Abstract:
Embodiments of the present invention comprise methods and devices for amplifying a signal by amplifying a first signal and by then amplifying a second signal only if the first signal exceeds a predetermined threshold. The first and second amplified signals are then combined, and the combination is fed back to a signal source and used to control the values of the first and second signal. The combination is further transmitted to a load. In the preferred embodiment, the first amplified signal is transmitted through an impedance inverter before it is combined with the second amplified signal.

Description:
The present application is a continuation of U.S. application Ser. No. 10/884,627 filed Jul. 2, 2004. Said application Ser. No. 10/884,627 is hereby incorporated by reference in its entirety. 

   FIELD OF THE INVENTION 
   The present invention relates generally to power amplifiers, and more specifically to Doherty power amplifiers. 
   BACKGROUND OF THE INVENTION 
   Referring to  FIG. 1 , a typical Doherty type amplifier known to the prior art includes a primary power amplifier  14  and an auxiliary power amplifier  18  whose input terminals  16  and  20  respectively are connected together at node  22 . The input terminal  20  of auxiliary amplifier  18  is connected to node  22  through a phase shifter  24 . Node  22  is the input terminal for an input signal  23  such as an RF signal. The output terminals  26 ,  28  of amplifiers  14  and  18  respectively are connected together at node  32  which is the output terminal for the amplifier pair. The output terminal  26  of primary amplifier  14  is connected to node  32  through an impedance inverter  30 . Output node  32  provides the amplified output signal to a load  34 . 
   In operation, the input signal  23  is amplified by primary amplifier  14  and passed through the impedance inverter  30  prior to being transmitted to load  34 . The auxiliary amplifier  18  is turned off at this point. As the voltage applied by the primary amplifier  14  increases, the auxiliary amplifier  18  turns on. Typically the auxiliary amplifier  18  is a class C amplifier. 
     FIG. 2  depicts a power graph for the Doherty amplifier shown in  FIG. 1 . As the voltage supplied by the primary amplifier  14  increases, the output voltage increases linearly until the primary amplifier  14  reaches its output limit,  36 . Eventually, the primary amplifier  14  reaches saturation, and its output voltage approaches its saturated limit. When a saturation point  37  is reached by the primary amplifier  14 , the auxiliary amplifier  18  is turned on and as the input voltage is increased the output voltage is also increased linearly. The transition period marked by the powering on of auxiliary amplifier  18  is typically non-linear. Thus, a need exists for a Doherty style amplifier capable of amplifying signals without the non-linearities introduced by the powering on of the auxiliary amplifier. 
   SUMMARY OF THE INVENTION 
   In satisfaction of this need, embodiments of the present invention comprise methods and devices for amplifying a signal by amplifying a first signal and by then amplifying a second signal only if the first signal exceeds a predetermined threshold. In one embodiment, the first and second amplified signals are then combined, and a portion of this combination is fed back to the signal source and used to control the phase and amplitude of the first and second signal. This process may be referred to as predistortion. This combination is then transmitted to a load. Additionally, in various embodiments, the first amplified signal is transmitted through an impedance inverter before it is combined with the second amplified signal. 
   In accordance with one aspect of the invention, an amplifier is provided comprising a primary amplifier, an auxiliary amplifier, and a signal source. The output of the primary amplifier and the auxiliary amplifier are connected in parallel. Furthermore, the signal source is in electrical communication with the input terminals of the primary amplifier and the auxiliary amplifier. The signal source preferably controls the input signals of the primary amplifier and the auxiliary amplifier in response to the output signals from the primary amplifier and the auxiliary amplifier. In some embodiments, an impedance inverter is between the output of the primary amplifier and the auxiliary amplifier. Also, in some embodiments, a phase shifter is preferably between the signal source and the auxiliary amplifier. In some embodiments, the phase shifter and the impedance inverter may- be a lumped impedance element or a quarter-wave impedance inverter. 
