An ideal capacitor’s impedance vs. frequency will decreasemonotonically with increasing frequency. Figure 3 shows theimpedance vs. frequency for two different 100 μF capacitors.One is an aluminum electrolytic type, the other amultilayered ceramic capacitor. At low frequencies, theimpedance drops off monotonically with increasingfrequency, as expected. However, because of the ESR, atsome frequency, this impedance reaches a minimum. Asfrequency continues to increase, the capacitor starts tobehave more like an inductor, and the impedance willincrease in frequency. The impedance vs. frequency curvesare called ‘bathtub’ curves, and all real capacitors behave inthis manner.
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