simulationsoftwarespice
Does anyone know of a freeware SPICE / circuit simulator?
SPICE (Simulation Program with Integrated Circuit Emphasis) is a general-purpose, open source analog electronic circuit simulator. It is a powerful program that is used in integrated circuit and board-level design to check the integrity of circuit designs and to predict circuit behavior. Wikipedia
I'm going to sorta disagree with Olin.
If you're using a simulator for something you can do with a calculator and a piece of paper in a few minutes then you're using the simulator for the wrong purpose. If you assume that your 'napkin math' analysis will hold up in reality you're likely working on extremely simple, basic circuits in the first place.
More importantly you're really saying that it's OK to skip what is really the most important stage of pre-prototype design verification. This is a really bad idea if you're working on anything even moderately complex and very much can come back to bite you even on simple circuits. I've seen even the simplest IR transmitter oscillate due to parasitics.
Additionally, a huge use case of simulation that is a real pain to do with just a calculator is Montecarlo analysis. Almost every simulator supports this and it is very important for production designs.
It's actually very rare that a simulator will not give you more insight into a real circuit than a 2-minute, mostly intuition-based, analysis of the circuit will. A couple of hours generating the simulation can easily save you days waiting to get a prototype back only to find out that through some awkward component or parasitic interactions your theoretically perfect transmitter is just a lousy oscillator.
As an example, taken from an Analog Devices app note:
On the left we see a basic op amp circuit. On the right we see what this circuit would look like if one considers basic PCB parasitic effects.
No question that with 60 seconds and a calculator you can figure out what the circuit on the left is doing.
However, that is no substitute for producing a more complex model of the real circuit in the real application such as the circuit to the right. The right hand circuit is far from easy to analyze manually without hand-waving away components as irrelevant.
Additionally a proper simulation is going to use more realistic models for components, rather than the ideal models, which for a circuit of any complexity or speed is critically important to understand and analyze.
As to the original question:
Most circuit simulators are at least related to SPICE and many share a compatible or close to compatible model format. Additionally there are many other simulators which specialize in particular fields. Notably RF/microwave simulation, digital logic simulation, etc.
The most common simulators I've run into:
Which one(s) you will see in a given company is usually a function of their field of specialty (analog, mixed-signal, RF, etc), what integrates well in their chosen development environment and what they are historically comfortable with.
I would say that depends heavily on what you need it for. Often the expensive spices are part of some e.g PCB design tool. I'm just trying out MultiSIm from NI (expensive orcad type tool), and it has lots of pretty virtual instruments (e.g scope, distortion analyser, etc) and monte carlo analysis (which LTspice does not have a "convenient" version of - it does have some functions you can use though as Vlad points out, here is a link on using them) but to be honest I find that 99% of things I could do on LTspice.
I find the LTspice setup is by far the quickest out of any spice I have tried, once you get used to the key commands. R for resistor, D for diode, is much easier than clicking the picture (or even selecting from a pop up box in MultiSims case, arghh) and dragging to the right place every time.
You can have a circuit done in seconds this way.
The manual is not as pretty either, but all the info you need is there regarding how to use, eg. the .param, .step and .measure commands for doing things like running an analysis many times and varying parameters. I just tried to run a transfer function analysis in MultiSim in this manner (i.e. run may times and vary a parameter then plot results) but despite reading/wrestling for hours with it, I couldn't manage it, but a quick addition of .step V2 -15 15 1 to the sheet made it possible in LTspice.
I'm sure some of the above is simply as I'm new to MultiSim, and no doubt I am missing something (as the above example simply must be possible in a tool like that) and no I don't work for LT :-) but it has been the only spice that I have used regularly for the last few years. The main point is that it will do all the normal stuff as good (and probably faster) than the expensive tools, but if you need the extras (e.g. monte carle, PCB level anaysis based on actual routing/IBIS models - Altium does this excellently) and all wrapped up in one design tool then you may need more than if can offer.
In my view it can't hurt much to have around even if you do need a more powerful tool anyway.
Best Answer
I thought that one of the other analog chip makers had a spice too but I can't remember who :(
I have been to a few talks on simulation given by physicists and EEs who have done chip design. Each of the talks seems to end like this ---
Also most of the free simulators are not distributed with models. Re-distribution of the models is usually a copyright violation. LTspice is distributed with models of the Linear Tech parts. I am not sure the quality of the models. Most manufacturers do not want to reveal too many details about their process.