Abstract:
According to an exemplary embodiment, a circuit arrangement includes a multi-mode bias circuit having a control voltage input, a mode control input for selecting between a linear mode and a saturation mode, and a bias output. The circuit arrangement further includes an amplifier having a bias input connected to the bias output of the multi-mode bias circuit, the amplifier having an RF input and an RF output. The multi-mode bias circuit causes the amplifier RF output power to be proportional to the RF input power when the mode control input selects the linear mode. Conversely, the multi-mode bias circuit causes the amplifier RF output power to be determined by the voltage at the control voltage input when the mode control input selects the saturation mode.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention is generally in the field of communications devices. More specifically, the invention is in the field of power amplifiers for communications devices. 
   2. Related Art 
   Wireless communication devices, such as cellular handsets, generally utilize two classes of power amplifiers (“PA”): linear PAs, where output power is controlled by input power; and saturated PAs, where input power is constant and output power is controlled by a control voltage, such as an analog power control voltage (“voltage for analog power control” or “VAPC”). Linear PAs are used in, for example, cellular handsets using code-division multiple access (“CDMA”) wireless communication standard. Cellular handsets using global system for mobile communications (“GSM”) wireless communication standard that use the enhanced data through GSM evolution (“EDGE”) modulation format also require a linear PA. 
   To ensure proper linear operation, a linear PA requires an appropriate DC bias, which can be provided by using a conventional current mirror circuit including a current source, a reference transistor, and a voltage follower transistor. In a PA comprising a radio frequency (“RF”) output transistor, a base-emitter voltage can be provided at the base of the RF output transistor by coupling the base of the RF output transistor to the base of the reference transistor and utilizing the current source to inject a known current into the collector of the reference transistor. The voltage follower transistor, which is coupled to the bases of the reference and RF output transistors, provides the necessary base current to the reference transistor and the RF output transistor. As a result, a constant DC bias is generated by the conventional current mirror circuit to appropriately bias the linear PA. In the linear PA discussed above, the output power of the RF output transistor is proportional to the input power of an RF input signal coupled to the base of the RF output transistor. 
   Saturated PAs are used in, for example, cellular handsets using a GSM wireless communication standard that utilizes Gaussian minimum shift keying (“GMSK”) modulation. In a saturated PA, the phase of RF input signal is varied to transmit information while the power of the RF input signal is held constant. In a saturated PA comprising an RF output transistor, DC bias can be provided by a bias circuit comprising a control voltage, such as VAPC, coupled to the base of the RF output transistor via a resistor. In the saturated PA, the output power of the RF output transistor is a monotonic function of VAPC applied to the base of the RF output transistor. 
   As discussed above, the saturated PA and the linear PA each require a different bias circuit. However, it is desirable for a PA to be able to operate in both a saturated mode and a linear mode to support multi-mode wireless applications. 
   Thus, there is a need in the art for a low-cost, easy to implement bias circuit that can effectively support both linear and saturated operating modes of a power amplifier. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to multi-mode bias circuits for power amplifiers. The present invention addresses and resolves the need in the art for a low-cost, easy to implement bias circuit that can effectively support both linear and saturated operating modes of a power amplifier. 
   According to an exemplary embodiment, a circuit arrangement includes a multi-mode bias circuit having a control voltage input, a mode control input for selecting between a linear mode and a saturation mode, and a bias output. The circuit arrangement further includes an amplifier having a bias input connected to the bias output of the multi-mode bias circuit, the amplifier having an RF input and an RF output. The multi-mode bias circuit causes the amplifier RF output power to be proportional to the RF input power when the mode control input selects the linear mode. Conversely, the multi-mode bias circuit causes the amplifier RF output power to be determined by the voltage at the control voltage input when the mode control input selects the saturation mode. 
   In one embodiment, the mode control input is connected to a controlled current source utilized in the multi-mode bias circuit. The mode control input enables the controlled current source in the linear mode and disables the controlled current source in the saturation mode. In one embodiment, the mode control input is connected to a switch, the switch being coupled to the control voltage input. The mode control input closes the switch in the saturation mode, thus allowing the control voltage input to determine the bias output of the multi-mode bias circuit in the saturation mode; and the mode control input opens the switch in the linear mode, thus allowing the controlled current source to determine the bias output of the multi-mode bias circuit in the linear mode. 
