Abstract:
A method and apparatus for an amplifier, such as a radio frequency amplifier embodied as an integrated circuit is disclosed. Embodiments provide for a wide range of operating powers with good energy efficiency at many power levels. Resonant components act to provide consistent operating parameters over the wide range of power levels used. The invention may operate in the microwave region or at other RFs.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention generally relates to electronics circuits. The invention more particularly relates to amplifier circuits, for example, RF (radio frequency) PA (power amplifier) circuits especially integrated circuits for microwave signals that are used to provide gain of an input signal.  
       BACKGROUND OF THE INVENTION  
       [0002]     RF power amplifiers of conventional design typically suffer from a significant efficiency reduction when the output power level is adjusted to values below the peak design output power by varying the input drive to the power amplifier. This is true for all classes of amplifiers: A, B, C, E, and F. Maintaining efficiency with RF output power cutback is an important requirement for radios that are designed to save battery power as a result of reduced power operation. Generally speaking, the power efficiency of power amplifiers operated at small signal levels will be poor unless the amplifier incorporates features expressly to remedy that incipient deficiency.  
         [0003]     One known option for varying output power while maintaining efficiency is to adjust the voltage supply to the amplifier stage. However, this option is inconvenient since a high efficiency DC/DC (direct current) converter is required in the case of a battery powered RF amplifier.  
         [0004]     Another option is to use one or more variations on the well-known Doherty Amplifier which comprises a carrier amplifier and a peaking amplifier. This option is relatively expensive when embodied as an integrated circuit due to the types of circuit required.  
         [0005]     A further option is described in U.S. Pat. No. 5,276,912 issued to Siwiak et al., wherein a multi-mode transformation network is used to vary an impedance presented at an output transistor as a means to vary power level. Typically such implementations suffer from lossy components such as PIN diodes in the multi-mode transformation network. Also, the output impedance at the output transistor is not at the optimal value for the power level when operated in the lower power mode. Moreover, all impedance mismatches have negative implications for voltage levels arising out of reflections thus giving rise to consequent reliability and longevity issues especially where the amplifier is implemented as an integrated circuit such as an analog CMOS (complementary metal-oxide semiconductor) circuit.  
         [0006]     Thus there is a need for an improved amplifier design capable of multiple power levels and a superior tradeoff between power efficiency and cost.  
       SUMMARY  
       [0007]     Accordingly, the invention provides amplifiers with superior performance and power efficiency. Such an amplifier may be implemented as an IC (integrated circuit) with CMOS or other semiconductor technologies such as GaAs (Gallium Arsenide) or InP (Indium Phosphate) or other III-IV semiconductor devices. High operating frequency (e.g., microwave) may be supported through LSI (large scale integration), as is well-known in the art. Superior performance results from aspects of the novel designs.  
         [0008]     According to a first aspect of the invention, a method for amplifying is disclosed, the method comprising operating first and second amplifiers with respective matching networks and having outputs summed together. In one operating mode both amplifiers are biased to be active and in a lower power operating mode one of the amplifiers is at least partially cut off. Due to the circuit configuration and the arrangement exploiting resonances at the operating frequency efficiency is preserved even when power is reduced as in the lower power operating mode.  
         [0009]     According to another aspect of the invention, an embodied circuit is disclosed which may exploit the method of the first aspect.  
         [0010]     Several variants of these aspects are also disclosed together with alternative exemplary embodiments.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention, and, together with the description, serve to explain the principles of the invention:  
         [0012]      FIG. 1  shows a schematic diagram of a part of an integrated circuit according to an embodiment of the invention.  
         [0013]      FIG. 2  depicts a small signal equivalent circuit of an RF PA embodied in  FIG. 1 .  
         [0014]      FIG. 3  depicts an alternative small signal equivalent circuit of part of the circuit of  FIG. 2  operating in a first mode.  
         [0015]      FIG. 4A  depicts an alternative small signal equivalent of part of the circuit of  FIG. 2  operating in a second mode.  
         [0016]      FIGS. 4B and 4C  depict other alternative small signal equivalents of part of the circuit of  FIG. 2  operating in a second mode.  
         [0017]      FIGS. 5A and 5B  depict alternative embodiments of bias circuits suitable for use with embodiments of the invention.  
