Abstract:
To improve operation during cutback power mode by reducing gain expansion, sections of a multi-section amplifier are selectively biased. During cutback power condition, fewer than all sections of the multi-section amplifier are biased. Selective biasing reduces power consumption and obtains desired output power. To reduce gain expansion, a bias resistor is provided between one or more sections to establish a small or leakage bias current into non-enabled or non-biased sections. This leakage bias current weakly biases the non-enabled sections allowing small signal amplification by the non-enabled sections. The combined amplification of the enabled section and the weakly biased section provide greater initial gain at lower power input signal levels thereby reducing gain expansion.

Description:
FIELD OF THE INVENTION 
   The invention relates to power amplifiers and in particular a method and apparatus for maintaining linearity in cutback power conditions. 
   RELATED ART 
   Numerous electronic devices utilize power amplifiers to increase the magnitude of an electrical signal for transmission of the signal via an antenna or other medium. Wireless devices, which are often battery powered, are designed to minimize power consumption. Likewise, wireless devices often operate under a particular standard or specification in unison with other wireless devices and one or more base stations. As such, it is important for the wireless device to operate within accurate tolerances and maintain accurate signal reproduction, particularly with regard to output power and linearity. 
   In general, power amplifiers in wireless devices typically operate at power levels significantly lower than their saturated output (max) power condition. This condition is commonly referred to as the cutback power condition. This may be done to reduce power consumption, maintain operation in a desired output power range, or both. At these power levels, the efficiency of the power amplifier is essentially determined by the quiescent current (bias current with no RF applied). It is therefore desirable to provide a mode of operation corresponding to reduced quiescent current for the cutback condition. 
   One such solution is to simply reduce the quiescent collector current which is accomplished by reducing the base bias in a bipolar transistor amplifier. While a reduced quiescent current mode of operation is a well known technique, there is a fundamental trade-off between reducing the quiescent current for a given transistor area and the linearity or gain flatness of the resulting amplifier. In general, for an amplifier with a design optimized for operation near its saturated output power, “leaning out” the bias will reduce the current density within the transistor which will result in a characteristic referred to as gain expansion where the output power of the amplifier increases at a rate greater than the increase in input power. Gain expansion is a type of non linear distortion that is generally undesirable because an amplifier with excessive gain expansion tends to have poor inter-modulation, Adjacent Channel Power Ratio (ACPR), and Error Vector Magnitude (EVM) performance. Inter-modulation, ACPR, and EVM are common figures of merit used to measure the linearity of an amplifier used in wireless communication. 
   One proposed possible solution is to reduce the area of the transistor by physically “switching out” a portion of the transistor cells that typically comprise a power amplifier output stage. This technique reduces the quiescent current and the transistor area at the same time so as to not reduce the current density of the transistor amplifier thereby not resulting in gain expansion. In order to completely “switch out” these cells, they must be isolated at DC and at the RF frequency of operation. It is however, extremely difficult to realize an effective RF switch that does not add undesirable parasitic losses or degrade the linearity of the amplifier. If only the DC path of these cells are isolated, the RF signal will still excite the “switched out” cells at some power level and result in the undesired gain expansion mentioned above. As a result, this proposed solution does not fully provide a workable circuit architecture to address the problem. 
   As a result, there is a need for a bias circuit that provides a means of disabling or significantly reducing the DC bias to a portion of the amplifier cells when the amplifier cells comprise an output stage for low power or cutback power modes of operation. Any such proposed solution should minimize the gain expansion effect that is typically associated with reduced quiescent current operation. The circuit needs to have minimal parasitic loss and should not degrade the linearity of the amplifier. 
   SUMMARY 
   Disclosed herein is a method and apparatus that overcomes the drawbacks of the prior art by providing an amplifier system which may operate in full or maximum power mode while still managing operation at cut back power. Gain expansion is reduced thereby maintaining amplifier performance. 
