The present invention relates to wireless communications, and in particular, to a unique technique for controlling amplifier performance in multiple communication modes by varying a supply voltage for the amplifier.
The present invention is particularly useful in mobile terminals, such as personal communication assistants, pagers, headsets, wireless modems, analog and digital cellular telephones, and the like. Since many of these devices are battery-powered, amplifier efficiency is preferably maximized to extend battery life. When amplifiers are designed for their highest efficiency in converting DC energy into RF energy, parasitic losses are minimized, bandwidths are reduced to a bare minimum, harmonics are terminated, and high-Q matching networks are employed. Unfortunately, these design goals are counter to current approaches used to implement multi-mode amplifiers capable of operating at multiple frequencies.
Most RF power amplifiers are designed to operate over a single band of frequencies. If coverage is desired for one or more additional frequency bands, a multi-band amplifier is typically created. For a dual-mode application, the most straightforward approach is to simply use two amplifiers and switch between them to select a desired band for transmission. If a single amplifier configuration is desired, then matching networks that provide the proper impedance transformation for both frequency bands are required. These matching networks are implemented in a number of ways. The use of series and parallel resonant elements, typically inductors and capacitors, in the matching networks are selected such that at one frequency band, the combination appears inductive and at the other band, it appears capacitive. Clever combinations of series and shunt element pairs may allow a creation of networks that deliver the desired properties over multiple frequency bands. These approaches typically result in limited bandwidth for each band and difficulty in tuning and maintaining performance over each of the bands.
If the amplifier is designed to operate in one band at a time, one or more switches are typically used to add elements to or remove elements from the matching networks for the appropriate band. This works well and has been employed in many applications. The drawbacks are the additional energy needed to operate the switches and the losses the switches add to the networks. If a change in the amplifier mode is desired, such as changing linearity, efficiency, or power, then the load of the amplifier may be switched by adding or removing elements from the matching network. Examples of these techniques are disclosed in U.S. Pat. Nos. 5,438,684 and 5,673,287, which are assigned to Motorola, Inc. Again, the switching techniques reduce system efficiency, which results in decreased battery life. As such, there is a need for an improved and efficient multi-mode amplification technique that does not require inefficient and complicated matching networks and amplifier designs.
The present invention eliminates the need for complex impedance networks or parallel amplification stages for multi-mode mobile terminals. A wideband power amplifier is configured to amplify signals in different frequency bands corresponding to different operating modes. The supply voltage of the wideband power amplifier is adjusted in light of the load impedance of radiating circuitry to achieve a desired output power for the respective operating modes.
Accordingly, the present invention relates to a radio frequency communication system including radiating circuitry, a wideband power amplifier output stage, a variable power supply, and a control system. The radiating circuitry, which includes an antenna, has a load impedance. The wideband power amplifier output stage is coupled to the radiating circuitry and is adapted to amplify a radio frequency signal in each of a plurality of frequency bands corresponding to a plurality of operating modes. The variable power supply is adapted to provide a selected supply voltage for supplying power to the wideband power amplifier output stage in response to a power control signal. The control system is adapted to generate the power control signal based on a selected one of the plurality of operating modes. The selected supply voltage and the load impedance for the frequency band of the selected operating mode determine the power delivered to the radiating circuitry from the wideband power amplifier output stage for each of the plurality of operating modes.
The power level for any given operating mode may be further controlled by controlling a signal level for the radio frequency signals to be amplified by the wideband power amplifier. Alternatively, controlling the selected supply voltage may control the power level.
The wideband power amplifier output stage is preferably made of transistors configured to operate in a saturation region when in one of the selected operating modes and in a linear region when in another of the selected operating modes. Preferably, one or more wideband intermediate amplifier stages are coupled in series to the wideband power amplifier output stage. The intermediate amplifier states are fed by combining circuitry for passing signals within the frequency band of the selected operating mode.
Bias circuitry is used to provide bias to the wideband power amplifier output stage. The bias is configured to optimize efficiency of the wideband power amplifier output stage at different supply voltages for each of the operating modes. Further, the radiating circuitry may include a selectable impedance component to adjust the reactance of the load impedance for at least one of the operation modes to optimize signal transmission. The radiating circuitry may be configured to provide a load impedance having a first impedance for a first frequency band and a second impedance for a second frequency band. The first and second impedances may be substantially the same or may vary based on design considerations. In operation, the supply voltages for the wideband power amplifier output stage will set the output power levels.
Those skilled in the art will appreciate the scope of the present invention and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures.