B² Spice

Case Study: Amplifier Distortion Testing in B² Spice

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Distortion
Measuring distortion in amplifier topologies is one of the SPICE’s main features. B2 A/D Spice provides two measuring techniques. The first is DISTO, which is the quick and dirty measure of distortion. Much like the small signal AC sweep test, this test looks into the SPICE models for distortion information, rather than actually measuring the actual dynamic distortion. And much like the small signal AC sweep test, this test is better suited to large, complex circuits than small, simple circuits.

Well, what if we want full monty of distortion information, then the Fourier test is the better place to look and it is found in Transient Sweep test setup. The first step is to confirm input signal’s frequency match the Fourier test’s fundamental frequency, otherwise the results will be nonsense. The next step is to specify an adequate period of time for the test frequency to unfold.

After running the test, the graph will display the Fourier analysis’ results.

Well, this does not look very promising. The problem is that too much information is offered: the peak magnitude, the normalized output (the peak value is “normalized” by treating its value as 1 and then all the other lower values are displayed relative to 1), the phase of each harmonic’s value, and the normalized display of the phase information. Why so much information? B2 A/D Spice does not know what you are looking for, so it default is to display all of the relevant to the circuit under test. It is better, however, to have too much information, than too little, as we can easily switch off the display of unneeded results.

Right-mouse button clicking on the graph brings up its popup menu, and from there, we select the “Edit Plot List” dialog window. Next we turn off the display of all the default plots in the graph. Then we press the “Add New Custom Plot to Graph…” button to add a new plot of our own design.

The new plot must have a name and a color that will show up against the graph’s background and, most importantly, an expression that defines what the plat displays. In this case, the plot will display the harmonics of the test frequency. The fundamental will be notched out, as the dB expression of 1 is 0. Below, we the 2nd through 9th harmonics display as being so many dBs down relative to the fundamental frequency, which is exactly what we want.

This offers more information than a simple THD percentage, as it displays each harmonics contribution to the distortion figure. Translating the dBs into percent of distortion is easy enough.

The actual formula used is: Percent = 100 x 10(dB/20)

Remember that the dBs are negative in this case. How do we get the total harmonic distortion figure for the amplifier, rather the figure for each harmonic? The easiest way is select both graph and table at the bottom of the Transient Sweep test setup dialog box.

When this option is selected, a spreadsheet like table is created that allows easy viewing of the individual harmonics’ contribution to the total distortion.

First of all, do not confuse the numbers at the extreme left as representing the harmonics number, as the numbers are off by 1 because they only refer to the row count. The next step is to ignore the fundamental (+1.000) and add all the remaining harmonics together and multiply this sum by 100. In the example above, the total comes in at 0.010571% THD.