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The distributed amplifier topology has been adapted to form a nonreciprocal coupler which is small in size and broadband. The coupler has been implemented in MMIC form using pHEMT devices and a coupling factor of 5 dB is demonstrated over 2-20 GHz bandwidth.
Introduction: Directional couplers are difficult to realise in MMIC form because they rely on transmission lines of lengths which are a significant fraction of a wavelength requiring large and, therefore, expensive circuit area. The area can be reduced by using lumped element and quasi-lumped element techniques [1, 2] which, however, result in a reduced operating bandwidth. A technique has been proposed [3] to use a distributed amplifier topology to form a nonreciprocal coupler. Although the technique is not directly applicable in all circumstances where a directional coupler would be employed, the fact that it is broadband and readily implemented on an MMIC, renders it worthy of further treatment. The MMIC implementation of a non-reciprocal coupler including the design process based on [1] and measured data for the MMIC circuit is presented.
Coupler design: The coupler design employed is based on a distributed amplifier topology [3, 4]. The distributed amplifier would conventionally have 50 O gate and drain line terminations. For a four-port coupler configuration, removal of these terminations permits the gate line to be used as the through path and the drain line as the coupled path. A series capacitor is incorporated in the gate line to set the coupling factor (Fig. 1).
The coupler has been implemented on an MMIC using the Bookham Technology H40P GaAs process and pHEMTs (0.2 mm gate lengths...