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Engineering LibreTexts

23.1: Theory Overview

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  • As the signal frequency extends to very high or very low frequencies, capacitive effects on gain can no longer be ignored or idealized as shorts or opens. At low frequencies, coupling and bypass capacitors in conjunction with surrounding resistance create lead networks that cause a reduction in voltage gain. At higher frequencies, small shunting capacitances associated with individual devices circuit wiring create lag networks. These will also create a reduction in voltage gain. In general, the highest critical frequency among the lead networks creates the amplifier’s lower limit frequency, \(f_1\). In contrast, the lowest critical frequency among the lag networks creates the amplifier’s upper limit frequency, \(f_2\). These points are defined as the half-power points and can be determined experimentally by finding those frequencies at which the output voltage (and hence, voltage gain) has fallen to 70.7% of the midband value. The values are found theoretically by Thevenizing the circuitry around the capacitor in question, reducing it to a single resistance, and solving for the critical frequency, \(f_c\).

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