Abstract:
A power amplifier circuit comprising a transistor for receiving a signal to be amplified at an input and for outputting an amplified signal at an output; a modulated power supply connected to the transistor output; and a resistive element connected at the transistor output such that a low impedance is maintained at the transistor output across a range of operational frequencies.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 13/361,421, with a U.S. filing date of Jan. 30, 2012, which is a continuation U.S. patent application Ser. No. 12/985,938, with a U.S. filing date of Jan. 6, 2011, now U.S. Pat. No. 8,106,713, which is a continuation of U.S. application Ser. No. 10/597,944, with a U.S. filing date of May 8, 2007, now U.S. Pat. No. 7,868,698, is a 371 filing of PCT/IB2005/000338, filed on Feb. 9, 2005 which claims priority of British Application Number GB 0403021.9, filed on Feb. 11, 2004. 
     Furthermore, U.S. Ser. No. 13/361,421 and GB 0403021.9 are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to power amplifiers, and in particular a power amplifier provided with a modulatable power supply. 
     BACKGROUND TO THE INVENTION 
     A known technique for providing an efficient means of power amplifying a non-constant envelope signal is to use envelope modulation. Envelope modulation is typically provided by means of a highly efficient modulatable power supply. 
     However, in such an arrangement the power amplifier presents an impedance to the modulatable supply. In particular, this impedance can change rapidly with frequency due to a presence of a high Q series resonance, making accurate high bandwidth modulation very difficult. In a typical transistor output stage, the transistor device capacitance resonates with the inductance of the supply feed to form such a series resonance. This series resonance presents problems if the supply modulation is to be applied at high bandwidths. 
     One solution is to redesign the power output stage to move the resonance to a higher frequency. However it is difficult to achieve this without compromising the output stage with respect to some other parameter. For example reducing the capacitance can only be achieved by changing the output device technology from, for example, LDMOS to GaAs mesfet or Phemt. This could potentially have a major implication on the cost of such units, as well as the ability to manufacture such units. Reducing the inductance of the supply feed, as an alternative, would affect the load presented to the transistor at RF and its harmonics. This could compromise the RF stage efficiency, gain or bandwidth. 
     Thus a problem exists with current power amplifier output stages in that at high frequencies a series resonance is presented at the power supply terminal of the amplifier as a consequence of the output capacitance of the transistor forming a high Q resonant network with the supply feed inductance. This network creates a source of potential instability, and creates a long time constant energy storage means that contributes to memory effect, and renders any form of wideband envelope modulation difficult to apply. 
     It is an aim of the present invention to provide an improved technique which addresses one or more of the above stated problems. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention there is provided a power amplifier including a resistive element connected at an output thereof to maintain a low impedance at the output across a range of operational frequencies. 
     The power amplifier may further include a transistor for receiving a signal to be amplified at an input and for providing an amplified signal at the output. 
     The output may be adapted for connection to a modulated power supply. The output may be adapted for connection to a modulated power supply via a supply feed inductance. 
     Said resistive element may comprise a resistor. 
     A reactive element may be connected in series with said resistive element. The resonant element may comprise a capacitive element or an inductive element in series with a capacitive element. Said inductive element may comprise a conductive element of said circuit. The conductive element may comprise a part of a conductive track or a bond wire. Said inductive element may comprise an inductor. Said capacitive element may comprise a capacitor. 
     The signal to be amplified may be a radio frequency signal. 
     In a further aspect the invention provides a power amplifier circuit comprising a transistor for receiving a signal to be amplified at an input and for outputting an amplified signal at an output; a modulated power supply connected to the transistor output; and a resistive element connected at the transistor output such that a low impedance is maintained at the transistor output across a range of operational frequencies. 
     The invention still further provides a method of maintaining a low impedance across a range of operational frequencies in a power amplifier, the method including providing a resistive element at an output of the power amplifier. 
     The method may further comprise step of providing a reactive element connected in series with said resistive element. 
