Audio Amplifi er Performance 333factor. This causes HF instability, which has to be cured by increasing the compensation
capacitance. This is turn reduces the slew-rate capability, resulting in a sluggish, indolent,
and generally bad amplifi er.
The obvious fl aw in this argument is that the amplifi er so condemned no longer has a high
NFB factor because the increased compensation capacitor has reduced the open-loop gain
at HF; therefore feedback itself can hardly be blamed. The real problem in this situation
is probably an unduly low standing current in the input stage; this is the other parameter
determining slew rate.
NFB may reduce low-order harmonics but increases the energy in the discordant
higher harmonics. A less common but recurring complaint is that the application of
global NFB is a shady business because it transfers energy from low-order distortion
harmonics—considered musically consonant—to higher order ones that are anything but.
This objection contains a grain of truth, but appears to be based on a misunderstanding
of one article in an important series by Peter Baxandall^24 in which he showed that if you
took an amplifi er with only second-harmonic distortion and then introduced NFB around
it, higher order harmonics were indeed generated as the second harmonic was fed back
round the loop. For example, the fundamental and the second harmonic intermodulate
to give a component at third-harmonic frequency. Likewise, the second and third
intermodulate to give the fi fth harmonic. If we accept that high-order harmonics should
be numerically weighted to refl ect their greater unpleasantness, there could conceivably
be a rise rather than a fall in the weighted THD when NFB is applied.
All active devices, in Class-A or -B (including FETs, which are often erroneously thought
to be purely square law), generate small amounts of high-order harmonics. Feedback
could and would generate these from nothing, but in practice they are already there.
The vital point is that if enough NFB is applied, all the harmonics can be reduced to a
lower level than without it. The extra harmonics generated, effectively by the distortion
of a distortion, are at an extremely low level, providing a reasonable NFB factor is used.
This is a powerful argument against low feedback factors such as 6 dB, which are most
likely to increase the weighted THD. For a full understanding of this topic, a careful
reading of the Baxandall series is absolutely indispensable.
A low open-loop bandwidth means a sluggish amplifi er with a low slew rate. Great
confusion exists in some quarters between open-loop bandwidth and slew rate. In truth,