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Case Study: Modeling Headphone Amplifiers in B² Spice

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But how clean is this FET amplifier and how much voltage and current can it swing and how much gain does have? To get these answers, we need output run a Transient Sweep test, with Fourier analysis. Rather than begin at zero time, the test is allowed to run for 1mS before the results start to be tabulated and after 3mS, the test is over. (In the real world of solder and scope probes, many thousands of milliseconds would pass before the circuit’s performance would evaluated, giving the circuit time to stabilize.) The Fourier setup is easy enough: the test is set to 1kHz and the signal source that feeds the amplifier’s input is set to .1 volt and 1kHz.

To get the graph to display Fourier analysis with the fundamental notched out (so that it only displays the harmonics’ amplitude  relative to the fundamental’s amplitude) requires defining a new plot line:

The normalized magnitude scales the fundamental and its harmonics relative to the fundamental; thus, the fundamental amplitude becomes 1 and all the harmonics are scaled accordingly. Now if we convert the normalized values to dBs, the fundamental becomes 0, as 20Log(1) = 0; and the harmonics are represented as being so many negative dB down from the fundamental.

What are the results? Not too good. The gain is a miserable 0.73 and the distortion harmonics are too high by anybody’s reckoning, the second harmonic being only –21dB down from the fundamental. What went wrong? What we have here is a perfect example of how easy it is to base a conclusion on too little evidence. The time window that we had specified, 1mS to 3mS was far too short to allow the SPICE engine to resolve the circuits inner workings, i.e. the establishing of bias points and the charging of capacitors.