Tr
M	MOSFET	VTO, KP, LAMBDA, GAMMA, PHI
J	JFET	VTO, BETA, LAMBDA, RD, RS
V	Voltage Source	DC, AC, PULSE, SIN, EXP, PWL
I	Current Source	DC, AC, PULSE, SIN, EXP, PWL

The accuracy of the simulation depends heavily on choosing the right model for each component and extracting the correct parameters from datasheet curves, measurements, or process data. Many vendor websites and open source repositories provide pre-made SPICE models for common parts.

With SPICE as the underlying engine, a circuit simulator provides a powerful tool for virtual prototyping and verification of electronic designs. By carefully modeling components, applying realistic stimulus, and analyzing the results, designers can gain valuable insight into circuit behavior and performance prior to building physical hardware.

FAQ on Free Circuit Simulation

What is the best free SPICE simulator?

There is no single “best” free SPICE simulator as it depends on the user’s specific needs and preferences. Some of the most popular free and open source SPICE simulators are ngspice, Xyce, and SPICE OPUS. When evaluating simulators, consider factors like performance, device model library, platform compatibility, ease of use, and community support.

Can free simulators handle large, complex circuits?

Yes, many free and open source simulators can handle large circuits with thousands of components. For example, Xyce from Sandia National Labs is designed for parallel execution on high-performance computing clusters to simulate very large analog, digital, and mixed-signal circuits. However, the simulation time and memory usage will increase with circuit size and complexity.

How accurate are free SPICE simulators compared to commercial ones?

The accuracy of a SPICE simulator depends more on the device models and parameters used than on whether it is free or commercial. Many free and open source simulators use the same underlying SPICE 3f5 engine as commercial ones and can provide similar results for well-characterized components. However, commercial simulators may offer broader model libraries, better parameter extraction tools, and more extensive verification and support.

Are there any web-based circuit simulators?

Yes, there are several web-based circuit simulators that allow you to create, edit, and simulate circuits entirely in a browser without installing any software locally. One popular example is CircuitLab which provides a full schematic capture and SPICE simulation environment. Another is the Falstad Circuit Simulator which has a simple user interface and fast real-time visualization of voltages and currents.

What are some good resources for learning circuit simulation?

There are many great free resources available online for learning circuit simulation and SPICE modeling. Some recommendations:

• SPICE: A Guide to Circuit Simulation and Analysis Using PSpice by Paul Tuinenga – A classic textbook on PSpice that covers SPICE basics and modeling techniques. Older editions available as free PDFs.
• Ngspice User Manual – Comprehensive documentation for the ngspice simulator including tutorials and model references.
• SPICE Circuit Handbook from Analog Devices – Collection of SPICE models and simulation examples for common analog building blocks.
• Ken Kundert’s Designer’s Guide Community – Forum and knowledgebase for advanced SPICE modeling techniques and industry best practices.

By leveraging the wealth of free and open source circuit simulation tools and educational materials, any student, hobbyist, or professional can start exploring the power of virtual circuit design and analysis.