     A series resonance at the power supply is equivalent to a parallel resistance at the junction between the inductance and the capacitance. This junction is the drain of the transistor. The low impedance at the power supply terminal is equivalent to a high impedance at the transistor terminal. This provides a solution of presenting a low impedance to the transistor terminal to thereby damp out the series resonance appearing at the power supply terminal. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The invention is now described with reference to particular embodiments as illustrated by the accompanying figures, in which: 
         FIG. 1  illustrates a physical representation of a RF power stage, including example locations of a control circuit implemented in accordance with embodiments of the invention; 
         FIG. 2  illustrates the equivalent circuit of a packaged power transistor such as shown in  FIG. 1 ; 
         FIGS. 3( a ) to 3( c )  illustrate curves of impedance, phase, and amplitude respectively for various frequencies, exhibited by the resonance at the transistor terminal with and without a control circuit in accordance with embodiments of the invention; 
         FIGS. 4( a ) and 4( b )  illustrate the electrical equivalent circuit of a power amplifier such as shown in  FIG. 1 , including the implementation of a control circuit in accordance with embodiments of the invention; and 
         FIG. 5  illustrates the implementation of a particular embodiment of a control network in accordance with the invention in a power amplifier output network. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to  FIG. 1 , an example physical representation of a RF power stage with which embodiments of the invention is implemented is illustrated. 
     With reference to  FIG. 1 , reference numeral  140  generally represents a transistor package. Reference numeral  102  generally represents the top surface of the transistor package. The jagged line  142  represents the opening of the top surface of the transistor package to show at least part of the internal structure thereof. The internal structure of the transistor package includes an array of transistor power amplifiers  114  and a corresponding array or bank of matching capacitors  108 . The plurality of transistor power amplifiers  114  are connected to the corresponding plurality of capacitors  108  via a plurality of bond wires  110 . The plurality of capacitors  108  are connected, via a further plurality of bond wires  112 , to a metallisation  116  which represents the connection of the bond wires  112  external to the transistor package  140 . The transistor package additionally is provided with a metallisation  106  which is connected to ground, as represented by reference numeral  104 . A plurality of inputs are provided to the transistor package, forming the inputs for the plurality of transistor power amplifiers  114 . The plurality of inputs are generally represented by reference numerals  138 . 
     As the invention is concerned with the output of the transistor amplifiers, as further described hereinbelow, the input stages of the transistor package  140  are not shown in any detail in  FIG. 1 . 
     As further illustrated in  FIG. 1 , there is provided a metallisation pattern  132  on a printed wiring board or printed circuit board on which the transistor package  140  is mounted. The metallisation  116  connected to the bond wires  112  connect to the metallisation layer  132  on the printed wiring or circuit board. 
     The metallisation  132  is configured to have a bias feed strip  138  to which a DC supply voltage is supplied, a transistor drain strip  134 , and an output strip  136  at which the radio frequency output signal RF OUT  is provided. 
     The DC feed  138  provides a defined load for the RF transistor and the RF frequency and harmonics thereof. Ideally, this load would be a short at baseband frequencies. However in order to provide a satisfactory load at the radio frequency and harmonics thereof, the baseband impedance is required to be significant. 
     In order to provide an acceptable match to the low internal load impedance of the output of the transistor, an internal match network is typically provided internal to the transistor. This is best illustrated by an equivalent circuit of the power amplifier circuit, as represented in  FIG. 2 . 
       FIG. 2  shows a typical equivalent circuit of the output of a power amplifier circuit including the elements internal to the transistor package  102 . 
