A pole (Figure 8) is defined as a point where the slope of the gain curve changes by -20 dB/decade (with reference to the slope of the curve prior to the pole). Note that the effect is additive: each additional pole will increase the negative slope by the factor ÒnÓ X (-20 dB/decade), where ÒnÓ is the number of additional poles. The phase shift introduced by a single pole is frequency dependent, varying from 0 to -90° (with a phase shift of -45° at the pole frequency). The most important point is that nearly all of the phase shift added by a pole (or zero) occurs within the frequency range one decade above and one decade below the pole (or zero) frequency.
NOTE: a single pole can add only -90° of total phase shift, so at least two poles are needed to reach -180° (which is where instability can occur).
ZEROES
A zero (Figure 9) is defined as a point where the gain changes by +20 dB/ decade (with respect to the slope prior to the zero). As before, the change in slope is additive with additional zeroes. The phase shift introduced by a zero varies from 0 to +90°, with a +45° shift occurring at the frequency of the zero. The most important thing to observe about a zero is that it is an Òanti-poleÓ, which is to say its effects on gain and phase are exactly the opposite of a pole. This is why zeroes are intentionally added to the feedback loops of LDO regulators: they can cancel out the effect of one of the poles that would cause instability if left uncompensated.作者: kilolo 時間: 2011-7-6 11:47 AM
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