Investigation of drain-line loss and the S22 kink effect in capacitively coupled distributed amplifiers

This paper investigates the practical limits of the capacitively coupled distributed amplifier (DA) in terms of common-source output impedance. It is shown that the output impedance of the common-source device is considerably affected by the input coupling circuit. The S22 kink effect is more pronounced in the case of the capacitively coupled circuit. The effect on drain-line loss is very marked, and becomes the practical limitation of the technique. The effect is clearly illustrated by practical measurements on a 45-MHz-20-GHz GaAs monolithic-microwave integrated-circuit amplifier. The kink effect is also shown to be relevant to the output impedance and stability of the common-gate stage in the cascode DA topology.