11.101 An op amp has a dc gain of 120 dB and three poles: ωP1 = 105 rad/s, ωP2 = 107 rad/s, and ωP3 = 108 rad/s. The frequency response must be compensated so that the gain has a uniform −20 dB/decade rolloff down to 0 dB and the 3-dB frequency as high as possible.
Miller compensation is used around one of the stages of the amplifier as shown in Fig. 11.41. Assume that without Cf connected, the input circuit is responsible for the first pole and the output circuit is responsible for the second pole. Let C1 = 1 pF, C2 = 0.05 pF, and gm = 10 mA/V. When Cf is connected, assume that the second pole moves to a frequency higher than that of the third pole.
(a) Provide a neat and clearly labeled sketch of the Bode plot of the magnitude of the open-loop gain of the uncompensated amplifier. The frequency axis should extend from 10 rad/s to 109 rad/s.
(b) On the same figure, sketch the Bode plot for the magnitude of the gain of the compensated amplifier. Specify the frequency of the dominant pole and the unity-gain bandwidth of the compensated amplifier.
(c) Give the required value of Cf .
(d) What is the frequency to which the second pole moves?
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