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WHAT IS LT SPICE
And why should I care?

So you have an idea in your head for a new circuit that you have never seen before and can't find in the literature. You wonder if it will even work but you would like to try it to find out. It uses many parts and would take a lot of time and work to breadboard. If you knew LT Spice you could easily test it without picking up a single resistor, capacitor, etc. Spending an afternoon with your computer can save you several days of breadboarding and testing.

The fancy schmancy generic name for such a program is circuit simulation software. Don't be afraid, it doesn't byte. The screen shot below shows just one of the simpler things it can do.

screen shot .

Figure 1 LTspice with tone control.

What all can you do with LTspice?

What can't you do.

YOU CAN DO,

These are a few I could think of off the top of my head. I'm sure there are many more I haven't thought of and may not even be aware of. A temperature test requires more than a household refrigerator. The last thing on the list is practically impossible in the real world. The book noted at the bottom of this page contains literally hundreds of applications for LTspice. It is 1.43 inches (36.4 mm) thick, not including the hard covers.

Anyone over a certain age who has enrolled in an electronics lab has likely done a frequency response by taking data points and plotting them on graph paper using a French curve. I can hear you, using words like antiquated, ancient, primitive, outdated, etc. I lived through those times. Instead of making fun of me consider yourself lucky you didn't.

At the top I seemed to be saying that LTspice can take the place of a Breadboarded version of the circuit. It is not meant for that although in some cases it might be. The normal progression of a circuit would proceed from idea, to LTspice, to breadboard, to prototype, to production. This procedure should always be followed when practical. Some high HF and VHF circuits probably can't be breadboarded successfully. An example might be the wide band amplifier in a 200 MHz oscilloscope. Such a circuit can only be tested on the printed circuit board which will be the final production version. LTspice can be used to test the DC and low frequency behavior of the amplifier but the only way to test the high end is to make a prototype PC board and test it in a mockup of the final product. It may take several iterations of the board to get everything right. My point in all this is, don't depend on LTspice, or any other circuit simulator, to be the final test of a circuit with the POSSIBLE exception of an audio amplifier.

History.

1969 to 1971.

The very first version of SPICE was not even called that. It was written at Berkley and was called CANCER (Computer Analysis of Nonlinear Circuits Excluding Radiation). A bit of a stretch. It was written in FORTRAN and for those too young to remember the input was by means of punched cards and the output from a line printer. That meant absolutely no graphics except maybe a crude graph maid by printing star characters on the printer. Today we think of that as primitive but when it's the only thing you've got and the alternative was a slide rule or an electro-mechanical calculator it looks like the best thing since sliced bread.

1972 to 1974.

These years brought the addition of models for transistors and operational amplifiers, as well as a name change. The somewhat questionable name became spice (Simulation Program with Integrated Circuit Emphasis). Better. It was introduced to the press in 1973.

1974 to 1983.

This period saw the addition of models for newly developed devices such as the MOSFET. 1983 was also the year of the last version written in FORTRAN.

1984 to 1990.

Spice was rewritten in C which permitted it to be run on PCs which had not been possible before. The new version was not completely compatible with the earlier version requiring model suppliers to make available versions for Old Spice or New Spice.

1990 to 1999.

In 1990 spice fell into the public domain. MicroCap and Pspice were products based on the original spice. In these years an improved schematic capture and graphical output was developed. Linear Technologies started a development program to make spice useful to their own engineers for the design of switch mode power supplies.

1999 to 2008.

Sometime during this period Linear Technologies began to make its version called LTspice available for free to its customers. It became so popular that they eventually made it available free and open to all. God bless em! The forgoing based on THE LTSPICE IV SIMULATOR
By
Gilles Brocard

This page last updated February 12, 2020.

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