   To improve operation during cutback power mode by reducing gain expansion, sections of a multi-section amplifier are selectively biased. During cutback power condition, fewer than all sections of the multi-section amplifier are biased. Selective biasing reduces power consumption and obtains desired output power. To reduce gain expansion, a bias resistor is provided between one or more sections to establish a small or leakage bias current into non-enabled or non-biased sections. This leakage bias current weakly biases the non-enabled sections allowing small amplification by the non-enabled sections. The combined amplification of the enabled section and the weakly biased section provide greater initial gain at lower power input signal levels thereby reducing gain expansion. At higher power levels, the bias current of the non-enabled sections is limited by the bias resistor since increased current flow through the bias resistor will result in increased voltage drop across the bias resistor which in turn reduces the bias voltage applied to the non-enabled cells. 
   In one embodiment, a method for amplifying an input signal while minimizing gain expansion is disclosed wherein the amplifier comprises at least a first section and a second section with the outputs of the first and second sections connected to a common output. In this method during a high power condition the amplifier receives a bias signal from a bias node of the first section and the second section and receives an input signal to the first section and the second section. The amplifier then amplifies the input signal with the first section and the second section to create an amplified signal at the common output. 
   During a cutback power condition the amplifier receives a bias signal from a bias node at only the first section or the second section, such that a portion of the bias signal flows through a resistor, interconnecting the first section and the second section, to the section which is not otherwise provided a bias signal. The amplifier receives the input signal to the first section and the second section and then amplifies the input signal with either the first section or the second section to create an amplified signal at the common output. 
   In one embodiment, the bias signal is provided by a voltage source. It is also contemplated that the first section and the second section may comprise one or more transistors. During cutback power condition, the portion of the bias signal that flows through the resistor weakly biases the section that does not receive a bias signal from a bias node, which in turn reduces gain expansion. As discussed herein, cutback power condition comprises operation with reduced quiescent current. This method may also comprise receiving a control signal and responsive to the control signal, providing a bias signal from a bias node to only one of the first section or the second section. 
   Also disclosed herein is an amplifier having at least a first section and a second section comprising in combination an input configured to receive an input signal and a first bias node configured to selectively receive a first bias signal. Also part of the amplifier is a second bias node configured to selectively receive a second bias signal. The amplifier also includes a common output configured to combine the amplified signals of the first section and the second section. As discussed below in more detail, the amplifier further comprises a first amplifier section configured to receive the input signal, selectively receive the first bias signal, and provide an amplified input signal to the output. Likewise, the amplifier also includes a second amplifier section configured to receive the input signal, selectively receive the second bias signal, and provide an amplified input signal to the output. To reduce gain expansion, this embodiment also includes a bias resistor connecting the first amplifier stage and the second amplifier stage. 
   In one embodiment, during a first mode of operation, the first amplifier stage is connected to the first bias node and the second amplifier stage is not connected to the second bias node. During a second mode of operation, the first amplifier stage is not connected to the first bias node and the second amplifier stage is connected to the second bias node. During a third mode of operation, the first amplifier stage is connected to the first bias node and the second amplifier stage is connected to the second bias node. For example, during the first mode of operation and the second mode of operation current flows through the bias resistor to the amplifier stage that is not connected to a bias node. 
   The configuration described above may further comprise one or more DC blocking capacitors. Moreover, this configuration may further comprise additional amplifier sections and additional biasing resistors. A bias circuit comprised of current mirrors may be configured to selectively bias the first bias node and the second bias node. 
   Also disclosed herein is a method of reducing gain expansion when amplifying a signal during cutback power mode. In one embodiment the method comprises providing an input signal to an amplifier such that the amplifier comprises at least a first section and a second section, and then selectively providing a bias signal to a first bias node or a second bias node to thereby bias either the first section or the second section to create a biased section and a non-biased section. Thereafter, the method, from the circuit configuration, provides a leakage bias signal to the non-biased section from the biased section through a bias resistor network to thereby weakly bias the non-biased section. Then, this method amplifies the signal with the first section and the second section, either of which is weakly biased, to create an amplified signal. The resulting amplified signal is provided on an amplifier output. 
