Abstract:
Circuits and methods for use in amplifying amplitude and phase modulated signals. A circuit uses a combiner with dual parallel signal amplifiers feeding it. The signal amplifiers have a low output impedance while the combiner does not provide any isolation between its inputs from the signal amplifiers. As in other Chireix architectures, the signals from the signal amplifiers are phase modulated prior to being fed to the combiner. The combiner then combines these two signals and, depending on how these two signals are combined, the resulting output of the combiner is amplitude modulated. The signal amplifiers may be Class D or Class F amplifiers to provide high efficiency amplification of the signals.

Description:
FIELD OF THE INVENTION 
     The present invention relates to signal processing and is particularly applicable but not limited to circuits for amplifying amplitude and phase modulated signals. 
     BACKGROUND TO THE INVENTION 
     The communications revolution of the 1990&#39;s has led to an increasing need for further and better means of transporting both data and voice communications. One offshoot of this revolution has been the burgeoning growth in wireless communications as more and more data is being transmitted by wireless means. For wireless handsets, wireless PDAs (personal digital assistants), and other wireless devices, one overarching concern is power consumption—the less power a device consumes the more desirable it is. To this end, higher efficiency components, such as amplifiers, are desirable in these wireless devices. 
     One type of architecture which was used in the past but has fallen out of recent favor is the so called Chireix architecture. First suggested by Henry Chireix in 1935, the technique also known as “outphasing” involves separately phase modulating two signals and recombining them in a combiner or combining network. By judiciously adjusting the phase modulation of the two signals, the combined resulting signal can become amplitude modulated as well as phase modulated. This technique enables the use of saturated amplifiers or switching amplifiers for amplitude modulated signals. 
     Also called “ampliphase” by the RCA Corporation when used in some of their radio transmitters, the technique has recently fallen out of favor due to its seeming inapplicability when amplifying signals. Previous attempts used common forms of linear and saturating amplifiers such as Class A and Class AB amplifiers to try and amplify the two signals prior to their being combined. Unfortunately, these efforts have yielded unacceptable results as the resulting circuits were found to be insufficiently efficient. A Chireix based architecture, if properly working with sufficient efficiency, would offer advantages in not only power consumption but in other areas as well. 
     Based on the above, there is a need for, methods or circuits which overcome or at least mitigate the drawback of the prior art. Such a solution should provide the advantages of a Chireix or outphasing architecture while providing sufficient amplification efficiency to be useful. 
     SUMMARY OF THE INVENTION 
     The present invention provides circuits and methods for use in amplifying amplitude and phase modulated signals. A circuit uses a combiner with dual parallel signal amplifiers feeding it. The signal amplifiers have a low output impedance while the combiner does not provide any isolation between its inputs from the signal amplifiers. As in other Chireix architectures, the signals from the signal amplifiers are phase modulated prior to being fed to the combiner. The combiner then combines these two signals and, depending on how these two signals are phase modulated, the resulting output of the combiner is amplitude modulated as well as phase modulated. The signal amplifiers may be Class D or Class F amplifiers to provide high efficiency amplification of the signals. 
     In a first aspect, the present invention provides a circuit for providing amplification to signals, the circuit comprising: 
     a signal combiner; 
     at least two signal amplifiers each receiving and amplifying a signal, said signal amplifiers being coupled in parallel to said combiner, each of said signal amplifiers acting as a voltage source and having a low output impedance, 
     wherein 
     each signal received and amplified by each of said signal amplifiers is phase modulated; 
     outputs of each of said signal amplifiers are added by said combiner to produce a resulting signal; and 
     amplitude modulation and phase modulation of said resulting signal from said combiner is achieved by an addition of said outputs. 