   In various embodiments, the input terminals of the primary amplifier and the auxiliary amplifier are both in electrical communication with a single signal source output terminal. Alternatively, in other embodiments, the input terminal of the primary amplifier and the input terminal of the auxiliary amplifier are in electrical communication with a first output terminal of the signal source and a second output terminal of the signal source, respectively. The auxiliary amplifier, in some embodiments, has a control input terminal in electrical communication with a control output terminal of the signal source. The signal source controls the auxiliary amplifier output using the voltage detected at either or both of the output terminals of the primary amplifier and the auxiliary amplifier. In the preferred embodiment, the primary amplifier, the auxiliary amplifier and the signal source are all in electrical communication with a load. Also, in various embodiments, the DC supply to the primary amplifier or auxiliary amplifier or both may be in electrical communication with a resistor, which in turn may be in electrical communication with a voltage source. The signal source may detect the voltage across this resistor. The voltage developed across this resistor may be used by the source to determine the amount of power being absorbed by the amplifiers. 
   In accordance with another aspect of the invention, a method is provided for amplifying a signal. The method comprises amplifying a first signal to produce a first amplified signal, and if this first amplified signal exceeds a threshold value, amplifying a second signal to produce a second amplified signal. The method further includes combining the first amplified signal with the second amplified signal to produce an amplified output signal, and controlling the values of the first signal and the second signal using the amplified output signal. In the case that the first amplified signal does not exceed the threshold value, the auxiliary amplifier remains off and thus there will be no second amplified signal; therefore, the amplified output signal will be simply the first amplified signal. 
   Embodiments of this method may also include transmitting the first amplified signal through an impedance inverter before the first amplified signal is combined with the second amplified signal. In various embodiments, the method includes transmitting the second signal through a phase shifter before it is amplified by the auxiliary amplifier. In other embodiments, both the impedance inverter and the phase shifter are a lumped impedance element or a quarter-wave transmission line. Embodiments of this method may further comprise controlling the value of the first signal using the amplified output signal. The method also includes transmitting the amplified output signal to a load. Furthermore, in various embodiments, the method includes controlling the voltage of the second signal amplification using the amplified output signal. Finally, in other embodiments, the first signal and the second signal are the same signal. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     These and other aspects of this invention will be readily apparent from the detailed description below and the appended drawings, which are meant to illustrate and not to limit the invention, and in which: 
       FIG. 1  depicts a Doherty type amplifier known to the prior art; 
       FIG. 2  depicts a power graph for the Doherty amplifier shown in  FIG. 1 ; 
       FIG. 3  depicts an embodiment of an amplifier circuit of the invention; 
       FIG. 4  depicts another embodiment of an amplifier circuit of the invention; and 
       FIG. 5  depicts yet another embodiment of an amplifier circuit of the invention. 
   

   In the drawings, like reference numbers generally refer to corresponding parts throughout the different views. 
   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 3 , in one embodiment of the invention, there are two amplifiers  14 ,  18  as in the Doherty type system known to the prior art. However in this embodiment both input terminals  16  and  20  respectively are connected directly to a signal source  50 . Again the output terminals  26 ,  28  of primary amplifier  14  and auxiliary amplifier  18  respectively are connected together at node  32 . The output terminal  26  of primary amplifier  14  is connected to node  32  through an impedance inverter  30 . The node  32  acts as an output terminal supplying the amplified signal to load  34 . The output signal is fed back through a feedback connection  64  to the signal source  50 . The DC voltage  54  for primary amplifier  14  is connected to primary amplifier  14  through a resistor  56  and the voltage drop across the resistor  56  is monitored by the signal source  50  using connection  60 . Similarly, in other embodiments, a voltage source  55  is connected to auxiliary amplifier  18  through a resistor  57  and the voltage across this resistor  57  is measured by signal source  50  using connection  61 . By measuring the voltage drop across resistors  56  and  57  in connection with primary amplifier  14  and auxiliary amplifier  18 , respectively, signal source  50  can determine the power being used by these amplifiers. This power consumption information may then be used by signal source  50  to optimize its output for efficiency. 