   In one embodiment, first section and the second section comprise one or more transistors. It is contemplated that the first section and the second section may have a different number of transistors or cells. In addition, the method may also include generating at least one control signal, such as with a processor or controller. The control signal may determine bias connections or selectively provides a bias signal to a first bias node, a second bias node or both. In one embodiment, the leakage bias signal flows through a resistor that is connected between a base node of the first section and a base node of the second section. In one configuration, the bias node of the unbiased section is floating. More than one bias node may be utilized. 
   Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. The features and elements described herein may be enabled in any combination or individually. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. In the figures, like reference numerals designate corresponding parts throughout the different views. 
       FIG. 1  illustrates a single prior art power amplifier output stage which suffers from gain expansion at reduced quiescent current conditions. 
       FIG. 2  illustrates an example plot of gain as related to power output in a “normal” or high quiescent current bias state. 
       FIG. 3  illustrates an example plot of gain verses power for the same amplifier as used to generate the plot of  FIG. 2 , but with reduced quiescent current in the output stage. 
       FIG. 4  illustrates an example circuit having divided sections within an amplifier output stage. 
       FIG. 5  illustrates an example plot of gain verses power characteristic for the amplifier with the output transistor stage shown in  FIG. 4  with both DC ports biased at the same level. 
       FIG. 6  illustrates a signal plot gain verses power characteristic for the amplifier configuration having the output transistor stage shown in  FIG. 4  but with only the ¼ cell DC port biased and the ¾ cell Vbias node floating. 
       FIG. 7  illustrates a signal plot generated by the circuit of  FIG. 4 , except the ¾ cell&#39;s base connection is tied to ground. 
       FIG. 8  illustrates an example output stage configuration having a bias resistor included to reduce gain expansion. 
       FIG. 9  illustrates a signal plot showing the gain verses power characteristic for the amplifier with the output transistor stage shown in  FIG. 8 . 
       FIG. 10  illustrates a signal plot of the gain verses power characteristic for the amplifier with the output transistor stage shown in  FIG. 8  with only the ¼ cell DC port biased. 
       FIG. 11  illustrates a signal plot as shown in  FIG. 10  except the bias point for the ¼ cell has been reduced. 
       FIG. 12  illustrates a block diagram of a generalized example embodiment of the amplifier as disclosed herein. 
   

   DETAILED DESCRIPTION 
   To overcome the drawback of the prior art and to provide additional benefits as disclosed herein, a power amplifier is disclosed wherein during cutback power condition a small amount of base current flows to sections of the output stage that are deselected to thereby weakly bias these transistor cells. As discussed below in more detail, this configuration and method of operation prevents or reduces gain expansion while not adversely affecting other aspects of operation. 
     FIG. 1  illustrates a single prior art power amplifier output stage  100  which suffers from gain expansion. In this example prior art amplifier stage, an RF input  104  is configured to receive an input signal to be amplified by the output stage  106 . The input  104  connect to a capacitor  120 , of which the opposing side connects to a bias node  108  shown as V bias . The bias node  108  connects to the base of transistor cells or sections  130 A- 130 D. In this embodiment the transistor cells  130  comprise NPN type devices. The emitters of cells  130  connect to ground  116  as shown while the collectors are tied to a common output  112 , labeled as RF out . 
