An Improved Miller Effect Model for High-Frequency Behavior

The Miller effect model of a common emitter (and common source) amplifier is presented to students in beginning analog circuits courses [1]-[5] to help model the high-frequency characteristics of the amplifier. The standard Miller effect circuit is used to determine the dominant pole of the amplifier, but is inaccurate and not useful at higher frequencies. Presented here is an improved Miller effect model. Differing from the standard model only in the capacitor values, this model accurately reflects the amplifier frequency response with regard to the poles over all frequencies. The accuracy allows the Miller effect model to be useful in the classroom description of a number of important aspects. These aspects can include the second pole location, pole splitting using a compensation capacitor, and determination of the unity gain frequency. None of these can be addressed by the standard Miller effect model. Furthermore, depending on the background of the students, the improved Miller effect model is easier or no more difficult for the students to understand than the standard model. Inaccurate approximations used to "derive" the standard model are no longer necessary.