     In a second aspect, the present invention provides a circuit for providing amplification to signals, the circuit comprising: 
     at least two signal amplifiers each receiving and amplifying a signal and each producing an amplifier output and acting as a voltage source; and 
     an signal combiner receiving each of said amplifier outputs in parallel, said signal combiner providing no isolation between said amplifier outputs, 
     wherein 
     each signal received and amplified by each of said signal amplifiers is phase modulated; 
     said amplifier outputs are added by said combiner to produce a resulting signal; and 
     amplitude modulation of said resulting signal from said combiner is achieved by an addition of said amplifier outputs. 
    
    
     BRIEF DESCRIPTION OF THE INVENTION 
     A better understanding of the invention will be obtained by considering the detailed description below, with reference to the following drawings in which: 
     FIG. 1 is a block diagram of an amplifier system using a Chireix architecture according to one aspect of the invention; 
     FIG. 2 is a block diagram of a modified balanced magnetic transformer which may be used as a combiner in the system of FIG. 1; 
     FIG. 3 is a block diagram of a modified Wilkinson combiner which may be used as a combiner in the system of FIG. 1; 
     FIG. 4 is a block diagram of a Class-F power amplifier which may be used as an amplifier in the system of FIG. 1; and 
     FIG. 5 is a block diagram of a Class-D power amplifier which may also be used as an amplifier in the system of FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, a block diagram of a circuit  10  according to one aspect of the invention is illustrated. Signal amplifiers  20 A,  20 B feed a combiner  30  which produces an output  40 . The signal amplifiers  20 A,  20 B are low output impedance amplifiers while the combiner  30  is a combiner appropriate for Chireix architectures. 
     As in well known Chireix architectures, the signals received and amplified by the signal amplifiers  20   a ,  20 B are phase modulated signals. These phase modulated signals, after being amplified by the signal amplifiers  20 A,  20 B, are combined or added by the combiner  30  to result in the output signal  40 . By judiciously phase modulating the signals using appropriate phase modulators (not shown), the resulting signal  40  from the addition of the two signals is an amplitude modulated signal. Not only that, but the resulting signal is, in effect, an amplified version of a phase and an amplitude modulated signal. 
     It should be noted that similar architectures as that illustrated in FIG. 1 have been attempted in the past. However, the low efficiency of the circuitry of the previous attempts stem from their use of low efficiency amplifiers such as Class A and Class AB amplifiers which have significant output impedances. Such linear and saturating amplifiers did not respond correctly to the dynamic adjustment of the load impedance being presented to them due to their significant output impedance. 
     The dynamic nature of the load impedance is a result of the appropriate combiner for a Chireix architecture. Such an appropriate combiner not only reinserts the amplitude modulation to the resulting signal  40 , it also provides a dynamic adjustment of the load impedance presented to each one of the signal amplifiers. This outphasing adjustment of the load impedance is such that the DC current through each signal amplifier decreases as the combined output amplitude decreases thereby maintaining high efficiency. 
     Two appropriate combiners are illustrated in FIGS. 2 and 3. FIG. 2 illustrates a modified balanced magnetic transformer. As is known in the art, magnetic transformer combiners normally have a center tap-to-ground connection. In the modified transformer of FIG. 2, the tap-to-ground connection is not present. As can be seen, each of the two terminals  50 A,  50 B of an input winding  60  is to be coupled to a signal input or, in this case, a corresponding output of a corresponding signal amplifier. The output  70  is taken from one terminal of the output winding  80  while the other terminal  90  is coupled to ground  100 . 
     Another appropriate combiner is a modified Wilkinson combiner  110  as illustrated in FIG.  3 . As is known, a Wilkinson combiner has an isolating resistor which is normally coupled between the inputs. In the modified Wilkinson combiner of FIG. 3, neither of the inputs  120 A,  120 B are coupled to the isolating resistor  130 . This isolating resistor is effectively not present in the modified combiner of FIG.  3 . The resistor  130  is provided merely for illustration. Other than this modification relating to the non-coupled resistor  130 , the modified Wilkinson combiner  110  is similar in couplings and characteristics to other well known Wilkinson combiners. 