   In this embodiment signal source  50  comprises a digital RF source. One skilled in the art will readily recognize that signal source  50  may also comprise a Digital Signal Processor (DSP) or a variety of similar devices. A technique for predistortion is described in U.S. patent application Ser. No. 10/613,372 entitled “Adaptive Predistortion for a Transmit System.” Additionally, amplifiers  14  and  18  may comprise one or more of any of the standard classes of amplifiers. However, in one embodiment, primary amplifier  14  comprises a class F amplifier and auxiliary amplifier  18  comprises an inverse class F amplifier. Furthermore, in some embodiments the impedance inverter  30  may comprise a quarter-wave transmission line or lumped impedance elements. Such lumped impedance elements are described in U.S. patent application Ser. No. 10/610,497 entitled, “Integrated Circuit Incorporating Wire Bond Inductance,” the entire content of which is incorporated herein. 
   In operation, at low power levels, primary amplifier  14  amplifies a first signal from the signal source  50 , received at terminal  16 , and in turn transmits this amplified signal though the impedance inverter  30  to load  34 . At higher power levels, as primary amplifier  14  begins to saturate, auxiliary amplifier  18  turns on and amplifies a second signal from the signal source, received at terminal  20 , and transmits this amplified signal to load  34  via node  32 . In typical embodiments, auxiliary amplifier  18  is biased so that it does not begin to operate until primary amplifier  14  has reached its saturation point. As auxiliary amplifier  18  becomes more active driving more power into load  34 , its output current gradually reduces the effective load impedance as seen by primary amplifier  14 , thus allowing primary amplifier  14  to deliver even more power at the same output voltage at saturation. Thus, in effect, primary amplifier  14  is able to deliver a higher power output at its saturation point. 
   In this embodiment, the combined amplified signals from amplifiers  14  and  18  are transmitted to signal source  50  via feedback connection  64 . In one embodiment, signal source  50  receives feedback directly from output terminal  26 . Moreover, in a second embodiment, signal source  50  receives feedback directly from output terminal  28 , and in a third embodiment signal source  50  receives feedback directly from output terminal  31 . Signal source  50  may use the received feedback to modify the signals being transmitted to at least one of amplifiers  14  and  18 . In this way the predistortion is used to reduce non-linearities in the amplification. Finally, in a fourth embodiment, signal source  50  receives no feedback. 
   In one embodiment, the output voltage of primary amplifier  14  is determined by signal source  50 , by measuring the voltage across resistor  56 . Signal source  50  may then use this voltage information to adjust the signal being transmitted to either or both of amplifiers  14  and  18 . The signal source  50  may also use this voltage information to measure the power consumption of primary amplifier  14  to determine when primary amplifier  14  has reached saturation. In a second embodiment signal source  50  may determine the output voltage and power consumption of auxiliary amplifier  18  in a similar fashion by measuring the voltage across a resistor in electrical communication with auxiliary amplifier  18 . Signal source  50  may then optimize its output for efficiency by using the power consumption information from primary amplifier  14  and auxiliary amplifier  18 . 
   Referring to  FIG. 4 , in another embodiment of the amplifier, there are again two amplifiers  14 ,  18  and in this embodiment both input terminals  16  and  20  respectively are again connected directly to the signal source  50 . Again the output terminals  26 ,  28  of amplifiers  14  and  18  respectively are connected together at node  32  and the output terminal  26  of primary amplifier  14  is connected to node  32  through the impedance inverter  30 . The node  32  again acts as an output terminal supplying the amplified signal to load  34 . The output signal is fed back through a feedback connection  64  to signal source  50  as in the previous embodiment. Additionally, in this embodiment, auxiliary amplifier  18  has a control terminal  68  which is connected to the signal source  50  that allows the operating point of amplifier  18  to be optimized. In various embodiments not shown here, primary amplifier  14  may also have a control terminal connected to signal source  50  that allows the operating point of amplifier  14  to be optimized. 
