B² Spice

Case Study: Amplifier Testing in B² Spice

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Testing an amplifier usually involves seven core measurements: distortion, frequency response, input impedance, power output, output impedance, PSRR, and loop gain. In B2 A/D Spice, these measurements are easy to perform.

Audio amplifiers differ from other electronic circuits, such as, timer circuits, oscillators, and voltage regulators. Most electronic circuits are but cookie cutter efforts, lifted from a few established sources, such as manufacturer white papers and data books, circuit encyclopedias, and textbooks; they provoke little controversy and gain few advocates or detractors. Audio amplifiers, on the other hand, they have more character. They are often personal design statements that do find advocates or detractors. For example solid-state and tube amplifiers offer different technologies, different sounds, different emotional responses, and different partisans. The difference lies in the subjective evaluation that audio amplifiers invite, as we all have a set of ears and a viewpoint. While a SPICE program cannot help with subjective evaluation, it can help with objective evaluation. In SPICE’s virtual test bench, an amplifier’s performance can be readily evaluated with a few techniques.

Frequency Plots
Flat from 20Hz to 20kHz is specification everyone wants to see, as it covers the range of human hearing (your dog would prefer a 20Hz to 50kHz specification and a bat would prefer…). We have the range, but what is being measured? For an audio amplifier, the answer is output voltage; for a loudspeaker, sound pressure. A flat output usually means the range of frequencies that do not deviate by more than –3 dB from the output at some reference frequency, usually 1kHz. SPICE offers a quick and simple frequency response test in the form of a small signal AC sweep. Here the DC operating point is first calculated, and then all non-linear elements are replaced with their small signal models. Small signal models? The Berkeley SPICE engine efficient and it takes every short cut it can. Why? In part, because when SPICE was developed computers were much slower than today’s computers with gigahertz CPUs and because circuits can grow amazingly complex.

So rather than actually simulating the entire circuit, SPICE engine simply cascades each individual parts’ frequency response into the next part down the line. The results are close to the amplifier’s actual performance with small magnitude signals, but without the processing overhead. (To see this trick in action run an AC Sweep test with an op-amp that has no connection to a power supply! The frequency response will be plotted nonetheless, as the SPICE engine didn’t need the power supply information.) If this scenario bothers you, you are not alone. However, once you realize that SPICE’s function is not to exactly reproduce reality, but to offer same results as reality, you will appreciate this design decision. Furthermore, SPICE is best for helping you with what you don’t know, not what you already know. For example, there is no point to modeling a complete power supply, with transformer, diodes, and filter capacitors, if you know that the actual power supply has an output voltage of +30 volts and a 100mV of 120Hz ripple; instead, just use a voltage source with the same attributes. Unless, that is, you need to know how the power supply will behave in the first half second of operation.

Having said all that, we come upon a wrinkle when the device under test is a power amplifier. An amplifier’s frequency response is usually taken at two output levels: at 1 watt and at full output, not at some undefined small signal. So how do get the results we want at that those two output voltages? First we have find out what those two voltages are. While each amplifier will have its own maximum power output, we do know that 1 watt into an 8-ohm load equals 2.828 volts peak and 0.3535 amperes peak. The next step is to place voltage source in series with the amplifier’s input and to specify a sine-wave whose amplitude is equal to the amplifier’s gain divided by 2.828. But at what frequency?

If we run a small signal AC Sweep, we will have a good idea where the amplifier departs from flat. So, for example, if the –3dB points are 30Hz and 30kHz, then we should start first at 1kHz and then begin stepping down at 40Hz (in 1Hz increments) and then climbing from 20kHz (in 1kHz increments). Now we must run a set of Transient Sweep tests to get the spot frequency values.

Full-power bandwidth testing requires finding the maximum power at 1kHz, and then measuring the peak voltage. This voltage is then divided by the amplifier’s gain and then to specify this value as the voltage source’s value. Next all the spot frequency test are run and the results are tabulated.