9 Common-Mode Distortion in the 5532
In a series-feedback amplifier with a gain of 3.2 times the CM voltage is 3.1 Vrms for a 10 Vrms output. See Figure 13.9. The trace labelled “0R” (i.e., zero source resistance) is the same as the “No load” trace of Figure 13.8. The source resistance seen by the inverting input is not zero, because of the impedance of the feedback network, but this is only 2K2 in parallel with 1 KΩ; in other words 687Ω. Figure 13.9 implies that this will have only a very small effect, but more on that later. When we add some source resistance Rs, the picture is radically worse, with serious midband distortion rising at 6 dB/octave, and roughly proportional to the amount of resistance added. We will note it is 0.0015% at 10 kHz with Rs = 10 kΩ. The horizontal low-frequency parts of the traces are raised by the Johnson noise from the source resistances and also by the opamp current noise flowing in those resistances.