330 Chapter 10
- The supply currents can be kept out of the ground system. A single-rail AC
amplifi er has half-wave Class-B currents fl owing in the 0-V rail, which can
have a serious effect on distortion and cross talk performance.
10.6 Negative Feedback in Power Amplifi ers ...............................................................
It is not the role of this book to step through elementary theory, which can be found easily
in any number of textbooks. However, correspondence in audio and technical journals
shows that considerable confusion exists regarding NFB as applied to power amplifi ers;
perhaps there is something inherently mysterious in a process that improves almost all
performance parameters simply by feeding part of the output back to the input, but infl icts
dire instability problems if used to excess. This chapter therefore deals with a few of the
less obvious points here.
The main uses of NFB in amplifi ers are the reduction of harmonic distortion, the
reduction of output impedance, and the enhancement of supply-rail rejection. There are
analogous improvements in frequency response and gain stability, and reductions in DC
drift, but these are usually less important in audio applications.
By elementary feedback theory, the factor of improvement for all these quantities is
Improvement ratioAβ (10-1)where A is the open-loop gain and β is the attenuation in the feedback network, that is, the
reciprocal of the closed-loop gain. In most audio applications the improvement factor can
be regarded as simply open-loop gain divided by closed-loop gain.
In simple circuits you just apply NFB and that is the end of the matter. In a typical power
amplifi er, which cannot be operated without NFB, if only because it would be saturated
by its own DC offset voltages, several stages may accumulate phase shift, and simply
closing the loop usually brings on severe Nyquist oscillation at HF. This is a serious
matter, as it will not only burn out any tweeters that are unlucky enough to be connected,
but can also destroy the output devices by overheating, as they may be unable to turn off
fast enough at ultrasonic frequencies.
The standard cure for this instability is compensation. A capacitor is added, usually in
Miller-integrator format, to roll off the open-loop gain at 6 dB per octave, so it reaches
unity loop gain before enough phase shift can build up to allow oscillation. This means