         [0018]      FIGS. 6A, 6B ,  6 C,  6 D show alternatives embodiments of matching network fragments. 
     
    
       [0019]     For convenience in description, identical components have been given the same reference numbers in the various drawings.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]     In the following description, for purposes of clarity and conciseness of the description, not all of the numerous components shown in the schematics and/or drawings are described. The numerous components are shown in the drawings to provide a person of ordinary skill in the art a thorough, enabling disclosure of the present invention. The operation of many of the components would be understood and apparent to one skilled in the art.  
         [0021]      FIG. 1  is a schematic diagram of a part of an integrated circuit  200  (IC) according to an embodiment of the invention. As shown, IC  200  implements an analog RF PA (power amplifier) circuit. As a power amplifier, IC  200  may produce relatively high power levels such as might typically be needed in connection with a transmitter driving a radiating antenna. IC  200  may be implemented or incorporated as part of a semiconductor chip using well-known technologies such as MOS (metal-oxide semiconductors). MOS technologies are commonly used to embody RF PAs for use with signals in the microwave region. NMOS transistors (n-channel metal-oxide semiconductor field-effect transistors) are shown in the circuit but their use is exemplary only and comparable circuits may be constructed using PMOS transistors (p-channel metal-oxide semiconductor field-effect transistors), BJTs (Bipolar junction transistors, typically Silicon based) or other active solid state devices within the scope of the invention.  
         [0022]     Major components of IC  200  may be two amplifiers  200 A,  200 B having outputs that are summed at a summing node  200 N. In this exemplary embodiment, two subsystem impedance matching networks  200 P,  200 Q may also be identified.  
         [0023]     In IC  200 , NMOS transistors  201 ,  202  are active devices that function to amplify an input signal, typically an RF or IF (intermediate frequency) signal, presented at an input signal terminal  250 . The circuit may be energized via a power supply rail  215  and a ground  216  in the usual manner.  
         [0024]     The input signal may be coupled via DC blocking capacitors  263  to control terminals of transistors  201 ,  202 . The DC bias of each transistor  201 ,  202  may be set independently by 0 Hz bias voltages introduced at Bias ports  261  and  262 . Each transistor  201 ,  202  may have its own bias voltage independent of the other transistor. Optional inductors  266 ,  267  may serve as chokes to prevent stray RF voltages entering through the bias ports  261 ,  262 .  
         [0025]     The two transistors  201 ,  202  operate essentially in parallel and their outputs at their respective current terminal circuit nodes  203 ,  204  may be coupled by capacitors  220  to output port  280 . Inductor  230  may function with capacitors  220  as a high-pass filter which may also operate to perform impedance transformation. An output load  290 , such as a 50-Ohm radiating antenna, may be coupled to output port  280 , but such load is typically not within IC  200 .  
         [0026]     Inductors  210  are circuit elements within IC  200  whose functions are described below in the discussion of equivalent circuits.  
         [0027]     The IC  200  of  FIG. 1  may be usefully operated in at least two modes: 
        In a first mode transistors  201  and  202  may be biased in an active or fully-on region.     In a second mode transistor  202  may be biased in an active or fully-on region and transistor  201  may be cut off, or vice versa.     In other possible intermediate modes, transistors  201  and/or  202  may be operated so as to be partially cut off by reducing their idle or quiescent current.        
 
         [0031]     In the first mode transistors  201  and  202  act in parallel and both amplify the input signal to produce an output signal at the output signal port  280 . In the second mode transistor  202  alone amplifies the input signal and circuit node  203  is effectively open circuit to RF signals and thus effectively disconnected from an active device (i.e. transistor  201 ). In the second mode the overall gain and the maximum output power of the RF PA is less than in the first mode. In the possible intermediate modes, the gain and the maximum output power will typically have values lying between those of the first and second modes.  
         [0032]     Indeed if the two transistors  201  and  202  are identical and the associated components are also identical then the two transistors will contribute equally to the amplified output when the bias voltages are equal thus placing the circuit in the first mode. Thus, the two arms of the circuit are essentially equal or symmetrical when operated in the first mode.  