     As represented in  FIG. 2 , the equivalent circuit includes a transistor  210 . A capacitor  206  is connected in parallel across the source and drain of the transistor. The capacitor  206  represents the internal parasitic capacitance to the bulk of the transistor  210 . The output of the transistor  210 , namely the transistor drain, is taken on line  212  and connected to the DC power supply. It should be noted that the DC supply is connected to the output of the transistor via a supply feed inductance (not shown in  FIGS. 1 and 2 ). A series inductance, represented by inductor  202 , is connected to the drain of the transistor  210 . This inductor  202  is representative of the inductance of the bond wire  110  connecting the output of the transistor power amplifier to a capacitor of the capacitor bank  108 . A capacitor  208  is connected in parallel across the inductor  202  and the source of the transistor  210 . The capacitor  208  represents one of the capacitors of the capacitor bank  108 . A further inductor  204  is connected in series with the inductor  202 . The inductor  204  represents the inductance of the bond wire  112  connecting the capacitor  208  to the metallisation  116 . The source of the transistor  210  is connected to ground, represented by reference numeral  214 . In  FIG. 2 , the terminal  216  represents the connection to the metallisation  116 , and the terminal  218  represents a ground connection. 
     In transistor power amplifier implementations, the capacitor bank  108  may not always be provided. The capacitor bank  108  provides an impedance transformation capacitor for each transistor power amplifier. Where the capacitor bank  108  is not provided, use of the invention, as further described hereinbelow, is still advantageous, because there is still substantial capacitance associated with the transistor structure, as represented by capacitor  206  in  FIG. 2 . 
     As can be seen from  FIG. 2 , the equivalent circuit of the transistor package provides a resonant circuit. The resonant circuit has contributions to capacitance from the bulk capacitance  206  and the matching capacitor  208  from the capacitor bank  108 . The inductance includes inductors  202  and  204  representing the inductance of the bond wires  110  and  112 , as well as the supply feed inductance. In addition, the inductance is further contributed to by the transistor drain strip  132  and the bias feed  138 . This inductance all appears across the supply in series with the capacitors  206  and  208 . This parasitic resonant circuit results in parasitic resonance at high impedance, when a high impedance is presented at the transistor output. 
     The structure of the power amplifier circuitry provides a series resonant network at the power supply terminal, leading to the presentation of a very low impedance load to the power supply at high frequencies. This low impedance presented to the power supply transforms into a high impedance presented to the transistor drain. The resonance is undesired. In order to provide damping to this undesired resonance, a low resistance can be provided to the drain of the transistor for the same range of frequencies that the bias network presents a high impedance. However, this must be achieved without loading the output of the transistor at radio frequencies. If envelope modulation is to be applied, the network must also not provide significant loading at the modulation frequency. 
     The inventive solution is to provide a resistor in series with the DC feed to reduce the Q of the resonance. This is an effective solution, but a resistor set at a value that significantly reduces the resonance may consume an unexpectedly large portion of the power fed to the amplifier. 
     The invention thus provides a solution by providing a second resonant network, which may be considered to be a resonance compensation network or circuit or a control network or circuit, that connects to the output of the transistor. As the impedance of the transistor output terminal rises due to the bias resonance, the control resonant network impedance lowers. 
     The problem to which the invention is addressed, and the solution provided by the invention, is further discussed with reference to  FIGS. 3( a ) to 3( c ) , before further describing the implementation of the solution with regards to  FIGS. 4( a ) and 4( b )  and  FIG. 5 . 
       FIG. 3( a )  illustrates a plot of impedance against frequency for a transistor power amplifier with a modulatable supply. Reference numeral  302  designates the plot of impedance against frequency of such an amplifier output stage without any compensation in accordance with the invention. As can be seen, a spike  312  occurs at a particular range of frequencies, causing a significantly increased impedance at such frequencies. The impedance characteristics provided by the compensation circuit, the structure of which is further described hereinbelow, is represented by the curve  304 . As can be seen, the curve  304  represents a significantly lower impedance at the range of frequencies at which an impedance problem occurs. As a result, the overall effect of the control circuit is such that at the range of frequencies where the impedance ordinarily is significantly increased, the impedance is reduced. As illustrated in  FIG. 3( a ) , the effect of the control circuit between points  306  and  308  on the curve  302  is denoted by the dash line  310 , and it can be seen that the impedance is substantially reduced. 
