Shunt Capacitance and High Frequency Response Electronics Help

admittance consists of the conductance component and the capacities sustenance component we(1 – A,)Note that this input admittance is exactly the same  where Equation IO-AX shows that the input as it would be if a capacitance having value Cc(I – A) were connected between input and ground. instead of capacitance C(, connected between input and output.between input and output has the same effect as that capacitance “magnified” by the factor (1 – At.) and connected so that it shunts the input. This magnification of feedback capacitance, reflected to the input, is called the Miller effect, and the magnified value CA.) is called the Miller capacitance, CM Miller capacitance is relevant only for an inverting amplifier, so A. is a negative number and the magnification factor (1 – A.) equals one plus the magnitude of A By a derivation similar to the foregoing, it can be shown that capacitance in the feedback path is also reflected to the output side of an amplifier. In this case, the effective shunt capacitance at the output is Once again, At. is negative, so the magnitude of the reflected capacitance is Since the increase in capacitance’s inversely proportional.to gain, the effect is much less significant than that of the capacitance reflected to the input. The computation of the value of the Miller capacitance, CM, is complicated by the fact that the gain At. itself depends on CA. At high frequencies, the Miller  capacitance reduces the gain, just as any other shunt capacitance does, and the gain reduction in turn reduces the Miller capacitance. As a first approximation, the mid band gain can be used to compute CM CM Ce(1 ~ Am). This computation , will always be conservative, in the sense that it wilt predict an upper cutoff frequency that is less than the actual value of f The total shunt capacitance at.the input ‘is the-sum of the Miller capacitance and any other input-to-ground capacitance that may be present. Also, the total shunt capacitance at the output is the sum of the reflected capacitance and any other output-to-ground .capacitance present. The effect of Miller capacitance is illustrated in the next example.The inverting amplifier shown in Figure 10-22 has midland gain Find its upper cutoff frequency.

Solution We will use the Midland gain to determine the Miller capacitance. Notice that this is the gain between the points where the 20-pF capacitance is connected. The Miller capacitance is, therefore, CM [l – (-200)4020 pF. The total capacitance C shunting the input is then CII = (40 pF) + (4020 pF) 4060 pF. The cutoff frequency due to C, is The upper cutoff frequency is the smaller of the two: J2 = 411.6 kHz.

Posted on November 19, 2015 in FREQUENCY RESPONSE

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