A signal to be amplified is provided in polar format comprising an amplitude component ( 210 ) and a phase component ( 212 ). The amplitude component ( 210 ) is used to bias the phase component ( 212 ). The gain of amplifier ( 220 ) depends upon the bias of the signal that it receives. Therefore, the bias given to the phase component ( 212 ) provides the envelope characteristics of the output signal ( 224 ) of amplifier ( 220 ). Both (or either) of the amplitude component ( 210 ) and the phase component ( 212 ) may be predistorted ( 226, 310 ) to eliminate distortion appearing in the output signal ( 224 ).
1. Apparatus for amplifying an input signal provided in polar format comprising an amplitude component and a phase component to produce an output signal, the apparatus comprising a biaser configured to derive a bias signal from the amplitude component, a combiner configured to combine the bias signal with the phase component to produce a biased phase component signal, and an amplifier whose gain depends upon the bias provided to the signal it amplifies, wherein the amplifier is supplied with the biased phase component signal to produce the output signal.
2. The apparatus of claim 1 , further comprising a linearizing scheme configured to counteract distortion in the output signal.
3. The apparatus of claim 2 , wherein the linearizing scheme comprises a subtractor for subtracting modulation information obtained from the output signal from modulation information that is used to create the phase and amplitude components.
4. The apparatus of claim 3 , wherein the subtractor is arranged to subtract amplitude information obtained from the output signal from amplitude modulation information that is used to create the amplitude component.
5. The apparatus of claim 3 , wherein the subtractor is arranged to subtract phase information obtained from the output signal from phase modulation information that is used to create the phase component.
6. The apparatus of claim 2 , wherein the linearizing scheme comprises a predistortion scheme configured to predistort the input signal.
7. The apparatus of claim 6 , wherein the predistortion scheme comprises an amplitude predistorter configured to predistort the amplitude component of the input signal.
8. The apparatus of claim 6 , wherein the predistortion scheme comprises a phase predistorter configured to predistort the phase component of the input signal.
9. The apparatus of claim 6 , wherein the predistortion scheme operates on at least one of the components of the input signal in the digital domain.
10. The apparatus of claim 6 , wherein the predistortion scheme monitors residual distortion in the output signal and adapts the predistortion of the input signal to reduce the residual distortion.
11. The apparatus of claim 1 , wherein the input signal is provided as a modulated carrier signal and the apparatus comprises a limiter configured to limit the modulated carrier signal to produce the phase component.
12. The apparatus of claim 1 , wherein the input signal is provided as a modulated carrier signal, and the apparatus comprises a detector configured to detect the envelope of the modulated carrier signal to produce the amplitude component.
13. The apparatus of claim 1 , wherein the input signal is provided as a quadrature signal and the apparatus comprises a converter configured to convert the input signal from quadrature to polar format.
14. The apparatus of claim 1 , wherein the input signal is provided as a channel of amplitude modulation information and a channel of phase modulation information, and the apparatus comprises a converter configured to derive the amplitude and phase components from the amplitude and phase modulation information channels, respectively.
15. The apparatus of claim 1 , wherein the biaser is configured to amplify envelope information from the amplitude component to derive the bias signal.
16. A method of amplifying an input signal to produce an output signal, comprising the steps of:
(a) providing the input signal in polar format comprising an amplitude component and a phase component;
(b) deriving a bias signal from the amplitude component,
(c) combining the bias signal with the phase component to produce a biased phase component signal; and
(d) amplifying the biased phase component signal to produce the output signal using an amplifier whose gain depends upon the bias provided to the signal which it amplifies.