Digital predistortion for amplifiers

Details Digital predistortion for amplifiers Digital predistortion for amplifiers

2000-06-12

2001-10-16

Pascal, Robert (Department: 2817)

Amplifiers

Hum or noise or distortion bucking introduced into signal...

C330S002000, C375S296000, C375S297000

Reexamination Certificate

active

06304140

ABSTRACT:

RELATED APPLICATIONS
The subject matter of the present application is related to the subject matter of co-pending U.S. patent application entitled “Extended Base Band Multicarrier System,” Thron et al., which shares a common assignee with the present application and is incorporated by reference herein.
FIELD OF THE INVENTION
The present application is related to the field of signal amplification and more particularly to a system for predistorting a signal prior to amplification to achieve a more linear amplification.
BACKGROUND OF THE INVENTION
In an ideal linear amplifier, the output power (V
OUT
2
) is equal to the input power (V
IN
2
) times a constant K that does not vary with the input power. Similarly, the input signal phase (&THgr;
IN
) is equal the output signal phase (&THgr;
OUT
). In an actual amplifier, however, both the output power and the output phase vary from the ideal output power and the ideal output phase. Typically, the variation from the ideal output power and phase is a function of input power. Referring to
FIGS. 1 and 2
, diagrams of output power and carrier phase rotation as a function of input power for a non-ideal (real) amplifier are depicted. In
FIG. 1
, the output power of an actual amplifier is diagramed as a function of input power. The response of an ideal amplifier is represented by the straight line
100
where the slope of line
100
is equal to the desired gain of the amplifier denoted by K.
FIG. 1
further indicates a real response curve
102
representing the output power of a real amplifier as a function of input power.
Typically, real response curve
102
includes three sections as indicated by reference numerals
104
,
106
, and
108
. A first region
104
, referred to herein as linear region
104
, typically includes portions of response curve
102
representing input powers in the vicinity of zero. In first region
104
, the response curve
102
closely tracks the ideal response curve
100
. Thus, in linear region
104
, the real amplifier represented by response curve
102
closely resembles an ideal amplifier. As the input power is increased, however, a real amplifier typically enters a second (compression) region
106
in which response curve
102
begins to roll off of ideal response curve
100
. As the input power is further increased, the real amplifier represented by response curve
102
enters a third (saturation) region
108
in which the output power is essentially independent of input power as the real amplifier reaches a maximum obtainable output power.
Referring now to
FIG. 2
, response curve
201
represents the carrier phase rotation of a real amplifier as a function of input power. From inspection of response curve
201
, the carrier phase rotation, which indicates the differential between the input signal and the output signal phase, is substantially equal to zero at low input powers indicating little or no phase shift. As the input power is increased, however, the carrier phase rotation increases in magnitude as indicated by the descending slope of response curve
201
.
Typically, it is highly desirable to eliminate the non-linearity of real amplifiers represented by response curves
102
and
201
in
FIGS. 1 and 2
respectively. Therefore, it would be highly desirable to implement a circuit, method, and system to compensate for the non-linearity associated with real amplifiers such that the output of the amplifier would more closely resemble the output of an ideal amplifier. It would be further desirable if the implemented circuit, method, and system did not significantly increase the cost, complexity, and reliability of the amplification system.


REFERENCES:
patent: 4462001 (1984-07-01), Girard
patent: 5049832 (1991-09-01), Cavers
patent: 5627857 (1997-05-01), Wilson
patent: 5732333 (1998-03-01), Cox et al.
patent: 5929704 (1999-07-01), Proctor, Jr. et al.
patent: 5937011 (1999-08-01), Carney et al.
patent: 6112062 (2000-08-01), Hans et al.
Lohtia, “Power Amplifier Linearization using Cubic Spline Interpolation”, IEEE, pp. 676-679 (1993).
Kenington, “Linearised RF Amplifier and Transmitter Techniques,” Wireless Systems International Ltd. pp. 2-22 (1998).

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