B² Spice

Case Study: Modeling Headphone Amplifiers in B² Spice

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If we run the test again with a much wider time window, we will get a better picture of what is going on in the circuit. So, let’s set the time aperture to span from 0 to 200mS, which is still only .2 seconds after all.

It took over 1 minute for this new test to finish on a fast machine (AMD 1800), which makes sense as many 1µS slices fit in 200mS. Now, we can see what had been going on in our circuit: the internal capacitors had to be charged to their quiescent values before the circuit’s output could settle down to 0 volts average, i.e. (Vmax + Vmin) / 2 = 0V. At the very end of the 200mS run, we see a symmetrical output swing and a much reduced distortion harmonic content (almost a hundredfold improvement), as shown below.

Is there a way to speed the test up? In fact, there is. If we click on “Show Steady State” from the “View” menu set, we get the follow results.

(The part labels and part values were turned off in the menu’s “Edit” » “Options” dialog box.) Notice that the internal coupling capacitor has no voltage across its plates and the output coupling capacitor has 4.5V across its leads. Steady-state analysis in SPICE ignores the capacitors altogether and reveals only the DC aspect of the circuit.