         [0033]      FIG. 2  depicts a “Small Signal” equivalent circuit of the RF PA embodied as IC  200  in  FIG. 1  operating in the first mode, i.e. when both transistors are biased into the active or fully-on region. Referring to  FIG. 2  input port  250  is shown connected to equivalent input impedances Z 362  and Z 363 . Voltage-dependent current source I 302  may have a mutual transconductance g M  representing the amplifying properties of corresponding transistor  201  (of  FIG. 1 ).  
         [0034]     Similarly, voltage-dependent current source I 303  may have the same mutual transconductance g M  and represent the amplifying properties of transistor  202 . Inductances L 1  represent the properties of inductors  210  of  FIG. 1 , capacitances C 1  represent the properties of capacitors  220  of  FIG. 1  and inductance L 2  represents the property of inductor  230   FIG. 1 . Output port  280  and a resistive load R are also shown.  
         [0035]      FIG. 3  depicts an alternative small signal equivalent circuit of the RF PA embodied as IC  200  in  FIG. 1  operating in the first mode. That is to say  FIG. 2  and  FIG. 3  are equivalent in the condition that both transistors are active at the same operating point.  
         [0036]     Contrasting  FIG. 3  with  FIG. 2 , inductance L 2  has been divided into a pair of parallel inductances  2 L 2 , each having twice the inductance L 2  as conforms to the well-known properties of inductors connected in parallel. Similarly, load resistance R is envisioned as divided into obviously equivalent pair of parallel resistances  2 R 2  having twice the numeric ohmic value of load R of  FIG. 2  (and abbreviated to  2 R herein).  
         [0037]     Still referring to  FIG. 3 , by symmetry, the conductor  303  carries no current, thus it is valid to analyze each half of the circuit separately. In the normal operating condition, each pair of components C 1  and  2 L 2  may be substantially resonant at the nominal operating (carrier) frequency of the RF PA. This resonant structure provides a suitably lower load impedance (lower than  2 R that is, and of perhaps roughly three ohms in a typical embodiment) to the current source I 302 , I 303  as appropriate. Simultaneously the resonant structure develops a suitably high voltage across inductance  2 L 2  in order to drive the relatively high impedance load  2 R which may typically be around 100 Ohms.  
         [0038]     In this circuit-operating mode, the inductance L 1  acts to supply power while permitting small signal AC to be generated at the transistor current terminals. Provided L 1  is of sufficiently large inductance to avoid excessive loading of the voltage controlled current source I 303  or I 302 , then the actual value of L 1  is not critical in the first operating mode.  
         [0039]      FIG. 4A  depicts a small signal equivalent circuit of the RF P A of  FIG. 1  when operated in the second mode, i.e. when transistor  201  ( FIG. 1 ) is biased so as to be cut-off Since transistor  201  ( FIG. 1 ) is biased to be cut off the corresponding voltage-dependent current source is represented by open circuit I 301  i.e. a current “source” of zero amps.  
         [0040]     Comparing and contrasting  FIG. 3  with  FIG. 4A , it is apparent that the input impedance is substantially the same but the one active voltage-controlled current source I 303  will supply current to both load resistances  2 R. Significant current may flow in conductor  303 . Moreover, the equivalent circuit of  FIG. 4A  will produce significantly less gain than that of  FIG. 3 .  
         [0041]     The equivalent circuit of  FIG. 4A  may be redrawn as that of  FIG. 4B , showing the two load resistances combined as one resistance R (typically about 50 Ohms) and the Open Circuit omitted for clarity.  
         [0042]     Considering  FIG. 4B , the series circuit leg  401  consisting of C 1  in series with L 1  can be taken as approximately the same as C 1  alone if the circuit is designed such that 
 
ω o   L   1 &lt;&lt;1/ω o   C   1  . . . where ω o  is the center operating angular frequency. 
