Amplifier Stability and Instability
Amplifier stability can be one of the more challenging areas of design. Instability can refer to unwanted oscillations at either HF or LF, but the latter is very rare in solid-state amplifiers, though still very much an issue for valve designers. Instability has to be taken very seriously, because it may not only destroy the amplifier that hosts it, but also damage the loudspeakers.
Instability at middle frequencies such as 1 kHz is virtually impossible unless you have a very eccentric design with roll-offs and phase-shifts in the middle of the audio band.
HF instability is probably the most difficult problem that may confront the amplifier designer, and there are several reasons for this:
- The most daunting feature of HF oscillation is that under some circumstances it can cause the destruction of the amplifier in relatively short order. It is often most inadvisable to let the amplifier sit there oscillating while you ponder its shortcomings. BJT amplifiers will suffer overheating because of conduction overlap in the output devices; it takes time to clear the charge carriers out of the device junctions. Some designs deal with this better than others, but it is still true that subjecting a BJT design to prolonged sine wave testing above 20 kHz should be done with great caution. Internal oscillations may of course have much higher frequencies than this, and in some cases the output devices may be heated to destruction in a few seconds. The resistor in the Zobel network will probably also catch ﬁre. FET amplifiers are less vulnerable to this overlap effect, due to their different conduction mechanism, but show a much greater tendency to parasitic oscillation at high frequencies, which can be equally destructive. Under high-amplitude oscillation, plastic-package FETs may fail explosively; this is usually a prompt failure within a second or so and leaves very little time to hit the off switch.
- Various sub-sections of the amplifier may go into oscillation on their own account, even if the global feedback loop is stable against Nyquist oscillation. Even a single device may go into parasitic oscillation (e.g., emitter-followers fed from inappropriate source impedances) and this is usually at a sufficiently high frequency that it either does not ﬁght its way through to the amplifier output, or does not register on a 20 MHz scope. The presence of this last kind of parasitic is usually revealed by excessive and unexpected non-linearity.
- Another problem with HF oscillation is that it cannot in general be modelled theoretically. The exception to this is global Nyquist oscillation (i.e., oscillation around the main feedback loop because the phase-shift has become too great before the loop gain has dropped below unity) which can be avoided by calculation, simulation, and design. The forward-path gain and the dominant pole frequency are both easy to calculate, though the higher pole frequencies that cause phase-shift to accumulate are usually completely mysterious; to the best of my knowledge virtually no work has been done on the frequency response of audio amplifier output stages. Design for Nyquist stability therefore reduces to deciding what feedback factor at 20 kHz will give reliable stability with various resistive and reactive loads, and then apportioning the open-loop gain between the transconductance of the input stage and the transresistance of the VAS.
The other HF oscillations, however, such as parasitics and other more obscure oscillatory misbehaviour, seem to depend on various unknown or partly known second order effects that are difficult or impossible to deal with quantitatively and are quite reasonably left out of simulator device models. This means we are reduced to something not much better than trial-and-error when faced with a tricky problem.
The CFP output stage has two transistors connected together in a very tight 100% local feedback loop, and there is a clear possibility of oscillation inside this loop. When it happens, this tends to be benign, at a relatively high frequency (say, 2e10 MHz) with a clear association with one polarity of half-cycle.
Amplifier instability at LF (motorboating) is largely a thing of the past now that amplifiers are almost invariably designed with DC-coupling throughout the forward and feedback paths. The theoretical basis for it is exactly as for HF Nyquist oscillation; when enough phase-shift accumulates at a given frequency, there will be oscillation, and it does not matter if that frequency is 1 Hz or 1 MHz. It can be as destructive of bass drivers as HF oscillation is of tweeters, especially with bass reﬂex designs that impose no cone loading at subsonic frequencies.
At LF things are actually easier, because all the relevant time-constants are known, or can at least be pinned down to a range of values based on electrolytic capacitor tolerances, and so the system is designable, which is far from the case at high frequencies. The techniques for dealing with almost any number of LF poles and zeros were well known in the valve era, when AC coupling between stages was usually unavoidable, because of the large DC voltage difference between the anode of one stage and the grid of the next.
The likeliest cause of LF instability is probably a mis-designed multi-pole DC servo. Oscillation at LF is very unlikely to be provoked by awkward load impedances. This is not true at HF, where a capacitive load can cause serious instability. However, this problem at least is easily handled by adding an output inductor.
Excerpt from Audio Power Amplifier Design, 6th Edition by Douglas Self © 2013 Douglas Self.