     Regarding a proper choice of amplifiers for use in the block diagram of FIG. 1, it has been found by the inventors that either Class D or Class F power amplifiers can correctly respond to the dynamic nature of the load impedance as mentioned above. These Class D or Class F power amplifiers provide the desirably very low output impedances that allows the Chireix architecture to amplify both phase and amplitude modulated signals. Such a Class F power amplifier is illustrated in FIG.  4 . This Class F amplifier  150  consists of a resistor load  160  coupled between ground  170  and a capacitor  180 . An inductor  190  also coupled in series with the capacitor  180  forms a resonator which passes the fundamental harmonic of the signal to resistor load  160 . Between the inductor  190  and ground are coupled, in series, a high pass filter  200  and another resistor  210 . The resistor  210  has a much higher resistance value than the load resistor  160 . A shorted, quarter wave transmission line  220  effectively shorts all even harmonic voltages while all odd harmonic voltages, including the fundamental, are passed on. A switching voltage input  230  is provided through a transistor  230 . 
     As is known, Class F amplifiers provide a good approximation to a voltage square-wave across the output terminals of a device by “shorting” all even-harmonic voltages and “supporting” all odd-harmonic voltages. As a result, the voltage waveform across the output terminals of a device contains only odd-harmonic components. In addition, this sorting of odd- and even-harmonics results in a current passing through the output terminals of the device that contains the fundamental, and only even-harmonic components. 
     This “shorting” and “supporting” of harmonics is conveniently achieved with a shorted quarter-wave shunt-stub connected across the device&#39;s output terminals, a shown in FIG.  4 . The stub will “short” the even harmonics and “support” the odd harmonics. Since each harmonic will contain only a voltage component or a current component, the device will not absorb power, except at the fundamental. 
     In a practical situation, with a stub effectively shorting only the second harmonic voltage, and passing only the fundamental and third harmonic voltage, the power-added efficiency can be more than 85%. 
     As an example of a Class D amplifier which may be used with the invention, FIG. 5 illustrates a block diagram of a Class D amplifier. As can be seen, the Class D amplifier is somewhat similar in structure to the Class F amplifier of FIG.  4 . Inductor  190 , capacitor  180  and resistor load  160  form a series circuit. The inductor  190  is coupled in series with one end of a transformer coil  240  while the other end of the transformer coil  240  is coupled to ground  170 . The other transformer coil  250  is tapped by a voltage source (Vcc) coupled to a grounded capacitor  260 . This other transformer coil  250  is coupled at each end to a switching voltage input  270 A,  270 B by way of transistors  280 A,  280 B. 
     For such Class D (voltage switching) amplifiers, the active device passes no current between its output terminals when the voltage across its output terminals is at the voltage rail, and passes maximum current between its output terminals when the voltage across its output terminals is zero. As a result, the device does not absorb any power, and all power taken from the bias supply is converted into the output signal (100% power-added efficiency). 
     It should be noted that the amplifiers in the amplifier pair  20 A,  20 B in FIG. 1 are to be of the same type. As such, if one amplifier  20 A is a Class D amplifier, the other amplifier  20 B should also be a Class D amplifier. Similarly, if one amplifier is a Class F amplifier, the other should also be a Class F amplifier. These two amplifier classes provide the requisite switch mode operation along with the very low output impedance. Other amplifier classes may have the switch mode operation but their output impedance is not of the desired very low value. 
     It should also be noted that while the circuit of FIG. 1 only illustrates two signal amplifiers in parallel, multiple signal amplifiers may be used in parallel with an appropriate combiner. Such a configuration may use multiple parallel signal amplifiers of the same type or class, with all parallel signal amplifiers feeding a single multiple input port combiner. However, as noted above, these multiple signal amplifiers are to have the requisite switch mode operation and the low output impedance. Ideally, such a multiply parallel arrangement would only use Class D or Class F power amplifiers. 
     A person understanding this invention may now conceive of alternative structures and embodiments or variations of the above all of which are intended to fall within the scope of the invention as defined in the claims that follow.