Abstract:
A system and method is provided to compensate an amplifier circuit for changes in a load impedance in order to maintain a substantially optimum performance for the amplifier. More specifically, if the load impedance increases, then the amplifier is reconfigured to produce an output impedance that is likewise increased. One way of reconfiguring the amplifier for a load impedance increase is to increase the supply voltage to the device. The increase in the supply voltage to the device increases the rail to rail operation of the device. This would allow more dynamic range for the system performance. Assuming the current is substantially constant, the impedance seen the output of the amplifier will increase and be multiplied up to the impedance desired by the load resulting in a more optimum power transfer. Other parameters, such as the input drive and bias voltage to the amplifier can be changed in order to improve the performance of the amplifier.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a divisional patent application of Ser. No. 09/552,284, filed on Apr. 19, 2000, entitled “SYSTEM AND METHOD FOR CLOSED LOOP VSWR CORRECTION AND TUNING IN RF POWER AMPLIFIERS,” which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention generally relates to radio frequency (RF)/microwave amplifiers, and in particular, a system and method for automatically improving the impedance match between a load and the output impedance of an output power stage. 
     BACKGROUND OF THE INVENTION 
     When an RF power amplifier is designed, the load variations are taken into account when determining the quiescent biasing condition, or “Q” point, for the device. Depending on how poor the match can be with a given load, the resultant “Q” point ends up being compromised. When the condition exists that the magnitude of the load is less than the optimal or characteristic impedance expected by the amplifier, power transfer is no longer optimum. 
     To achieve the same output power at the load, in the mismatched condition, much more power is dissipated across the output stage. This increase in the power dissipated causes excess heat to be generated in the amplifier which results in an elevated device junction temperature. The elevated junction temperature has an exponential relationship with reliability. In addition to reliability degradation, more power must be supplied to the amplifier than would be required for a properly matched condition. For portable applications which rely heavily on battery capacity, this can drastically increase the time required between recharges. 
     FIG. 1 illustrates a block diagram of a prior art amplifier output circuit  100 . The amplifier output circuit  100  consists of a field effect transistor (FET)  102  having a grounded source (S), and a high RF impedance bias network  104  which directs the power to the output stage. The inherent output impedance for power device  102  is usually much less than the impedance of the load  108 . Therefore, for optimum performance, the impedance seen to the left of aa′ is transformed to substantially match the impedance at the load  108 . Typically, an impedance matching network  106  is employed to multiply up the output impedance of the transistor  102 . 
     The actual impedance of the transistor  102  is calculated by dividing the voltage drop across the transistor by the current flowing from drain (D) to source (S) at the quiescent bias point with no RF input. This would equate basically to the supply voltage divided by the supplied current to the stage in question. This is true for class A operation. 
     If the load impedance were to change for some reason, a mismatched condition would exist. This would decrease the efficiency of the system since the criteria for optimum power transfer has been violated. The following discusses the effects of the mismatched conditions when the load impedance increases and decreases. 
     If the load impedance increases, then the transistor&#39;s output impedance seen at aa′ will be too low for a matched condition. The ac current flowing through the transistor  102 , about the “Q” point current will decrease. The ac voltage, seen at the output, will increase across the load constant. This can only occur until the device starts to approach rail to rail operation. At this point, distortion begins to evidence itself and even though the power may start to approach the desired output power level, the spectral density of this power may become very undesirable since the energy is no longer confined to the desired signal but to intermodulation products as well. 
     If the load impedance decreases, then the transistor&#39;s output impedance seen at aa′ will be too high for a matched condition. The ac current flowing through the transistor  102 , about the “Q” point current, will increase. The ac voltage, seen at the output, wants to decrease. This can continue only until the device starts to saturate in its ability to provide increased current. At this point, distortion begins to evidence itself and, even though the power may start to approach the desired output power, the spectral density of this power may become very undesirable since the energy is no longer confined to the desired signal but to intermodulation products as well. In addition, there will be an increased voltage drop across as well as current through the output transistor  102 . This will cause an increase in junction temperature which will lead to increased stress on the transistor  102  as well as performance degradation. 
     Accordingly, there is a need to mitigate some of the problems stated above. 
     SUMMARY OF THE INVENTION 
     One aspect of the invention is a method to compensate an amplifier circuit for changes in a load impedance in order improve the performance of the amplifier. More specifically, if the load impedance increases, then the amplifier is reconfigured to produce an output impedance seen at aa′ that likewise increased. One way of reconfiguring the amplifier for a load impedance increase is to increase the drain (FET) or collector (bipolar) voltage to the device. The increase in the drain (FET) or collector (bipolar) voltage to the device increases the rail to rail operation capability of the device. This would allow more dynamic range for the system performance. Assuming the drain (FET) or collector (bipolar) current is substantially constant, the impedance seen at aa′ will increase and be multiplied up, by the impedance matching network, to the impedance desired by the load resulting in more optimum power transfer. 
     Similarly, if the load impedance decreases, then the amplifier is reconfigured to produce an output impedance seen at aa′ that likewise decreases. One way of reconfiguring the amplifier for a load impedance decrease is to decrease the drain (FET) or collector (bipolar) voltage to the device. Assuming the drain (FET) or collector (bipolar) current is substantially constant, the impedance seen at aa′ will decrease and be multiplied up, by the impedance matching network, to an impedance closer to that desired by the load for a more optimum power transfer. 