   In general in prior art embodiments a power amplifier output stage may be partitioned into two or more sections, each section  130  containing an arbitrary number of transistor cells. For example, four sections  130  are shown in  FIG. 1 . The sections  130  may or may not contain equal numbers of transistor cells. The collectors of all cells in all sections  130  are connected together at a main collector node which serves as the RF output node  112  of the amplifier stage. The emitters of all cells in all sections  130  are likewise connected together and tied to a common ground node  116  thereby creating a common emitter amplifier stage. The bases of all cells within a section  130  are tied together at a common base node as shown. In this example, there are 4 sections of the array, each containing 25 individual cells. The input capacitor that couples the RF signal to the base of the output stage is also shown. 
   As a characteristic to this configuration of an output stage, gain expansion does not occur when quiescent current is optimized for operation near saturation, but gain expansion does occur during operation with reduced quiescent current.  FIG. 2  illustrates an example plot of power output as related to gain. As shown, the vertical axis  204  represents gain in dB while the horizontal axis  208  represents output power in dB milliwatts. A signal plot  212  shows the gain verses power characteristic for a three stage amplifier employing an output transistor stage shown in  FIG. 1  with quiescent current optimized for operation near saturated output power. In this example configuration, this comprises 191.2 mA. As can be seen from the plot  212  of  FIG. 2 , gain expansion is less than a few tenths of a dB. 
     FIG. 3  illustrates a signal plot of gain verses power for the same amplifier as used to generate the plot of  FIG. 2 , but with reduced quiescent current in the output stage. In this example plot  312  quiescent current was reduced by a factor of 4. In  FIG. 3 , the vertical axis  304  represents gain in dB while the horizontal axis  308  represents output power in dB milliwatts. Total gain expansion of 4.5 dB is evident and gain expansion in the back-off region is 3.2 dB (in this example the back-off region refers to power levels between 10 and 25 dBm). Quiescent current savings compared to high power mode is 144 mA, however, the gain expansion is undesirable. 
   To overcome these drawbacks in the prior art, the circuit configuration shown in  FIG. 4  may be adopted.  FIG. 4  illustrates an example circuit having divided sections within an amplifier output stage. As compared to  FIG. 1 , identical elements are labeled with identical reference numbers. Focusing on the differences in  FIG. 4  as compared to  FIG. 1 , the input node  104  branches to not only capacitor  120  but also capacitor  408 . The opposing end of capacitor  408  connects to a second bias node  404 . The base of sections  130 A connect to bias node  108  while the base of sections  130 B- 130 D connect to bias node  404 . 
   Bias node  108  and bias node  404  may be selectively energized such that both, or only one of the bias nodes is energized at a time. This allows the bias of ¼ of the transistor cells to be isolated or operated independently from the other ¾ of the transistor cells in the output array. The input capacitors  120 ,  408  have been partitioned and provided to provide the necessary DC isolation from the input. As an advantage to this embodiment the reduced gain expansion is evidence of the improvement. For numerous reasons this is an improvement including because the bias of the cells can be DC isolated. 
     FIG. 5  illustrates a signal plot  512  of gain verses power characteristic for the amplifier having the output transistor stage shown in  FIG. 4  with both DC ports biased at the same level. In this plot, the vertical axis  504  represent gain in dB while the horizontal axis  508  represents output power in dB milliwatts. Because bias nodes  108  and  404  are biased at the same level, the results are comparable to those shown in  FIG. 2 . 
   In contrast,  FIG. 6  illustrates a signal plot  612  of gain verses power characteristic for the amplifier configuration having the output transistor stage shown in  FIG. 4  but with only the ¼ cell DC port biased, namely bias node  108  is on while bias node  404  is off. Hence, in this example, the other ¾ cell&#39;s base connection is open circuited. In this plot, the vertical axis  504  represent gain in dB while the horizontal axis  508  represents output power in dB milliwatts. 
   Note there is less total gain expansion (3.2 verses 4.5 dB) and in the back-off region (2.2 vs 3.2 dB) as compared to the signal plot shown in  FIG. 3 . This level of gain expansion is an improvement over the configuration of  FIG. 1 , buy may be considered unacceptably high for certain specifications. Also note that the saturated output power is reduced, but this is acceptable since the reduced bias mode corresponds to lower output power operation. Quiescent current reduction compared to high power mode is 142 mA in this example configuration. 
