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来点E文的参考一下：
Zobel networks
All power amplifiers except for the most rudimentary kinds include a Zobel
network in their arrangements for stability. This simple but somewhat enig-
matic network comprises a resistor and capacitor in series from the ampli-
fier output rail to ground. It is always fitted on the inside (i.e., upstream)
of the output inductor, though a few designs have a second Zobel net-
work after the output inductor; the thinking behind this latter approach is
obscure. The resistor approximates to the expected load impedance, and is
usually between 4.7 and 10Ω. The capacitor is almost invariably 100 nF,
and these convenient values and their constancy in the face of changing
amplifier design might lead one to suppose that they are not critical; in fact
experiment suggests that the real reason is that the traditional values are
just about right.
The function of the Zobel network (sometimes also called a Boucherot cell)
is rarely discussed, but is usually said to prevent too inductive a reactance
being presented to the amplifier output by a loudspeaker voice-coil, the
implication being that this could cause HF instability. It is intuitively easy to
see why a capacitative load on an amplifier with a finite output resistance
could cause HF instability by introducing extra lagging phase-shift into
the global NFB loop, but it is less clear why an inductive load should be
a problem; if a capacitive load reduces stability margins, then it seems
reasonable that an inductive one would increase them.
At this point I felt some experiments were called for, and so I removed the
standard 10Ω/0.1μF Zobel from a Blameless Class-B amplifier with CFP
output and the usual NFB factor of 32 dB at 20 kHz. With an 8Ω resistive
load the THD performance and stability were unchanged. However, when
a 0.47mH inductor was added in series, to roughly simulate a single-unit
loudspeaker, there was evidence of local VHF instability in the output
stage; there was certainly no Nyquist instability of the global NFB loop.
I also attempted to reduce the loading placed on the output by the Zobel
network. However, increasing the series resistance to 22Ω still gave some
evidence of stability problems, and I was forced to the depressing conclu-
sion that the standard values are just about right. In fact, with the standard
10Ω/0.1μF network the extra loading placed on the amplifier at HF is not
great; for a 1 V output at 10 kHz the Zobel network draws 6.3 mA, rising
to 12.4mA at 20 kHz, compared with 125mA drawn at all frequencies by
an 8Ω resistor. These currents can be simply scaled up for realistic output
levels, and this allows the Zobel resistor power rating to be determined.
Thus an amplifier capable of 20 V rms output must have a Zobel resistor
capable of sustaining 248mA rms at 20 kHz, dissipating 0.62 W; a 1W
component could be chosen.
In fact, the greatest stress is placed on the Zobel resistor by HF instability, as
amplifier oscillation is often in the range 50–500 kHz. It should therefore be
chosen to withstand this for at least a short time, as otherwise faultfinding
becomes rather fraught; ratings in the range 3 to 5W are usual.
To conclude this section, there seems no doubt that a Zobel network is
required with any load that is even mildly inductive. The resistor can be
of an ordinary wire-wound type, rated to 5W or more; this should prevent
its burn-out under HF instability. A wire-wound resistor may reduce the
effectiveness of the Zobel at VHF, but seems to work well in practice; the
Zobel still gives effective stabilisation with inductive loads.
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发表于 2009-1-26 01:17
佐贝尔网络 