   This embodiment operates in an almost identical fashion to the embodiment described above in  FIG. 3 . However, in this embodiment the signal source controls the voltage of auxiliary amplifier  18  directly via a connection to control terminal  68  of auxiliary amplifier  18 . In typical embodiments, the signal source  50  uses the signal feedback along feedback connection  64  to control the voltage and/or voltage bias to auxiliary amplifier  18 . Furthermore, in one embodiment, the signal source  50  controls the voltage and/or voltage bias to auxiliary amplifier  18  based on information received about the output voltage of auxiliary amplifier  18  via measuring the voltage of a resistor in electrical communication with auxiliary amplifier  18 . Similarly, in a second embodiment, the signal source  50  uses the signal feedback, received from feedback connection  64 , to control the voltage and/or voltage bias of primary amplifier  14 . In a third embodiment, this control of the voltage and/or voltage bias of primary amplifier  14  is based on information received about the output voltage of primary amplifier  14  as measured across resistor  56 . By controlling the bias of primary amplifier  14 , the non-linearity caused by the turning on of the auxiliary amplifier  18 , and illustrated as point  37  in  FIG. 2 , may preferably be minimized; the magnitude of this non-linearity depends on the bias of the primary amplifier  14  and tends to vary with temperature load impedance and supply voltage. 
   Referring to  FIG. 5 , in yet another embodiment of the amplifier, there are again two amplifiers  14 ,  18  and in this embodiment both input terminals  16  and  20  respectively are again connected to signal source  50  through a common node  22 . The input terminal of auxiliary amplifier  18  is connected to node  22  through a phase shifter  24 . Again the output terminals  26 ,  28  of amplifiers  14  and  18  respectively are connected together at node  32  and the output terminal  26  of primary amplifier  14  is connected to node  32  through the impedance inverter  30 . The node  32  again acts as an output terminal supplying the amplified signal to load  34 . The output signal is fed back through a feedback connection  64  to the signal source  50  as in the previous embodiments and again in this embodiment the signal source  50  controls the voltage to auxiliary amplifier  18  directly. Similarly, in some embodiments, the signal source  50  controls the voltage of primary amplifier  14  directly via a connection to a control terminal of primary amplifier  14 . 
   In operation, a signal is transmitted from signal source  50  to both primary amplifier  14  and phase shifter  24  via node  22 . The signal is further transmitted through phase shifter  24  to auxiliary amplifier  18 . Primary amplifier  14  amplifies the signal and in turn transmits the amplified signal though impedance inverter  30  to load  34 . As primary amplifier  14  begins to saturate, auxiliary amplifier  18  turns on and amplifies the phase shifted signal transmitted via terminal  20 , and then transmits the amplified signal to load  34  via node  32 . In typical embodiments, auxiliary amplifier  18  is biased so that it does not operate until primary amplifier  14  has reached its saturation point. As auxiliary amplifier  18  becomes more active driving more power into load  34 , its output current gradually reduces the effective load impedance as seen by primary amplifier  14 , thus allowing primary amplifier  14  to deliver even more power at the same output voltage at saturation. Thus, as in the previously described embodiments, primary amplifier  14  delivers a higher power output at its saturation point. 
   Also, as in the previously described embodiments, in this embodiment the combined amplified signals from amplifiers  14  and  18  are transmitted to signal source  50  via feedback connection  64 . In the preferred embodiment, signal source  50  uses this feedback to modify the signal being transmitted to primary amplifier  14  and phase shifter  24 , so that non-linearities in the amplification may be reduced. Additionally, in various embodiments not shown here, signal source  50  also receives feedback directly from at least one of terminals  26 ,  28 , and  31 . 
   Embodiments of the devices and methods described herein offer several advantages over the prior art. As the primary and auxiliary amplifiers are independently controlled, they can both be optimized to remove non-linearities associated with the operation of the auxiliary amplifier. Furthermore, there are several different means of removing non-linearities in the present invention. Examples include controlling one or both amplifiers based on the signal received from at least one of terminals  26 ,  28 , and  31 , via the feedback connection  64 , and controlling the voltage and/or voltage bias of either or both of the primary amplifier  14  and the auxiliary amplifier  18  based on their respective output voltages. These extra degrees of freedom allow for optimized efficiency in the linearization process. Additionally, in embodiments utilizing a class F amplifier as the primary amplifier and an inverse class F amplifier as the auxiliary amplifier, the efficiency of the invention is increased over that of the prior art, especially when amplifying broadband signals. 
   It should be appreciated by those skilled in the art, that various omissions, additions and modifications may be made to the methods and systems described above without departing from the spirit of the invention. All such modifications and changes are intended to fall within the scope of the invention as illustrated by the appended claims.