     FIG. 7  illustrates a signal plot  712  which is also generated by the circuit of  FIG. 4 , except the ¾ cell&#39;s base connection is tied to ground. In this plot, the vertical axis  704  represent gain in dB while the horizontal axis  708  represents output power in dB milliwatts. Similar total gain expansion (3.4 dB) and back-off gain expansion (2.4 dB) is present as compare to  FIG. 6 . Quiescent current reduction compared to high power mode is also 142 mA. 
     FIG. 8  illustrates an example output stage configuration having a bias resistor  812  included to reduce gain expansion. As compared to  FIG. 4 , identical elements are labeled with identical reference numbers. It is contemplated that additional circuitry or elements, in addition to that shown, may be provided and included in  FIG. 8 . Focusing on the aspects of  FIG. 8  which differ from  FIGS. 1 and 4 , a bias node  808  connects to sections  130 B- 130 D and to DC blocking capacitor  804 . A resistor  812  connects bias nodes  108  and  808 . A number of series capacitors equal to the number of sections are used to couple an RF signal into the base nodes of each section. The resistor  812  is added between the base nodes of each section. 
   The resistor  812  allows a very small amount of base current to flow in the section of the output stage that is deselected, so that the transistor cells in that section are weakly biased on. The resistance  812  is of a sufficiently high value as to limit the base current that is allowed to feed into the deselected transistor cells which in turn limits the gain expansion of this portion of the amplifier. The remaining (selected) section of the amplifier operates in a normal fashion where the quiescent current is equal to the quiescent current used in a conventional amplifier scaled by the ratio of cells in that section to the total number of cells. 
   Stated another way, this resistor  812  allows the section that does not have bias applied in the cutback mode to draw a small amount of bias current through the resistor from the section that does have bias applied. This small amount of bias current helps to reduce variation in amplifier performance with respect to input impedance and small signal gain, but the resistor  812  prevents the non biased section from causing gain expansion as the RF signal increases. One or more switches (not shown) may be utilized to selectively enable one or more sections, such as for example by enabling voltage on the nodes  108  or  808 . A controller, processor, or other device may selectively control the switches or other means for selectively enabling the one or more sections. 
   When both sections of the output stage are selected, the amplifier operates as a typical device. The resistor  812  placed between the base nodes  108 ,  808  of the two sections has no effect on circuit operation since both base nodes are at the same potential and thus no current flows through the resistor. It is contemplated that in other embodiments the bias nodes  108 ,  808  may be established at different levels to establish any manner of output gain and gain compression curves that are desired. 
   The following signal plots shown in  FIGS. 9 ,  10 , and  11  illustrate the advantages of this configuration over other approaches. For these example plots, which are discussed below, the “back off” region is defined as power levels between 10 and 25 dBm which is roughly 10 dB lower than the saturated output power in the high power mode. 
     FIG. 9  illustrates an example plot  912  showing the gain verses power characteristic for the amplifier with the output transistor stage shown in  FIG. 8 . In this plot, the vertical axis  904  represent gain in dB while the horizontal axis  908  represents output power in dB milliwatts. In this plot, both bias nodes are established or biased at the same level. The results are comparable to those shown in  FIG. 2  because the effect of the resistor is negated due the identical potential on each resistor terminal. This indicates that the configuration shown in  FIG. 8  does not degrade the performance of the amplifier when operated at high power levels near the saturated output power. 
