FFT audio amplifier design

Posted on Feb 5, 2014

For all of the following tests, we used a high quality signal generator from BlackStar. The PicoScope trace below shows a pure 1 kHz tone from the signal generator. Please note: although the screenshots below show the THD, SFDR and SINAD measurements in a separate window these are now included within the PicoScope program and are displayed in the

FFT audio amplifier design
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main window ( see an example ). The diagram below shows a basic power amplifier circuit. The output stage shown is the simple and widely used emitter follower topology. The following tests show how the quality of the signal from output stage can be monitored. Modifications can then be made to the circuit and any improvements recorded. This output stage topology has a stage gain of just less than 1 so it can be easily moved outside the feedback loop as in the circuit below. Initially the circuit was constructed as above. The ADC-216 was connected to point A` in the circuit and the signal generator was connected to the input of the circuit. The PicoScope screenshot below shows the signal at point A`. It is clear that the op-amp is doing a reasonable job as could be expected with such a large amount of negative feedback applied. If we now look at the output at point B`, we can see excessive crossover distortion clearly visible on the scope trace. The harmonic information also indicates the problem with the third harmonic component being the largest of the harmonics. The load resistor connected to point B` was 2k2. It is obvious that the output stage is suffering from severe crossover distortion effects. If we continue to monitor point B but move the output stage inside the feedback loop by connecting the inverting input of the opamp to point B`, we notice a massive reduction in the output stage distortion. Indeed, if you were...

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