     With reference to  FIG. 3( b ) , there is illustrated a plot of phase against frequency for the transistor output amplifier. As can be seen, the plot  317  has a significant negative peak denoted by reference numeral  316 . By provision of the control circuit of the present invention, this peak is removed and the curve has a much reduced distortion between points  318  and  320  as represented by dash line  322 . Similarly in  FIG. 3( c )  there is illustrated a plot of amplitude against frequency. As can be seen the output amplitude dips significantly to a negative peak at point  314 . The curve of amplitude against frequency  319  is controlled by the control circuit of the invention between points  324  and  326  such that the negative peak is significantly reduced, as represented by the dash line  328 . 
     The invention, and the example implementation thereof, is now further illustrated with reference to  FIGS. 4( a ) and 4( b ) . In  FIG. 4( a ) , a transistor power amplifier is represented by reference numeral  402 . A capacitor  406 , representing a matching capacitance, is connected in parallel across the source and drain of the transistor  402 . Resistor  404  represents a load which the power amplifier is driving. Further represented in  FIG. 4( a )  are four inductors, denoted  412 ,  410 ,  408 , and  407 . The inductor  407  represents the inductance associated with the supply strip  138 , the inductor  408  represents the inductance associated with the metallisation  132  between the connection point to the metallisation and the supply strip, the inductor  410  represents the inductance of the metallisation between the connection point to the metallisation and the drain strip  134 , and the inductor  412  represents the inductance of the drain strip  134 . The points A, B, C, D are marked in  FIG. 4( a )  and in  FIG. 1 , and the inductors  412 ,  410 ,  408 ,  407  represent the inductances between such points, which are provided by metallisation. 
     The invention, in embodiments, is to provide a resonant circuit at the output of the transistor power amplifier. Such a resonant circuit is illustrated in  FIG. 4( b ) . The resonant circuit  426  comprises, in series, an inductor  420 , a capacitor  422 , and a resistor  424 . The resonant circuit  426  is connected between terminals  428  and  430 , and terminal  430  is connected to ground as represented by reference numeral  432 . The provision of the resistor  424  allows the power problem as described hereinabove to be avoided. The resonant circuit provided by the inductor  420  and the capacitor  422  are not required in order to overcome the power problem, but are preferably provided so as to ensure that the resistor  424  does not consume power. 
     In various embodiments, the control circuit of the invention, as represented by the resonant circuit  426  of  FIG. 4( b ) , may be connected at any one of the points A, B, C, D shown in  FIGS. 1 and 4 ( a ). Specifically, the terminal  428  may be connected to any one of the points A, B, C, D. These points represent the possible locations of the control circuit, or being locations of the output of the transistor power amplifier. 
     There are different advantages associated with the provision of the control circuit at ones of the various points. As can be seen from  FIGS. 1 and 4 ( a ), the closer the control circuit  426  is connected to the DC supply, the higher the value of the inductor  420  is required. A particularly advantageous location of the control circuit  426  is to connect the terminal  428  at point D, denoted by reference numeral  130  in  FIG. 4( a ) . In such location, the series inductance provided by the inductors  407 ,  408 ,  410 ,  412  is such that the inductor  420  can be eliminated, and the control circuit  426  may comprise simply a capacitor  422  and a resistor  424 . In such location of the control circuit, the inductors  407 ,  408 ,  410 ,  412  provide the inductance of the resonant circuit. Thus, it will be understood that whether the resonant circuit is required to specifically have an inductor component is dependent upon the size of the inherent inductance provided in the circuit implementation. The inductance of the control circuit may simply be provided by the inherent inductance within the circuit, and such inherent inductance becomes part of the circuit. 