   Other features and advantages of the present invention will become more readily apparent to those of ordinary skill in the art after reviewing the following detailed description and accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a circuit diagram of an exemplary conventional linear mode bias circuit coupled to a power amplifier. 
       FIG. 2  illustrates a circuit diagram of an exemplary conventional saturated mode bias circuit coupled to a power amplifier. 
       FIG. 3  illustrates a circuit diagram of an exemplary multi-mode bias circuit coupled to an exemplary power amplifier in accordance with one embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention is directed to multi-mode bias circuits for power amplifiers. The following description contains specific information pertaining to the implementation of the present invention. One skilled in the art will recognize that the present invention may be implemented in a manner different from that specifically discussed in the present application. Moreover, some of the specific details of the invention are not discussed in order not to obscure the invention. 
   The drawings in the present application and their accompanying detailed description are directed to merely exemplary embodiments of the invention. To maintain brevity, other embodiments of the present invention are not specifically described in the present application and are not specifically illustrated by the present drawings. The present invention applies to a multi-mode bias circuit for power amplifiers used in wireless communication devices, such as cellular handsets, using different wireless communication standards, such as CDMA, time-division multiple access (“TDMA”), and GSM. 
     FIG. 1  shows circuit diagram of an exemplary conventional linear mode bias circuit coupled to an exemplary PA. In circuit diagram  100 , linear mode bias circuit  102  is coupled to PA  104  via inductor  106 . Linear mode bias circuit  102  includes current source  108  and transistors  110  and  112  and PA  104  includes transistor  114 , output matching circuit  116 , and inductor  118 . 
   As shown in  FIG. 1 , a first terminal of current source  108  is coupled to supply voltage  120  and a second terminal of current source  108  is coupled to the gate of transistor  110  and the collector of transistor  112  at node  122 . Supply voltage  120  may be a supply voltage having a constant DC voltage with no AC component, such as VDD. In the present application, VDD generally refers to the voltage at the positive terminal of a DC power supply, typically a battery&#39;s positive terminal. Current source  108  provides a constant current into the collector of transistor  112 , which can be an NPN bipolar transistor. Transistor  110  can be, for example, an N-channel field effect transistor (“NFET”). Also shown in  FIG. 1 , the emitter of transistor  112  is coupled to reference voltage  124 , which can be a ground voltage (“ground”). 
   Further shown in  FIG. 1 , the base of transistor  112  is coupled to the source of transistor  110  and a first terminal of inductor  106  at node  126 . Inductor  106  can be, for example, an RF choke. The drain of transistor  110  is coupled to supply voltage  120 . Also shown in  FIG. 1 , a second terminal of inductor  106  is coupled to a first terminal of capacitor  130  and the base of transistor  114  at node  128 , which is also referred to as “PA input” in the present application. Transistor  114  is configured to operate as a power amplifier and can be, for example, an NPN transistor. The second terminal of capacitor  130  is coupled to PA input signal  132 , which can be an RF signal. 
   Also shown in  FIG. 1 , the collector of transistor  114  is coupled to a first terminal of inductor  118  and a first terminal of output matching circuit  116  at node  134 . Output matching circuit  116  provides impedance matching between the collector of transistor  114  and an output circuit (not shown in  FIG. 1 ) coupled to PA output  136 . A second terminal of inductor  118  is coupled to supply voltage  120 . Further shown in  FIG. 1 , the emitter of transistor  114  is coupled to reference voltage  124 . 
   The function and operation of linear mode bias circuit  102  will now be discussed. Current source  108  inputs a controlled current into the collector of transistor  112 , which causes transistor  112  to have a corresponding base-emitter voltage. Since the base of transistor  114  is coupled to the base of transistor  112 , the base-emitter voltage of transistor  112  is also applied to transistor  114 , which causes a collector current to flow in transistor  114  that is proportional to the collector current in transistor  112 . Base current is provided to transistors  112  and  114  from supply voltage  120  via transistor  110 , which is coupled to the base of transistor  112  and transistor  114  at node  126 . Thus, bias circuit  102  generates a constant DC bias at the base of transistor  114 , which enables PA  104  to operate in a linear mode. Thus, since DC bias is constant, i.e. has a fixed voltage, in linear mode, the power output of PA  104  at PA output  136  is proportional to the input power provided by PA input signal  132  at the PA input. 
