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 looking at just the scope trace on a conventional scope, you would not see any problems. This is where the power of the Pico Spectrum Analysis combined with the sensitivity of the ADC-216 are crucial. The problem is still evident in the spectrum view (Figure 5).
If we connect the ADC-216 back to point ‘A’ but keep the output stage in the feedback loop, we can see what kind of corrections the opamp is having to apply in order to remove the error generated by the output stage. The opamp follows the sinusoidal signal for the positive and negative peaks of the waveform but around the crossover points, it has to work very hard to cover up the output stage error. It has to quickly traverse the point where the output stage does not conduct. It is clear then that the opamp needs to have a much higher slew rate, than that which text books may initially suggest, in order to compensate for the poorly designed output stage. This makes the design of earlier gain stages much more difficult and expensive than they need to be (Figure 6).
Next the output circuit was improved by applying a bias voltage between the bases of the output devices using the simple diode drop technique. The view below shows the output at point ‘B’ again. It can bee seen that the addition of a small amount of biasing has improved the THD reading by almost 10 dB (Figure 7).
If we look again at point ‘A’ with this added bias voltage, we can see that the op-amp does not require such a high slew rate since it does not have to work as hard to cover up the output stage imperfections (Figure 8).
This tech note has described a simple application whereby the performance of an amplifier output stage design can be evaluated using the powerful FFT spectrum plot in PicoScope. With some experience, many problems can be identified from their characteristic spectrum plots which might otherwise go unnoticed but lead to a poor or coloured sound reproduction.
Figure 5
Figure 6
Figure 7
Figure 8