     It should be noted that in  FIG. 1  the points A, B, C, D are provided by way of example. As discussed hereinabove, point D has significant advantages insofar as that the inductance provided by the metallisation between the DC supply and point D is such that no specific inductor  420  may be required in a control circuit  426 . It will also be understood that the points A, B, C, D are not the only points at which the control circuit may be applied. The control circuit may be applied at any point at the output stage. The points A, B, C, D are represented by reference numerals  126 ,  124 ,  128 ,  130  in  FIG. 4( a ) . It will be understood that if any of these points move along the metallisation, either away from or toward the DC supply, then the inductance values of the inductors  407 ,  408 ,  410 ,  412  will vary. It should be noted that point B, denoted by reference numeral  124 , represents the point at which the bond wire  112  connects with the metallisation layer  132  on the printed wiring or circuit board. 
     A further embodiment of the invention is illustrated in  FIG. 5 , which more fully illustrates an implementation of the output stage of a transistor power amplifier. In  FIG. 5 , the transistor power amplifier is denoted by reference numeral  502 . Reference numeral  504  denotes a matching network connected to the output of the transistor through which the radio frequency output of the power amplifier is generated on line  512 . The matching network comprises a series pair of inductors  508  and  506 , and a parallel capacitor  510  connected to ground. The modulatable supply is denoted by reference numeral  514 , and generally comprises an amplifier  516  having a feedback connection to a summer  518 , which combines the output of the amplifier with an input signal. The modulatable DC supply is provided on line  520 . An inductor  522  is connected between the output of the transistor power amplifier  502  and the modulatable power supply on line  520  represents the supply feed inductance. 
     In accordance with the invention, a control circuit  530  is provided comprising a resonant circuit and a resistive load. The resonant circuit includes an inductor  532  and a capacitor  534 , and a load  536  is provided. The control circuit  530  is connected to the output of the transistor power amplifier  502  via an RF blocking circuit  524 , comprising an inductor  528  and a capacitor  526  connected in parallel. 
     The control circuit  530  is connected to ground as represented by  540 . 
     Reference is made herein to high frequencies. The range of values of such high frequencies is application dependent. However in general reference to high frequencies is reference to high modulation frequencies. In a preferred implementation of the invention, in radio frequency applications, the range of high frequencies is at the upper part of the modulation spectrum, but substantially less than the frequency of the radio frequency signal. 
     Reference is also made herein to low impedance. The range of values of such low impedance is application dependent. In general, the impedance should not be substantially higher at any particular modulation frequency from the average impedance over a range of frequencies. In a preferred implementation, a low impedance is considered to be in the range of 1 to 10 ohms. 
     Thus, the invention and embodiments thereof provide a control circuit for use at the output of a power amplifier stage which utilises a modulated supply. The control circuit includes a resistor, and preferably a resonant circuit comprising an inductor and a capacitor. The inductor of the resonant circuit may be provided by inherent inductance contained in the output stage of the power amplifier. The provision of such a control circuit overcomes a problem associated with the power amplifier topology. 
     Embodiments of the invention have been described where the provision of the control circuit is “off-chip”, and specifically connected to the metallisation of a printing wiring or circuit board. The invention is not limited to its implementation to such locations. For example, the control circuit may be implemented “on-chip”. This gives direct access to the equivalent circuit such as shown in  FIG. 2 , and therefore may provide better performance. In  FIG. 1 , reference numerals  118  and  120  denote potential locations of the control circuit on-chip. 
     The specific values of the elements of the control circuit are implementation dependent. One skilled in the art will be able to choose appropriate values of the capacitor, inductor and resistor of the control circuit in accordance with the application in which it is being used. 
     In essence, the invention provides for maintaining a low impedance at the drain (output) node of the transistor in the power amplifier circuit such that parasitic resonance in the power amplifier circuit is damped. 
     The invention has been described herein by way of reference to particular embodiments and examples. The invention is not limited to any of the specific embodiments described. Modifications and variations to the embodiment presented herein will be understood by one skilled in the art. The scope of protection afforded by the invention is defined by the appended claims.