A feed-forward amplifier with an input arranged to receive wide-band signals and an output to supply a linearly amplified version of these has two parallel signal paths arranged between such input and output, one of the signal paths includes a cascade connection of a signal amplifier and a delay network and the second signal path includes an error amplifier, an error extraction circuit having a directional coupler in which an input side has a first pair of conjugate first and second ports and is connected partly to the input of the feed-forward amplifier and partly to an output of the signal amplifier and an output side has a second pair of conjugate third and fourth ports, the fourth port being connected to an input of the error amplifier, and an error injection circuit which has a first and a second input connected to an output of the delay network and to an output of the error amplifier, respectively and an output connected to the output of the feed-forward amplifier, the error extraction circuit having the third port of the directional coupler connected to an input of the signal amplifier and a summation circuit having a first and second input connected to the input of the feed-forward amplifier and to the output of the signal amplifier, respectively, and an output connected to the second port of the directional coupler.

This invention relates to a feed-forward amplifier with an input to receive 
wide-band signals and an output to supply a linearly amplified version of 
these having two parallel signal paths arranged between the input and the 
output of the feed-forward amplifier, one of the signal paths being a 
cascade connection of a signal amplifier and a delay network and the 
second signal path being an error amplifier, an error extraction circuit 
which includes a directional coupler in which an input side has a first 
pair of conjugate first and second ports and is connected partly to the 
input of the feed-forward amplifier and partly to an output of the signal 
amplifier and an output side has a second pair of conjugate third and 
fourth ports, the fourth port being connected to an input of the error 
amplifier, and an error injection circuit which has a first and a second 
input connected to an output of the delay network and to an output of the 
error amplifier, respectively, and an output connected to the output of 
the feed-forward amplifier. 
A known feed-forward amplifier of the above defined type that is described 
in the Swedish patent No. 7102055-6 requires besides said delay network a 
further delay network which is cascade connected to the error amplifier. 
The delay networks become expensive and bulky when they must delay 
wide-band signals. In addition, their phase shift should be adjustable. A 
description of the technical problems involved is given in greater detail 
in the Bell System Technical Journal, Vol. 47, pp 651-722, May-June 1968 
and Vol. 50, pp 2879-2916, November 1971. 
An object of the present invention is to provide a feed-forward amplifier 
which like the known feed-forward amplifier attains a linear gain of 
wide-band signals, but is different in that it achieves this result 
utilizing only one delay network.

The drawing shows a feed-forward amplifier which has an input 1 and an 
output 2 arranged to receive wide-band signals and to supply a linearly 
amplified version of these, respectively. Two parallel signal paths are 
arranged between the input 1 and the output 2. One of the signal paths has 
a cascade connection of a signal amplifier 3 and a delay network 4 and the 
second signal path comprising an error amplifier there 5. In the 
feed-forward amplifier is included an error extraction circuit which 
comprises a directional coupler 6 in which an input side has a first pair 
of conjugate first and second ports and is connected partly to the input 1 
of the feed-forward amplifier and partly to the output of the signal 
amplifier 3 and an output side has a second pair of conjugate third and 
fourth ports, the fourth port being connected to an input of the error 
amplifier 5, and an error injection circuit which according to the 
embodiment is constituted by a directional coupler 7 which has a first and 
a second port in a first pair of conjugated ports connected to the output 
of the delay network 4 and to the output of the error amplifier 5, 
respectively, and a third and a fourth port in a second pair of conjugate 
ports connected to the output 2 of the feed-forward amplifier and 
resistively connected to ground via a resistor 8, respectively. 
The error extraction circuit is according to the invention characterized in 
that it has the third port of the directional coupler 6 connected to an 
input of the signal amplifier 3 and comprises a summation circuit in the 
form of transformer 9 which has a first and a second input connected to 
the input 1 of the feed-forward amplifier and to the output of the signal 
amplifier 3, respectively, and an output connected to the second port of 
the directional coupler 6 and arranged to supply a difference signal to 
the coupler. Any error component generated by the signal amplifier 3 and 
supplied on is output and added to a linarly amplified signal component 
will on the third and fourth port of the directional coupler 6 appear 
substracted from the signal component. This fact permits a cancellation of 
the error component to take place in the directional coupler 7 after a 
suitable amplification in the error amplifier 5 and delay in the delay 
network 4. The two directional couplers 6 and 7 effectively isolate the 
two signal paths from each other at the inputs of the amplifiers 3 and 5 
and at the outputs of the delay network 4 and the amplifier 5, 
respectively. As a consequence of the fact that the feed-forward amplifier 
is arranged to amplify wide-band signals, the isolation that the 
directional coupler 6 provides between the two signal paths is essential 
in order to prevent reflection phenomena from spoiling the cancellation of 
the error. component. 
The cancellation of the error component is dependent on the fact that the 
error amplifier 5 introduces only a negligible distortion. In this respect 
the error amplifier 5 is more critial than the signal amplifier 3. 
Consequently it is advantageous if the transmission coefficient S.sub.24 
between the second and fourth port in the directional coupler 6 is chosen 
near the value one although the coupling coefficient S.sub.23 between the 
second and third port in the same directional coupler 6 than becomes small 
according to the relation .vertline.S.sub.23 .vertline..sup.2 + 
.vertline.S.sub.24 .vertline..sup.2 = 1 with the consequence that an 
increased signal gain is required in the signal amplifier 3. 
The above mentioned summation circuit is not particularly critical 
regarding the isolation between its inputs and it is in the preferred 
embodiment of the invention constituted by a transformer 9 which has a 
primary side connected to the input 1 of the feed-forward amplifier and a 
secondary side a first end of which is connected to the second port of the 
directional coupler 6 and an attenuation network 10 via which a second end 
of the secondary side of the transformer 9 is connected to the output of 
the signal amplifier 3. The first port of the directional coupler 6 is in 
this embodiment resistively connected to earth via a resistor 11. The 
attenuation network 10 may include reactive elements to give the 
feed-forward amplifier a desired frequency response, the error amplifier 5 
in a suitable manner being given a corresponding frequency response to 
ensure the cancellation of any error component generated by the signal 
amplifier 3. 
In the feed-forward amplifier according to the invention the delay network 
4 is for delaying the transfer of an error component to the first port of 
the directional coupler 7, the error component being generated by the 
signal amplifier 3 and appearing on its output added to a linearly 
amplified signal component. The delayed error component shall coincide in 
time with the same error component subtracted from the signal component 
and transferred to the second port of the directional coupler 7 via the 
attenuation network 10, the transformer 9, the directional coupler 6 and 
the error amplifier 5. To achieve an accurate coincidence the delay 
network 4 is suitably provided with an adjustable phase shifter. 
The directional coupler 7 adds on its third port the signal components from 
the two signal paths in order to cancel any error component generated by 
the signal amplifier 3. The signal power is partly transferred via the 
fourth port of the directional coupler 7 to the resistor 8 where it is 
dissipated in the form of heat. It is therefore suitable that the coupling 
coefficient S.sub.14 between the first and the fourth port in the 
directional coupler 7 be small. Then also the coupling coefficient 
S.sub.23 becomes small with the consequence that an increased gain is 
required in the error amplifier 5 which implies a greater cost for this 
amplifier. The directional coupler 7 may of course be replaced by a 
summation circuit in the form of a transformer of the same type as the 
transformer 9 in order to achieve a low power dissipation which advantage 
then is obtained at the price of a reduced isolation between the outputs 
from the delay network 4 and the error amplifier 5. A discussion of these 
problems is given in greater detail in for example the English patent 
specification No. 1,302,605.