An active transistorized directional coupler and its use in a bidirectional amplifier

In previous paper an active directional coupler using a triode 1, 2 and its use in a bidirectional broad band amplifier 3 have been described. It is obvious the importance of investigating how such a device can be realized by using semiconductor technique. The use of transistors in directional couplers makes new problems grow up due to the variability of transistor parameters. In this paper we describe a transistorized directional coupler that exhibits high directivity over a broad band and a tight coupling coefficient. The problems of stability and non-linear distorsion are discussed and overcome by using feedback. Noise problems are also treated. The use of such a coupler in bidirectional amplifiers is also proposed, and, in this use, the problem of the long term stabilization of the directivity of the active coupler has been overcome by means of a control system using a reference signal. Experimental results concerning directivity and bandwidth confirm the possibility of a pratical use of the proposed amplifier in bidirectional broad band communication systems on a single cable and on the same frequency band.