This invention relates to a phase and amplitude detector (160) required to identify small signal errors in a signal envelope having a large dynamic range, especially in the context of linearization of a power amplifier (122) arrangement employing a pre-distortion technique. A vector generator (300, 352, 372) responsive to a reference signal R (110) produces a frame of reference vectors R 1 - R n (274-280) generated by a combination of the reference signal R (110) with first A (270) and second P (272) offset vectors that provide an amplitude and phase displacement of the reference signal R (110). A signal combiner (290-296, 360-366, 390-396) is arranged to generate difference vectors E 1 - E n by combining the frame of reference vectors R 1 - R n (274-280) and the feedback signal F (124, 150), with the difference vectors E 1 - E n expressing the phase (p, 254) and the gain (a, 252) error terms relative to the reference signal R (110) and the first A (270) and second P (272) offset vectors. An error signal detector, (330-336) responsive to the difference vectors E 1 - E n and arranged to provide a measure of the phase (p, 254) and the gain (a, 252) error terms, provides signal amplitudes that can be combined to generate error signals (182 (Y), 184 X)). The error signals take the general form: X = P 1 - P 2 - P 3 + P 4 = -8 P ̲ p R ̲ ; Y = P 1 + P 2 - P 3 - P 4 = -8 A ̲ a R ̲