Class-D Audio Power Amplifier Design by Robert N. Buono by Unknown

Author:Unknown
Language: eng
Format: epub
Published: 2025-03-10T23:28:02+00:00

Practical Feedback Loop Measurement Tips

It can be appreciated that these measurements described above are being made in a high-noise environment. The switching mosfets are creating large transients, contributing to noise in the oscilloscope readings for PROBES A and B. Furthermore, the driving signal levels are kept low, to be sure that the amplifier is being measured in its linear range. If the signal levels are too large, you will see non-linearities in the sinusoidal signals. So this is a classic case of trying to measure small signal levels in the presence of high noise. To facilitate these measurements, an oscilloscope with an averaging function is very helpful. To use this function, a solid trigger signal is required. Typically, the DRIVE signal itself can be used as the oscilloscope trigger. It may also be helpful to use an isolation transformer with a large step-down ratio, such that the DRIVE signal has an even greater amplitude, as the trigger to the oscilloscope. When the averaging function is used, the switching noise of the Class-D amplifier will be averaged out, because the switching of the Class-D amplifier’s power stage is asynchronous to the sinusoidal DRIVE signal frequency (oscilloscope trigger). Please note that there are a few commercially available feedback loop analyzer solutions (see the References) that use this technique and provide the necessary hardware, as well as software, to drive the loop and measure the results, with the necessary noise suppression. However, it is possible to make these measurements manually, as long as the noise issues are handled, and the waveforms are constantly monitored for signs of non-linearity. It is helpful in terms of signal-to-noise in these measurements to increase the DRIVE signal level as much as possible, provided linearity is maintained. Although these manual measurements are tedious, it is helpful even if only the two measurements of unity gain crossover frequency (with phase margin) and gain margin are made.

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