Feedforward Amplifiers

This invention relates to feedforward amplifiers such as may be used in coaxial line transmission systems.

Feedforward amplifiers may be generally defined as having main and auxiliary amplifier stages in separate amplifying channels connected in parallel between common input and output terminals, each amplifier stage having its own negative feedback path, with a cross connection between the output of the main amplifier stage and the input of the auxiliary amplifier stage. The various connection between the two channels and the terminals are conveniently made by way of directional couplers which provide isolation between the different parts of the circuits where required.

In long distance coaxial line transmission systems, e.g. in submarine systems, it is necessary to reduce the effects of 3rd order intermodulation vectors. One convenient method of doing this is to introduce phase scattering of the vectors along the system. Alternatively intermodulation products could be cancelled at the end of the system.

An object of the present invention is to provide an improved feedforward amplifier.

A feature of the present invention is the provision of a feedforward amplifier comprising: a main amplifier stage disposed in a signal cancelling loop coupled to an input of the feedforward amplifier; an auxiliary amplifier stage disposed in an error cancelling loop coupled between the signal loop and an output of the feedforward amplifier; and control means coupled to said error loop for controlling signals coupled to said output.

Typically the means for controlling the amplitude and phase of the signals fed to the output directional coupler comprises a variable resistor and a variable capacitor connected between the auxiliary amplifier input and ground. In a remote amplifier these devices can be respectively a PIN diode and a varactor diode the values of which can be remotely controlled by, for example, small changes in the d.c. (direct current) supply current .

The above-mentioned and other features and objects of the present invention and the manner of obtaining them will become more apparent by reference to the following description taken in conjunction with the drawing, the single FIGURE of which is a block circuit diagram of a feedforward amplifier in accordance with the principles of the present invention.

The basic feedforward amplifier has a main amplifier stage 1 and an auxiliary amplifier stage 2. The input signal is fed via an input directional coupler 3 to the main amplifier stage 1 and also via a delay line 4 and a second directional coupler 5 to the auxiliary amplifier stage 2. The output of the main amplifier stage 1 is fed via directional coupler 6, delay line 7 and output directional coupler 8 to the output, and via coupler 6, attenuation network 9 and coupler 5 to the auxiliary amplifier stage 2. The two delay lines delay the signals by amounts of time roughly equal to the transit times through each of the two amplifying stages so that the delays of paths 3-6-9-5 and 3-4-5 are equal but with a phase difference of 180.degree. and similarly for paths 6-7-8 and 6-9-5-2-8. Each of the amplifying stages 1 and 2 is a conventional feedback amplifier in its own right. In operation, a comparison is made between the amplified signal from the main amplifier stage 1 and the delayed "reference" signal via delay line 4, and the "error" signal so formed is the input to the auxiliary amplifier stage 2. The error signal is then amplified in the auxiliary amplifier stage 2 to a suitable level for interference with the main amplifier stage 1 output, suitably delayed, in the output directional coupler 8.

To provide additional control of the amplitude and phase of the error signal a variable resistor 10 and a variable capacitor 11 are connected across the input to the auxiliary amplifier stage 2. In the case of an intermediate repeater in a long haul system, e.g. a submarine coaxial line transmission system, remote control of the amplitude and/or phase of the error signal can be achieved if the variable resistor and variable capacitor are a PIN diode and a varactor diode, respectively. The performance of the system can then be improved by remotely controlling the feedforward amplifiers to maximize the effect of phase scatter of 3rd order intermodulation vectors. This remote control can be either by small changes in the d.c. supply current to the repeaters or by the use of pilot signals.

Another application of the invention is where a set of terminal amplifiers are controlled such that the error cancelling loops are deliberately unbalanced so as to cause distortion products to cancel or reduce those caused in the line amplifiers.

While reference has been made to the controlling of amplitude and phase of the error signals, it will be appreciated that it may be necessary to control only one of these parameters, thus requiring only the variable resistor or the variable capacitor alone.

While we have described above the principles of our invention in connection with specific apparatus it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of our invention as set forth in the objects thereof and in the accompanying claims.