     FIG. 2  shows a circuit diagram of an exemplary conventional saturated mode bias circuit coupled to an exemplary PA. In  FIG. 2 , PA  204 , inductor  206 , and capacitor  230  in circuit diagram  200  correspond, respectively, to PA  104 , inductor  106 , and capacitor  130  in circuit diagram  100  in FIG.  1 . In circuit diagram  200 , saturated mode bias circuit  240  includes control voltage  242  and resistor  244 . 
   As shown in  FIG. 2 , control voltage  242  is coupled to the PA input at node  228  by the series combination of resistor  244  and inductor  206 . Control voltage  242  can be an analog control voltage, such as VAPC. PA input signal  246 , which can be an RF signal, is also coupled to the PA input at node  228  via capacitor  230 . Also shown in  FIG. 2 , the base of transistor  214 , i.e. an output transistor, is coupled to the PA input at node  228 , the emitter of transistor  214  is coupled to reference voltage  224 , and the collector of transistor  214  is coupled to a first terminal of inductor  218  and a first terminal of impedance matching circuit  216  at node  234 . Further shown in  FIG. 2 , a second terminal of inductor  218  is coupled to supply voltage  220  and a second terminal of impedance matching circuit  216  is coupled to PA output  236 . 
   The function and operation of saturated mode bias circuit  240  will now be discussed. In a saturated mode, a DC bias can be applied to the base of transistor  214 , i.e. an output transistor, by control voltage  242 , which is coupled to the base of transistor  214  via resistor  244  and inductor  206 . Thus, the DC bias at the base of transistor  214  can be controlled by appropriately adjusting control voltage  242  such that PA  204  can operate in saturated mode, where the output power of PA  204  is proportional to control voltage  242 . 
     FIG. 3  shows a circuit diagram of an exemplary multi-mode mode bias circuit coupled to an exemplary PA in accordance with one embodiment of the present invention. Certain details and features have been left out of  FIG. 3 , which are apparent to a person of ordinary skill in the art. In circuit diagram  300 , multi-mode bias circuit  350  includes mode control signal  352 , controlled current source  354 , control voltage  356 , resistor  358 , switch  360 , and transistors  362  and  364 . PA  304  includes transistor  314 , impedance matching circuit  316 , and inductor  318  and is configured to operate in a linear mode and in a saturated mode. Although PA  304  is shown as having only a single PA stage, i.e. transistor  314 , for simplicity of illustration, PA  304  can have any number of PA stages. 
   As shown in  FIG. 3 , mode control signal  352  is coupled to a control terminal of controlled current source  354  through line  382 . Mode control signal  352  can be configured to enable controlled current source  354  in a linear mode and disable it, i.e. controlled current source  354 , in a saturated mode. Controlled current source  354  can be configured to output a constant current when enabled by controlled current source  354  in the linear mode. Also shown in  FIG. 3 , the input of controlled current source  354  is coupled to supply voltage  320 , which can be a reference voltage having a constant DC voltage with no AC current, such as VDD. The output of controlled current source  354  is coupled to the gate of transistor  362 , a first terminal of switch  360 , and the collector of transistor  364  at node  366 . In the present embodiment, transistor  362  can be an NFET and transistor  364  can be an NPN bipolar transistor. Switch  360  can be configured to be in an open position in a linear mode and in a closed, i.e. shorted, position in a saturated mode and can comprise, for example, a complimentary metal-oxide semiconductor (“CMOS”) pass gate or other appropriate switching device as known by a person of ordinary skill in the art. As shown in  FIG. 3 , mode control signal  352  is coupled to, and controls, switch  360  through line  384 . Mode control signal  352  can be configured to close switch  360  in a saturated mode and open switch  360  in a linear mode. 
   Further shown in  FIG. 3 , control voltage  356  is coupled to a first terminal of resistor  358  and a second terminal of resistor  358  is coupled to a second terminal of switch  360 . Also shown in  FIG. 3 , the drain of transistor  362  is coupled to supply voltage  320  and the source of transistor  362  is coupled to the base of transistor  364  and a first terminal of inductor  306  at node  368 . Node  368  provides the DC bias output for operation of PA  304 . Inductor  306  can isolate the DC bias outputted by multi-mode bias circuit  350  and an RF input signal at node  328  and can be, for example, an RF choke. Further shown in  FIG. 3 , the emitter of transistor  364  is coupled to reference voltage  324 , which can be, for example, a ground voltage (“ground”). 
