1. Field of the Invention
The present invention relates to amplification stages for providing a modulated supply voltage, and particularly to such amplification stages organised to provide higher power than can be achieved in a single stage by using an arrangement known as a ‘corporate structure’ amplifier.
2. Description of the Related Art
It is known to provide a modulated power supply for providing a supply voltage to an amplification stage, such as a radio frequency (RF) amplification stage. An example of a particularly advantageous modulated power supply stage can be found in United Kingdom Patent No. 2398648.
In this advantageous modulated power supply stage there is provided an efficient technique for tracking the supply voltage to an RF amplifier in dependence on the RF input signal to be amplified by the amplifier. A first control loop tracks the envelope of the input signal, representing a desired supply voltage for the amplification stage, and selects one of a plurality of available supply voltages in dependence thereon. A second control loop tracks the envelope of the input signal and the actual output signal, and generates an error signal representing the difference there between. This error signal is combined with the selected supply voltage to provide an adjusted selected supply voltage for the amplification stage. The first control loop is a low frequency loop, and the second control loop is a high frequency loop.
Such an improved modulated supply stage offers significant advantages and improvements in delivering a highly accurate tracking (or tracked) power supply to an amplifier, with associated efficiency improvements.
It is also known, in amplifying signals such as RF signals, to split the input signal into portions, and then amplify each portion in a separate power amplifier, before recombining the amplified portions. The use of multiple power amplifiers to amplify a signal in this way enables a more powerful signal to be delivered than could otherwise be delivered. Typically such an amplified signal is delivered to an antenna of an RF transmitter.
Arrangements in which a signal is split in this way for amplification by multiple parallel amplifiers, before recombining, are known as ‘corporate structures’.
In the prior art highly accurate tracking power supply arrangement found in United Kingdom Patent No. 2398648, if the power supply stage is adapted for a corporate structure to deliver a power supply to n power amplifiers, then the power of each of the low frequency loop and the high frequency loop must be increased by a factor n. This has a negative effect on performance. The loops must be made physically larger. Layout problems may arise on the circuit board, with longer tracks needed to some power amplifiers. The maximum number of amplifiers which may be combined may also be restricted due to physical constraints of the power amplifiers on the feed network through which the modulated supply is delivered.
Alternatively a single modulated power supply may be provided for each single amplifier stage. This requires a large amount of replication of circuits. This adds to the cost and size of the implementations, and thus introduces of its own inefficiencies. In this arrangement, to achieve the benefits of an efficient, highly accurate tracking power supply for each single amplifier requires a high component count and associated cost.
As noted above, in a corporate structure with n amplifiers and n associated modulated supplies, regardless of the specific modulated supply implementations, disadvantages of performance, cost and size may be encountered.
It is an aim of the invention to provide a technique to enable some or all of the benefits of high accuracy tracking power supplies to be obtained in arrangements in which multiple power amplifiers are implemented in parallel.