A control system for a linear feed-forward amplifier, using the ratio of measured spurious energy. Measuring receivers are coupled to two monitoring points to measure the spurious energy content in the error signal and at the main output of a feed-forward amplifier. The control system measures the ratio of these detected spurious, and uses this to optimize the settings of the second loop distortion cancellation. With the addition of an extra monitoring point, the same technique may also be used to control an adaptive predistorter prior to the main amplifier.