A Transistorized Directional Coupler

In previous papers an active directional coupler using a triode, [1],[2] and its use in a bidirectional broad-band amplifier, [3] has been described. It is obvious that it is important to investigate how such a device can be realized using semiconductor techniques. The use of transistors in directional couplers creates new problems due to the variability of transistor parameters. In this paper a transistorized directional coupler that exhibits high directivity over a broad band and a tight coupling coefficient is described. The problems of stability and nonlinear distortion are discussed and overcome by using feedback. Noise problems are also treated. The use of such a coupler in bidirectional amplifiers is also proposed. For 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 practical use of the proposed amplifier in bidirectional broad-band communication systems.