Amplifiers – Combined with automatic amplifier disabling switch means
Reexamination Certificate
1999-11-09
2001-09-11
Shingleton, Michael B (Department: 2817)
Amplifiers
Combined with automatic amplifier disabling switch means
C330S107000, C330S12400D, C330S129000, C330S149000, C455S126000, C455S103000
Reexamination Certificate
active
06288606
ABSTRACT:
TECHNOLOGICAL FIELD
The invention concerns generally the technology of power amplifiers or PAs used in radio frequency transmitters. Especially the invention concerns an arrangement for producing linear amplification effectively throughout a large dynamic range.
BACKGROUND OF THE INVENTION
A power amplifier is the component in a radio frequency transmitter that amplifies the signal to be transmitted to a power level high enough to be conducted to the output port (e.g. antenna port) of the transmitter. In systems employing digital linear modulation schemes the linearity of the power amplifier is a very important factor. Digital linear modulation means that the signal to be transmitted has a number of allowed phase- and amplitude states, whereby each allowed phase-amplitude combination represents a certain bit pattern in a bit stream used as the source of the modulation and constitutes a so-called constellation point in a phase-amplitude diagram. The power amplifier has to reproduce the phase-amplitude combinations of the signal to be transmitted so that the relative distances between the constellation points is not distorted; hence the strict requirements for linearity in the operation of the power amplifier.
A traditional linear power amplifier is a so-called Class A amplifier meaning that the amplifying component is not driven to output current cutoff at any allowed value of the driving voltage. If the driving voltage is a sinusoidal oscillation, the conduction angle of a Class A amplifier is said to be 360 degrees. The drawback of Class A amplifiers is their poor efficiency, which may theoretically be 50% but falls in most practical circuits to the order of 25 to 30%. The problem is made worse by the requirement of a large dynamic range which is typical to the mobile terminals of digital cellular radio systems. As an example we may consider a mobile telephone, where the difference between the lowest and highest values of transmitted power may be several tens of decibels. If the linear power amplifier is designed for maximal efficiency at one end (usually the high end) of the dynamic range by selecting the structural parameters of the amplifying semiconductor component in a suitable way, efficiency at the other end (the low end) of the dynamic range is bound to be poor. Correspondingly a relatively large amount of electric energy is wasted during operation in the latter end of the dynamic range. In portable radio devices like mobile telephones wasting energy means unnecessarily shortening the operating time before the next required recharging of batteries.
From the European patent application number 98660044.3, published in December 1998 and incorporated herein by reference, there is known an arrangement consisting of at least two parallel amplifier stages, of which only one is selected for use at any moment of time by setting the bias voltages of the amplifier stages to suitable values. In such a structure each of the parallel amplifier stages may have a different optimal power range which is a subset of the required dynamic range of the whole amplifier arrangement. A separate control circuit selects for use the amplifier stage whose optimal power range is closest to the momentarily required output power of the amplifier arrangement. A remarkable saving is achievable in comparison to a single linear power amplifier stage, but there remains the problem of the efficiency remaining under 50% even if continuous optimal operation is assumed. Additionally it has been shown that immediately after changing the selected amplifier branch a phase distortion occurs in the output signal, which is a detrimental effect in systems using continuous phase modulation.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a power amplifier arrangement with a relatively wide dynamic range and a good efficiency throughout it. It is a further object of the invention to realise the inventive amplifier arrangement with a small number of inexpensive components. Additionally it is an object of the invention to provide a method for operating such a power amplifier arrangement effectively throughout the dynamic range. A still further object of the invention is to reduce phase distortions in the power amplifier arrangement.
The objects of the invention are achieved by arranging the power amplifier arrangement to comprise at least two parallel amplifier branches, of which at least one comprises a nonlinear amplifier, and by predistorting the input signal of the nonlinear amplifier(s). The pursued reduction in phase distortions is achieved by using an adjustable phase shifter in at least one of the parallel amplifier branches.
It is characteristic to the power amplifier arrangement according to the invention that it comprises in its first parallel amplifier branch a series connection of a predistorter and a nonlinear amplifier, where said predistorter is arranged to compensate for the nonlinearity of said nonlinear amplifier.
The invention applies also to a radio communication device. The characteristic feature of the radio communication device is that it comprises
in a linear power amplifier arrangement,
a first parallel amplifier branch and a second parallel amplifier branch and selection means for conducting a radio frequency signal to be transmitted selectably to one of the parallel amplifier branches, and
in the first parallel amplifier branch a series connection of a predistorter and a nonlinear amplifier, where said predistorter is arranged to compensate for the nonlinearity of said nonlinear amplifier; and
a control block for controlling the operation of said selection means for selectively establishing the conduction of the radio frequency signal to be transmitted through either the first or the second parallel amplifier branch.
Additionally the invention applies to a method, having as its characteristic features the steps of
selecting a first parallel amplifier branch for operation and
in the first parallel amplifier branch, nonlinearly predistorting the signal to be amplified and subsequently nonlinearly amplifying it so that the nonlinearities of the predistortion and amplification substantially cancel each other.
The general idea of composing the power amplifier arrangement from a number of parallel selectable amplifier branches is known as such, as we may see e.g. from the European patent application cited above in the description of prior art. In the present invention this basic principle has been developed further by employing at least one nonlinear power amplifier in at least one of the parallel amplifier branches. By biasing an amplifier for nonlinear operation it is possible to obtain a higher efficiency than with traditional linear Class A amplifiers. To compensate for the nonlinearity of the amplifier proper a predistorter is inserted in front of it to implement a similar nonlinearity with an opposite sign. The distorting impacts of the predistorter and the nonlinear amplifier cancel each other to a large extent and their combination works like a linear power amplifier.
The invention does not limit the number of the parallel amplifier branches in the power amplifier arrangement. Also the number of branches with a nonlinear amplifier is not specified by the invention, as long as there is at least one such branch. A switching arrangement or a functionally equivalent circuit is provided for selectively conducting the signal to be amplified through one of the parallel amplifier branches at any given moment. One functionally equivalent circuit could be a biasing arrangement according to the European patent application cited above in the description of prior art. A control block controls the operation of the switching arrangement so that the selection of the amplifier branch takes place according to the required output power of the power amplifier arrangement.
To eliminate the phase distortion related to the moment of changing the selected amplifier branch the invention employs a phase detecting and correcting arrangement consisting of at least one phase detector and
Ekman Tapani
Rosnell Seppo
Vaisanen Risto
Nokia Mobile Phones Ltd.
Perman & Green LLP
Shingleton Michael B
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