On the Validity of Two-Port Analysis for Feedback Amplifiers

Two-port analysis is a well-known and widespread method for the study of negative feedback amplifiers, thanks to the intuitive approach and its design-oriented perspective. In some cases, its application requires attention, since the conditions that guarantee the validity of its results are not fully satisfied, thus causing deviations from the correct value of the closed loop gain and terminal impedances of the amplifier. In this contribution, one of these conditions is derived for input-series feedback amplifiers, by regarding two-port analysis as a particular case of application of the Rosenstark’s formula. Some examples of feedback amplifiers with input-series connection are then considered, thus explaining the validity or the good accuracy of the two-port analysis when this condition is fully or partially fulfilled and its failure when it is far from being satisfied.