     FIG. 10  illustrates an example plot  1012  of the gain verses power characteristic for an amplifier having the output transistor stage shown in  FIG. 8  with only the ¼ cell DC port biased, i.e. on. In this plot, the vertical axis  1004  represents gain in dB while the horizontal axis  1008  represents output power in dB milliwatts. This example plot  1012  shows significantly less total gain expansion (1.5 dB) and back-off gain expansion (1 dB) as compared to  FIGS. 3 ,  6 , and  7 . However, quiescent current is 21 mA higher than the low bias modes of  FIGS. 6 and 7  due to the base current that is allowed to leak into the “unbiased” ¾ cell through the added resistor. Quiescent current reduction compared to high power mode is 120 mA, which although slightly less than the embodiments of  FIGS. 1 and 4 , is offset in benefit by the improvement in gain expansion. 
     FIG. 11  illustrates an example plot  1112  of a signal from the same circuit and conditions as shown in  FIG. 10  except the bias point for the ¼ cell has been reduced such that the reduction in current relative to the high power mode is 136 mA for a more direct comparison of quiescent current verses gain expansion. In  FIG. 11 , the vertical axis  1104  represent gain in dB while the horizontal axis  1108  represents output power in dB milliwatts. As can be seen in  FIG. 11 , the total gain expansion is now only 2.2 dB compared to 3.2 and 3.4 dB for  FIGS. 6 and 7  respectively. As a further benefit, back-off gain expansion is only 1.6 dB compared to 2.2 and 2.4 dB for  FIGS. 6 and 7  respectively. As shown in these figures, the configuration of  FIG. 8  provides normal operation during saturation or high power condition, while providing the benefit of reduced gain expansion at comparable low quiescent current conditions, such as during cutback power condition. 
     FIG. 12  illustrates a block diagram of a generalized example embodiment of the amplifier as disclosed herein. In this example embodiment an input  1204  is configured to receive an input signal to be amplified. The input  1204  connects to DC blocking capacitors  1208 A- 1208 N, where N may comprise any whole number. The opposing terminal of the capacitor  1208  connects to a bias resistor network  1212 A- 1212 N as shown and to a bias node  1220 ,  1222 ,  1224 ,  1226 . Each of the bias resistor networks  1212 A- 1212 N may be configured identically or differently. The bias resistor networks  1212 A- 1212 N may comprise any configuration capable of isolating voltage from the bias nodes  1220 ,  1222 ,  1224 ,  1226  to the other amplifier cells to achieve operation as described above. The bias resistor networks  1212 A- 1212 N may comprise any elements including but not limited resistors, capacitors, inductors, control logic, other semiconductor devices, or combinations of the above. The bias resistor networks  1212  may interconnect in any manner as would be understood in the art or to achieve the benefits as set forth herein. For example, network  1212 N may connect in some fashion to network  1212 A. 
   The bias nodes  1220 ,  1222 ,  1224 ,  1226  may be selectively enabled or turned on/off to selectively provide a bias signal to its respective node. Any means may be utilized to enable a bias signal on a node  1220 ,  1222 ,  1224 ,  1226  including but not limited to switches or controlled bias circuits. 
   An amplifier cell  1230  connects to bias node  1220  and bias resistor network  1212 A. An amplifier cell  1232  connects to bias node  1222  and bias resistor networks  1212 A,  1212 B. An amplifier cell  1234  connects to bias node  1224  and bias resistor networks  1212 B,  1212 N. An amplifier cell  1236  connects to bias node  1226  and bias resistor network  1212 N. As discussed above, the network  1212 N may also connect to network  1212 A. The output of each amplifier cell  1230 ,  1232 ,  1234 ,  1236  connects to a common output  1208 . 
   Operation occurs as described above such that the V bias  nodes  1220 ,  1222 ,  1224 ,  1226  may be selectively enabled. During operation, some of the current from the enabled (on) cells associated with the enabled bias nodes will leak into the non-enabled cells thereby reducing gain expansion. The signal provided to the input is amplified as desired by the cells  1230 - 1236  and provided on the output  1208 . 
   While various embodiments of the invention have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible that are within the scope of this invention. In addition, the various features, elements, and embodiments described herein may be claimed or combined in any combination or arrangement.