Miller effect

In electronics, the Miller effect accounts for the increase in the equivalent input capacitance of an inverting voltage amplifier due to amplification of the effect of capacitance between the input and output terminals. The virtually increased input capacitance due to the Miller effect is given by In electronics, the Miller effect accounts for the increase in the equivalent input capacitance of an inverting voltage amplifier due to amplification of the effect of capacitance between the input and output terminals. The virtually increased input capacitance due to the Miller effect is given by where − A v {displaystyle -A_{v}} is the voltage gain of the inverting amplifier ( A v {displaystyle A_{v}} positive) and C {displaystyle C} is the feedback capacitance. Although the term Miller effect normally refers to capacitance, any impedance connected between the input and another node exhibiting gain can modify the amplifier input impedance via this effect. These properties of the Miller effect are generalized in the Miller theorem. The Miller capacitance due to parasitic capacitance between the output and input of active devices like transistors and vacuum tubes is a major factor limiting their gain at high frequencies. Miller capacitance was identified in 1920 in triode vacuum tubes by John Milton Miller. The Miller effect was named after John Milton Miller. When Miller published his work in 1920, he was working on vacuum tube triodes; however, the same theory applies to more modern devices such as bipolar and MOS transistors. Consider an ideal inverting voltage amplifier of gain A v {displaystyle A_{v}} with an impedance Z {displaystyle Z} connected between its input and output nodes. The output voltage is therefore V o = − A v V i {displaystyle V_{o}=-A_{v}V_{i}} . Assuming that the amplifier input draws no current, all of the input current flows through Z {displaystyle Z} , and is therefore given by