Radio frequency (RF) amplifiers are used in a wide variety of communications and other electronic applications. It is well known to use feed forward for distortion reduction in an RF amplifier. In a basic feed forward system, the instantaneous difference between an amplifier input and its, output, representing the amplifier distortion, is amplified and subtracted from the output. In effect, by knowing what the amplifier distortion is, it is possible to remove it by simple subtraction.
FIG. 1 shows the basic construction of a conventional feed forward amplifier. An input signal is provided on a line 10 to a directional coupler 12 which divides the input signal into two parts including a primary portion which is provided on a line 17 to a main amplifier 20 and a sampled portion which is provided on a line 22 to a delay line 25. The main amplifier 20 increases the level of the primary portion of the input signal, and in doing so introduces distortions. The delay line 25 is provided to compensate for the time delay of the primary portion of the input signal through the main amplifier, and the output of the delay line 25 is provided to a difference coupler 27.
The output of the main amplifier 20 is an amplified input signal which is provided to a directional coupler 30 where it is divided into two parts including a primary portion provided on a line 32 to a delay line 35 and a sampled portion which is provided via an attenuator 40 to the difference coupler 27. The attenuator 40 reduces the magnitude of the sample of the amplified input signal so that its magnitude is approximately the same as the magnitude of the delayed sampled portion of the input signal. The output of the difference coupler 27 is the difference between the output of the attenuator 40 and the delayed sampled portion of the input signal, which is a distortion signal that is primarily the distortion from the main amplifier. The distortion signal is provided on a line 43 to an error amplifier 47 the output of which is provided on a line 50 to a second difference coupler 52. The delay line 35 is provided to compensate for the time delay of the distortion signal through the error amplifier 47, and the output of the delay line 35 is also provided to the second difference coupler 52 via a line 54. The error amplifier increases the level of the distortion signal on the line 43 so that it can be subtracted from the amplified input signal in the difference coupler 52. Therefore, the feed forward amplifier of FIG. 1 provides a simple method for amplifying and subtracting out the distortion introduced by the main amplifier.
A problem with a feed forward amplifier of the type described above and illustrated in FIG. 1 is that its performance is dependent on circuit component characteristics and tolerances. The subtraction of the distortion signal must be done precisely, e.g., proper gain and phase, to be effective. Additionally, the feed forward amplifier represents an open loop process, meaning that there is no feed-back involved to make automatic adjustments for circuit errors. Therefore, as the gain and phase characteristics of the feed forward amplifier change with age and with changes in environmental conditions such as temperature and humidity, the circuit of FIG. 1 may not properly subtract the distortion from the output signal.