Feedforward is another linearization technique. It was invented by H. S. Black in 1928. In contrast to feedback it doesn’t suffer from stability problems. It is unconditionally stable. As a result it can be used to linearize wideband signals. The theory behind this technique is simple but it can be rather costly to implement it in hardware [4].
Feedforward linearization technique is depicted in Figure 2.6. As seen in the figure the input signal is split into two paths by a power splitter(divider). The signal in the upper path goes to the nonlinear power amplifier to be linearized, which is denoted as main amplifier in Figure 2.6. The output of the amplifier con- tains the amplified input signal plus the distortion generated by the amplifier. This signal is sampled by a coupler and then goes to the 180 hybrid coupler
passing through an attenuator. The signal in the lower path also goes to the same coupler after being delayed by a delay element. This delay is necessary to compensate for the delay introduced by the amplifier. These two signals are subtracted from each other by the 180 hybrid coupler and ideally the signal at
the output of the coupler only contains the distortion generated by the amplifier. The main signal is cancelled. This signal at the output of the 180hybrid coupler
is generally called the error signal. This error signal is amplified by an ampli- fier which is denoted as error amplifier in Figure 2.6. The output of the error amplifier, which contains the amplified distortion components of the main ampli- fier, goes to the error injection coupler. A delayed version of the main amplifier output also goes to the error injection coupler. This second delay compensates for the delay introduced by the error amplifier. These two signals are subtracted from each other by the error injection coupler. Ideally this subtraction cancels the distortion introduced to the input signal by the main amplifier. So to sum
up it can be said that the first loop cancels the main signal and isolates the dis- tortion of the main amplifier. In the second loop the isolated distortion is used to cancel the distortion of the main amplifier.
There are some issues that must be mentioned about feedforward lineariza- tion technique. Feedforward ideally cancels the distortion of the main amplifier completely but for this to occur, perfect gain matching is required in the signal and distortion cancellation loops. For gain matching in the signal cancellation loop, the total loss due to the sampling coupler and the attenuator must match the gain of the main amplifier. Also the delay of the delay line must match the group delay of the main amplifier to time align the main amplifier output and the signal in the lower path before subtracting them from each other. For gain matching in the distortion cancellation loop, the gain of the error amplifier must match the total loss due to sampling coupler, attenuator, hybrid coupler and error injection coupler to increase the error signal to the same level as the distortion component of the main amplifier output signal. Also the delay of the delay line must match the group delay of the error amplifier to time align the main amplifier output and error amplifier output before subtracting them from each other. When these conditions are not satisfied so that there is gain or delay mismatch in the system, complete cancellation of the main amplifier distortion will not occur. The system will have a finite distortion cancellation and the level of cancellation will depend on the level of gain/delay mismatch. The error am- plifier in the system is a critical component for distortion cancellation. It must be highly linear so that it doesn’t create additional distortion. It must provide sufficient gain. It must have a small group delay so that the required delay line length in the upper path is not large.
Figure 2.7: Multiple feedforward(2 loops) [1]
Figure 2.8: Adaptive feedforward [5]
The basic feedforward system shown in Figure 2.6 does not take the al- terations that can occur in the element responses with aging and temperature changes into account. But such changes occur and this degrades the linearization performance. One way to reduce such effects is to use multiple feedforward loops as seen in Figure 2.7 [2]. In this configuration a feedforward loop acts as main amplifier and is placed within another feedforward loop. This process can be continued and 3, 4, etc. feedforward loops can be used. But this also increases the complexity of the system fast, which is a disadvantage. Another way is to make the system adaptive(Figure 2.8). With adaptation the linearization per- formance is under control and any change in the element responses can easily be accounted for. But adaptation has also a disadvantage. It introduces feedback to the system and this can result in stability and bandwidth problems.
Figure 2.9: Envelope Elimination and Restoration Technique [6]