   Also shown in  FIG. 3 , a second terminal of inductor  306  is coupled to a first terminal of capacitor  330  and the base of transistor  314  at node  328 , i.e. the PA input of PA  304 . Transistor  314  can be, for example, a NPN bipolar transistor or other appropriate transistor. Further shown in  FIG. 3 , a second terminal of capacitor  330  is coupled to PA input signal  370 , which can be, for example, an RF input signal. Also shown in  FIG. 3 , the emitter of transistor  314  is coupled to reference voltage  324  and the collector of transistor  314  is coupled to a first terminal of inductor  318  and a first terminal of output matching circuit  316  at node  334 . Inductor  318  can isolate an amplifier RF signal at node  334  and supply voltage  320 , which is coupled to a second terminal of inductor  318 , and also provide DC bias to the collector of transistor  314 . Output matching circuit  316  provides impedance matching between the collector of transistor  314  and a device, such as an antenna, coupled to PA output  336 , which is coupled to a second terminal of output matching circuit  316 . PA  304  can be configured to provide an output signal, such as an RF output signal, at PA output  336 . 
   The function and operation of multi-mode bias circuit  350  will now be discussed. In the linear mode, switch  360  is opened and controlled current source  354  is enabled by mode control signal  352 . Since transistors  362  and  364  are configured and operate in a similar manner as transistors  110  and  112  in  FIG. 1 , multi-mode bias circuit  350  operates in a similar manner as linear mode bias circuit  102  discussed above. Thus, similar to linear mode bias circuit  102 , multi-mode bias circuit  350  generates a constant DC bias at the base of transistor  314 . Thus, since DC bias is constant, i.e. has a fixed voltage, in linear mode, the output power of PA  304  at PA output  336  is proportional to the input power of the input signal, i.e. PA input signal  370 . Therefore, in linear mode, the gain of PA  304  is constant with input power. 
   In the saturated mode, switch  360  is in a closed position and controlled current source  354  is disabled by mode control signal  352 . As a result, control voltage  356  is applied to the gate of transistor  362  and the collector of transistor  364  via resistor  358  and switch  360 . As control voltage  356  increases, the collector current in transistor  364  increases, which causes a corresponding increase in the base voltage of transistor  364 . As a result the collector current in transistor  314  increases since the base of transistor  364  is coupled to the base of transistor  314 . Transistor  362 , which functions as a voltage follower, allows base current to flow from supply voltage  320  through transistor  362  to the base transistor  314  When an RF input signal with a constant amplitude is applied at PA input  370 , the amplitude of the RF output signal will increase as the collector current in transistor  314  increases. Thus, in the saturated mode, the output power of PA  304  at PA output  336  is proportional to the amount of DC bias provided at the base of transistor  314 , by control voltage  356 , since the input power of PA input signal  370  is held constant. Thus, the gain of PA  304  is proportional to control voltage  356  in the saturated mode. 
   In other embodiments, transistor  362  could be a bipolar transistor, such as a heterojunction bipolar transistor, instead of a FET as shown in FIG.  3 . 
   Thus, as discussed above, the present invention achieves a multi-mode bias circuit that advantageously supports saturated and linear operating modes of a PA. Also, the present invention provides a multi-mode bias circuit that can easily be switched between saturated and linear modes by controlling a switch and disabling/enabling a current source. Additionally, the present invention&#39;s multi-mode bias circuit results in minimal increase in size and cost compared to a conventional linear mode bias circuit, since the additional resistor and switch the present invention requires have a small size and can be integrated on a die without significantly increasing die size. 
   It is appreciated by the above detailed description that the invention provides a multi-mode bias circuit for power amplifiers that is effective, easy to implement, and cost-effective. From the above description of the invention it is manifest that various techniques can be used for implementing the concepts of the present invention without departing from its scope. Moreover, while the invention has been described with specific reference to certain embodiments, a person of ordinary skill in the art would appreciate that changes can be made in form and detail without departing from the spirit and the scope of the invention. Thus, the described embodiments are to be considered in all respects as illustrative and not restrictive. It should also be understood that the invention is not limited to the particular embodiments described herein but is capable of many rearrangements, modifications, and substitutions without departing from the scope of the invention. 
   Thus, multi-mode bias circuit for power amplifiers has been described.