Amplifiers – With control of power supply or bias voltage – With control of input electrode or gain control electrode bias
Reexamination Certificate
1998-11-18
2001-02-20
Shingleton, Michael B (Department: 2817)
Amplifiers
With control of power supply or bias voltage
With control of input electrode or gain control electrode bias
C330S010000, C330S136000, C330S149000, C330S297000, C455S126000
Reexamination Certificate
active
06191653
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to RF amplifiers and, more particularly, to a circuit and method for linearizing amplitude modulation in a power amplifier.
BACKGROUND OF THE INVENTION
Radio frequency (RF) transmitters, such as used in cellular telephones, develop an RF signal to be transmitted through the air. Information is carried on the signal via some form of modulation such as frequency modulation, phase modulation, amplitude modulation, or a combination of these.
It may be desirable to create a modulated signal with both amplitude and phase modulation. With the necessity of developing small and lightweight devices, particularly cellular telephones, it is important that such amplifier circuits use a minimum of components. One way to satisfy this desire is to directly modulate an oscillator phase lock loop (PLL) to impart the phase modulation component directly on the signal and then to amplitude modulate the power amplifier stage connected to a voltage controlled oscillator/phase lock loop (VCO/PLL) combination with the amplitude component. VCO/PLL circuits exist that have sufficient bandwidth relative to the information bandwidth of the signal to cause the phase modulation to occur directly on the output signal without any up-conversion. It remains, however, to put an amplitude signal onto this phase modulated signal. This is preferably done in the power amplifier stage, as it will permit this stage to run at high efficiency in a non-linear mode. The amplitude information could be imparted via modulating the power amplifier supply voltage, much as was done with amplitude modulated transmitters previously. However, a difficulty arises in that the transfer function between power amplifier supply voltage and signal amplitude output may not be linear.
Previously, the above problems have been solved by various alternative modulation methods. One is the use of quadrature or I/Q modulation of the RF signal directly. This requires a linear power amplifier. Also, a linear RF power amplifier has used an amplitude feedback loop to make the RF output track the amplitude of the RF input signal. However, a linear power amplifier is not as efficient as a non-linear power amplifier. Also, either a linear or a non-linear power amplifier with an amplitude feedback loop has an inherent limit on the loop filtering that may restrict the amount of noise that can be suppressed from the varying voltage supply.
The present invention is directed to overcoming one or more of the problems discussed above in a novel and simple manner.
SUMMARY OF THE INVENTION
In accordance with the invention there is provided a circuit and method for linearizing the output of an amplitude modulated RF amplifier.
Broadly, there is disclosed herein an RF amplifier including an oscillator developing an RF input signal to be transmitted. A power amplifier circuit receives the RF input signal and amplifies the RF input signal to develop an RF output signal. An amplifier control circuit is operatively associated with the oscillator and the power amplifier circuit. The amplifier control includes means for developing a control signal representing a desired amplitude of the RF output signal. Memory means store correction information correlating actual amplitude of the RF output signal relative to the control signal. Control means vary the power amplifier circuit supply voltage using the control signal modified responsive to the correction information for the desired amplitude.
It is a feature of the invention that the oscillator develops a phase modulated RF input signal.
It is another feature of the invention that the amplifier control circuit comprises a processor circuit.
It is still another feature of the invention that the memory means stores a transfer curve of the power amplifier circuit RF output signal relative to the control signal.
It is still another feature of the invention to provide means coupled to the amplifier control circuit for monitoring the amplitude of RF output signal. The amplifier control circuit periodically updates the correction information using the monitored RF output signal amplitude and the desired amplitude.
It is still another feature of the invention that the control means comprises a switching regulator developing the power amplifier circuit supply voltage. Alternatively, the control means comprises a pulse density generator function such as a delta-sigma modulator and a Class D amplifier stage and a low pass filter developing the power amplifier circuit supply voltage.
It is still a further feature of the invention that the control means comprises a programmed processor controlling the oscillator. The oscillator comprises a high speed phase lock loop (PLL). The PLL includes a voltage-controlled oscillator (VCO) and a divider and a controller for the VCO controls the divider integer. In accordance with another aspect of the invention, another memory means stores a correction table correlating AM to PM conversion to the control signal and the processor sends a pre-distorted phase control signal to the PLL.
In accordance with a further aspect of the invention, the processor controls the VCO.
In accordance with still another aspect of the invention, the oscillator comprises a phase modulated voltage-controlled oscillator.
In accordance with a further aspect of the invention there is disclosed the method of linearizing amplitude modulation in a power amplifier of an amplifier circuit comprising the steps of developing an RF input signal to be transmitted, a power amplifier receiving the RF input signal and amplifying the RF input signal to develop an RF output signal, storing correction information correlating desired amplitude of the RF output signal relative to the power amplifier supply voltage, and varying the power amplifier supply voltage based on variation of the RF input signal to linearize amplitude modulation in the power amplifier.
Further features and advantages of the invention will be readily apparent from the specification and from the drawing.
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Arpaia Domenico
Boesch Ronald D.
Camp, Jr. William O.
Gore Charles
Schlang Jeffrey
Ericsson Inc.
Shingleton Michael B
Wood Phillips VanSanten Clark & Mortimer
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