I recently had a need to brush up on some analogue electronics and discovered, to my surprise, that there are some excellent free tools to help one learn and probably avoid some expensive mistakes before building actual circuits. Most of these tools are based on the famous SPICE simulation engine originally developed at University of California, Berkeley. Anyway, I wanted to share my discovery with anyone who may be in the same position as me and in want of some help. There are various commercial and free SPICE tools out there. I found that using the free with no-time-limit evaluation versions of commercial tools is a better bet than the free versions since these tools tend to be updated more often. One drawback with this is that vendors who offer the free to use no-time-limit evaluation versions of their tools tend to limit them in some way (obviously). However, for most small circuits they are an excellent alternative to nothing or dated free tools. One trick that may be handy when using these neutered versions of tools, for large circuits that break the limitations of the simulators, is to simulate the large circuit as mini sub-circuits. Once the behavior of the sub-circuit is determined one can replace the sub-circuit with a component (passive or active) for a particular condition one is trying to look at. It can get cumbersome but can help when no other avenues exist. However, this may not always be possible but I have found some use in doing so. It has also improved by intuition in circuit design since one starts looking at circuits as blocks and approximating their behavior for basic quick analysis.

Two simulators that I played with are 5Spice and SIMetrix/SIMPLIS Intro version. The former is page limited (see their FAQ entry for details on limitations and use restrictions) and the latter is node/parts limited, see here for details. Like all tools there is the obvious learning curve since each tool may do things differently, however, the pay-offs are worth the effort. Finally, if you are a student the one tool that you cannot go wrong with is the National Instruments student edition of MultiSim. I have been playing with the evaluation version of the tool and for $39.95 (student pricing, proof required) it is worth every penny and then some. Unfortunately, the base Pro version of Multisim is substantially more expensive (~$1500). Keep in mind that all these tools will simulate mixed mode circuits, that is, both digital and analogue circuits combined.

If commercial tools are not your thing then XSPICE by Georgia Tech may be worth looking into. It is public domain and based on the latest SPICE3. As far as I can tell, unfortunately (or fortunately!), it is provided as source and runs on Unix/Linux systems only. Spice OPUS is another that may be worth looking at, it is based off XSPICE with further optimizations added. Both these appear to be quite dated.

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