Power amplifier predistortion methods and apparatus
An embodiment of the invention is a predistortion approach to linearize a power amplifier by setting initially the static operation point of the power amplifier in the cut-off region. The architecture is based on the analog-digital combination by a multiplier and digital predistortion control circuit. The predistortion system generates an adaptive output signal to control the RF modulated signal that is the input signal of the power amplifier. The controlled and predistorted signal can contribute to drive a power amplifier with the cut-off static operation point into its amplifying region and simultaneously correct the non-linear distortion when the power amplifier operates to amplify an RF signal.
1. A method for linearizing an output of a power amplifier in a wireless broadband transmitter, the method comprising the steps of:
a) biasing the power amplifier to operate initially in a first bias point in a cut-off region;
b) determining a first set of predistortion lookup table entries from a predistortion lookup table;
c) applying the first set of predistortion lookup table entries to correct a radio frequency (RF) modulated signal while operating at the first bias point in the cut-off region;
d) biasing a static point of the power amplifier to a second bias point in a linear region of the power amplifier;
e) determining a second set of predistortion lookup table entries from the predistortion lookup table;
f) applying the second set of predistortion lookup table entries to the RF modulated signal, such that an error signal associated with a feedback component of the power amplifier and the RF modulated signal is reduced at the second bias point compared to the first bias point; and
g) repeating steps (d) through (f) until the error signal is adequately corrected.
2. The method of claim 1 wherein the first and second gains are at least partly determined based on a comparison between an ideal input and a feedback signal.
3. The method of claim 1 wherein the first predistortion control signal comprises substantial correction of non-linear signal distortion.
4. The method of claim 1 further comprising:
determining a first error signal comparing an ideal input of the power amplifier to a feedback signal of the power amplifier;
reducing the magnitude of the first error signal; and
adaptively adjusting and decreasing a gain of the predistortion processor in response to the error signal being reduced.
5. The method of claim 1 wherein determining the first and second predistortion control signals comprise use of a look-up table.
6. The method of claim 1 wherein the output of the power amplifier is v o (t)=v q (t)*G[x q (t)], where v q (t) is a quadrature modulated signal, G[x q (t)] is a gain of the power amplifier, and v o (t) is the output of the power amplifier.
7. The method of claim 6 wherein an input-output relation based on the output of the power amplifier is v o (t)=(v q (t)×v p )*G[x q (t)×v p ], where v p is a predistortion controller output component.