B² Spice

Case Study: Modeling Headphone Amplifiers in B² Spice

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How well does this amplifier work? If the initial results are to be believed, spectacularly well. Loaded with a 32-ohm load, the gain comes in at about 4 and the second harmonic is only 3.16% of the fundamental’s magnitude (with an input signal of 2V) and the output voltage is 15.6 volts peak-to-peak, which translates into .95 watts into 32 ohms! Well the results are not be believed. This time we have an example of how basing a conclusion on too little evidence can give overly optimistic results (given the choice, pessimistic results are always preferred, as it is better to be pleasantly surprised by better performance than to be disappointed by worse performance). If we widen the time window, we see the following results.

Applying the same procedure as we did before will straighten out the results. First we find the steady-state conditions and then we plug all the capacitors’ voltages into their initial condition edit box.

So capacitor C1’s intuitional voltage becomes 199V; C2’s, 2V; and C3’s, 102V. Now if we redo the tests, we will get some truer results. Unloaded, the gain comes in at about 8 and the second harmonic is only 0.22% of the fundamental’s magnitude (with an input signal of 0.2V). Loaded with a 32-ohm load, the gain drops to only 0.76 and the  second harmonic climbs to 5.4%. With a less excruciating load, 300 ohms, the gain becomes 4.1 and the second harmonic drops to 1.7%. If a sufficiently large input signal is applied, the amplifier will clip, which reveal the class-A boundaries of operation.

The biggest negative going voltage swing into a 32-ohm load is about 0.7 volts, which makes as it equals twice the idle current against the load impedance. (If a phase splitter were added and a conventional totem-pole output stage implemented, the peak output swing would easily double, as the amplifier could operate in class-B.) Figuring out the amplifier’s PSRR figure requires grounding the input and placing an additional voltage source in series with the power supply voltage source.