B² Spice

Case Study: Modeling Headphone Amplifiers in B² Spice

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Now that we have our desired frequency tailoring in place, how do we find out just how much voltage swing this amplifier can put into the 32-ohm load. The obvious way is to repeatedly increase voltage source V1’s AC magnitude until we see clipping. Alternatively, we can get fancy and run only one transient test.

Unfortunately, it isn’t possible to simply tell the SPICE engine to increase the input signal’s magnitude on each successive sweep; but there is a work around. Here’s how: we replace V1 with a “Nonlinear Dependent Voltage Source” from the “More Devices” submenu. This device B1 accepts a user-defined formula that will control the amount of voltage at its “output.” The equation we will insert is v =  v(N1,3) * v(3), which tells B1 to vary its voltage based on the voltage differential between nodes N1 and 3 (which equals 0Vdc and 0.1Vac) against the DC voltage present at node 3 (which is the parameter we will step up in value by 0.25V increments). Thus, the first sweep will give the amplifier an input signal of 0Vac, as 0.1 x 0 = 0; the next sweep, 0.025Vac; the next, 0.05Vac; the next, 0.075Vac …

Running the test gives the following results.

The sixth sweep seems to have the largest yet still clean waveform. The input signal on the sixth sweep is equal to 6 x 0.25Vdc x 0.1Vac or 0.15Vac.