 
         [0043]     The circuit design constraint upon inductance L 1  is that of being able to supply a sufficient DC component so that the transistor associated with active voltage-controlled current source I 303  can operate with sufficient headroom. The primary design constraint on C 1  is that the combination C 1  and  2 L 2  are substantially resonant at the operating frequency. Therefore it is feasible to choose to implement RF PA  200  with a sufficiently large value for C 1  and a sufficiently small value for L 1  so that this approximation holds. It may be noted that in much of the prior art the value chosen for components fulfilling a similar role to that of L 1  are often too large for this approximation to apply to those circuits. C 1  must be embodied with a sufficiently small value that the resonant  2 L 2  has a sufficiently large value to enable it to be embodied reliably and economically. The upper and lower constraints on C 1  may both be implemented with available semiconductor technologies. Certain embodiments of the invention may be possible even when the approximation ω o L 1 &lt;&lt;1/ω o C 1  does not hold, provided always that ω o L&lt;1/ω o C 1 .  
         [0044]     Still referring to  FIG. 4B , thus, with a judicious choice of circuit values for L 1  and C 1  then the lower sub-circuit  402  may be approximated as merely consisting of C 1  in parallel with  2 L 2  which, it may be recalled, is by design a resonant combination at the nominal operating frequency. Hence the small signal equivalent circuit may be elegantly simplified to that of  FIG. 4C .  
         [0045]     Now we may compare and contrast the small signal equivalent circuits of  FIGS. 2, 3  and  4 C corresponding to first and second modes of RF PA operation. It may be seen that the operating condition of transistor  202  ( FIG. 1 ) is much the same for either mode. However, only one current source is active in mode  2  and it therefore drives less power into the load without any change in the load itself (still nominally 50 Ohms). Thus, even though the delivered power to the load is lower in the second mode, the power added efficiency, the linearity and/or other electrical performance parameters of the device  202  have not significantly changed or degraded (as compared with operation in the first mode). The term power added efficiency is well known in the RF arts.  
         [0046]     Thus, the circuit  200  of  FIG. 1  can be designed for an optimal operating point, including good power efficiency of transistors  201  and  202 . Importantly, this same good power efficiency is sustained regardless of whether the control voltage present on bias input  261  biases the RF PA  200  to higher-gain with high-power or lower-gain with low-power.  
         [0047]      FIGS. 5A and 5B  depict alternative embodiments of bias circuits suitable for use with embodiments of the invention, such as IC  200  of  FIG. 1 .  FIG. 5A  shows a simple voltage divider arrangement,  FIG. 5B  sows an approach using a current mirror arrangement such as might be more cheaply embodied in various types of MOS technology.  
         [0048]     Actual component values for optimal quantitative compensation may be determined by circuit simulation techniques which are well known in the art. Various suitable circuit simulation software packages are commonplace in the art; for example, HSPICE™ may be used.  
         [0049]     Embodiments of the invention as described herein have significant advantages over previously developed implementations.  
         [0050]     Further embodiments of the invention may be extended to include other circuit configurations, not limited to the resonant high pass “L” match structure of  FIG. 1 .  FIG. 6A  shows a relevant fragment from  FIG. 1 .  FIGS. 6B, 6C ,  6 D show alternatives embodiments including a high-pass “π” match (or alternatively two cascaded high pass L matches), a low-pass “L” match and a bandpass match respectively. As will be apparent to one of ordinary skill in the art, other similar matching circuits are possible with good utility and within the general scope of the invention. Such similar matching circuits may, for example, have higher numbers of stages or even harmonic frequency traps.  
         [0051]     And as will be apparent to one of ordinary skill in the art, still further similar circuit arrangements are possible within the general scope of the invention.  
         [0052]     For example p-channel devices and n-channel devices may be interchanged with appropriate source-drain and polarity transpositions as is well known in the art. Further examples may include circuits embodied using discrete transistors or as integrated circuits, using metal-oxide semiconductors or other field effect transistors, and/or with Gallium Arsenide transistors or other technologies.  
         [0053]     Bipolar junction transistors or even thermionic tubes and other unidentified but active devices could also be used to construct an embodiment of the invention using the appropriate circuit arrangements.  
         [0054]     Also it is possible to replace analog circuit components with digital functional equivalents within the general scope of the invention. The embodiments described above are exemplary rather than limiting and the bounds of the invention should be determined from the claims.  
         [0055]     Although preferred embodiments of the present invention have been described in detail hereinabove, it should be clearly understood that many variations and/or modifications of the basic inventive concepts herein taught which may appear to those skilled in the present art will still fall within the spirit and scope of the present invention, as defined in the appended claims.