Abstract: A transmitter comprises an input terminal for receiving a signal to be amplified, a forward amplification path having a power amplifier output stage and a feedback path used to improve linearity, having two modes of operation:A) during a first mode of operation:i) at least occasionally measuring the DC conditions within the transmitter with the power amplifier disabled and using the results to minimize carrier output from the transmitter;ii) at least occasionally measuring the phase shift around the feedback loop required to achieve negative feedback;B) during a second mode of operation, using the measurement of phase shift to ensure negative feedback by applying phase compensation.

Abstract: A Cartesian amplifier is provided with a means of controlling its output power comprising a controllable attenuator in the amplifier forward path and a controllable amplifier in the feedback path. A control signal representing the required power level is applied to the attenuator and controllable amplifier and, in response thereto, the degrees of attenuation and amplification are varied in inverse proportion to one another.

Abstract: A Cartesian amplifier in which an input signal is pre-processed and split into two quadrature components. Both quadrature components are passed, in parallel, through an error amplifier (4), after which they are re-combined and up-converted to RF. The output of the amplifier is used to provide a feedback signal. This feedback is downconverted from RF to baseband and resolved into two quadrature components which are fed to the respective inputs of the error amplifier (4). Periodically the pre-processor (3) is switched into a calibration mode in which test signals are applied to the amplifier instead of the input signal. At these times the signal strength of the output of the power amplifier is measured and used to provide pre-distortion factors in the signal preprocessor (3) to improve amplifier linearity.