     In addition to changing the supply voltage to the amplifier for tuning its output circuit with the load, the drive input and the gate voltage (FET) or base current (bipolar) to the amplifier can also be changed to improve the impedance match between the output of the amplifier and the load. Specifically, if the load impedance increases, then the input drive to the amplifier is increased, and if the load impedance decreases, the input drive to the amplifier is decreased. Also, if the load impedance increases, the gate voltage (FET) or base current (bipolar) to the amplifier is changed to decrease the conduction current through the device, and if the load impedance decreases, the gate voltage (FET) or base current (bipolar) to the amplifier is changed to increase the conducting current through the device. 
     Other aspects of the invention includes an amplifier comprising an output amplification stage, a directional coupler for generating signals indicative of forward and reverse powers between the output amplification stage and a load, and a controller for determining an impedance match between the output amplification stage and the load from the signals indicative of forward and reverse powers. The controller is capable of changing an input drive to the output amplification stage in response to a change in an impedance of the load to improve the impedance match between the output amplification stage and the load. The controller can also change the drain and/or gate voltages (FET), or collector voltage and/or base current (bipolar) to the amplifier along with the input drive, individually or in any combination, to improve the impedance match between the output amplification stage and the load. 
     An additional aspect of the invention includes a method of tuning an amplifier, comprising the steps of changing an input drive to the amplifier along with the drain and/or gate voltages (FET), or collector voltage and/or base current (bipolar) to the amplifier, individually or in any combination, to improve the performance of the amplifier. The tuning of the amplifier by changing the variables listed above may be in response to a change in the load impedance. In such a case, it may be desirable to determine the impedance match between the output of the amplifier and the load. 
     Still, yet another aspect of the invention includes an amplifier comprising a plurality of cascaded amplification stages including an output amplification stage, a directional coupler for generating signals indicative of forward and reverse powers between the output amplification stage and a load, and a controller for determining an impedance match between the output amplification stage and the load from signals indicative of forward and reverse powers, and for changing the operating conditions of respective amplification stages to improve the impedance match between the output amplification stage and the load. The operating conditions of the respective amplification stages changed include the drain (or collector) voltage, input drive and/or the gate voltage (or base current). A directional coupler may be included between each of the stages of an amplifier, or between some of the stages. 
     Other aspects of the invention will become apparent from the detailed discussion of the invention as provided below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a block diagram of a prior art amplifier output circuit; 
     FIG. 2 illustrates a block diagram of an exemplary amplifier with an exemplary circuit for controlling the impedance match between the output of the amplifier and a load, in accordance with the invention; 
     FIG. 3 illustrates a block diagram of an exemplary multiple stage amplifier in accordance with the invention; 
     FIG. 4 illustrates a block diagram of yet another exemplary multiple stage amplifier in accordance with the invention; and 
     FIG. 5 illustrates a block diagram of an exemplary amplifier that is useful for various modes of operations, including a dynamic control, factory-set, user-selectable, and pre-programmed modes of operations. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2 illustrates a block diagram of an exemplary amplifier  200  with an exemplary circuit for controlling the impedance match between the output of the amplifier and the load in accordance with the invention. The amplifier  200  comprises an output amplifier stage  202 , a directional coupler  204  at the output of the amplifier stage  202 , a controller  206 , a controlled bias source  208 , and a voltage variable amplifier (VVA)  210 . The directional coupler  204  provides the controller  206  the forward and reverse power at the output of the amplifier stage  202 . The controller  206  can determine the VSWR or quality of the impedance match between the output of the amplifier stage  202  and the load (not shown). The controller  206  monitors the impedance match between the output of the amplifier stage  202  and the load, and reconfigures the amplifier stage  202  to improve its power performance. The load can include passive elements, such as an antenna, an input to a subsequent cascaded amplification stage or an input to another electronic device. 
     The controller  206  is coupled to the controlled bias source  208  by way of line “a”. Through line “a”, the controller  206  controls the controlled bias source  208  so that a desired bias configuration is applied to the amplifier stage  202 . In the preferred embodiment, the bias configuration to the amplifier stage  202  may be adjusted by relatively small step increments or decrements. Adjusting the bias configuration in steps allows the controller  202  to re-measure the output impedance match between steps, until a more optimum operating condition for the amplifier stage  202  is located. The bias configuration may include drain and/or gate voltage to a field effect transistor (enhanced or depletion mode), or a collector voltage and/or base current to a bipolar transistor. The controller  206  may also use line “a” to monitor the current (e.g. drain or collector current) drawn by the amplifier stage  202 . If too much current is drawn in a class AB amplifier stage  202 , compensation may become necessary by lowering the current to the amplifier stage  202 . 
     The controlling of the bias of the amplifier need not be limited to bipolar and field effect transistor technology. But, can be applicable to other devices that are developed in the future. In essence, what is being done here is the impedance of the active device is being changed to compensate for changes in the amplifier operating environment, including variations in the load impedance, temperature, and input drive. This operation need not be limited to bipolar or field effect transistor technology, and can encompass other technologies including future developed technologies. Also other devices within the family of bipolar and field effect transistor technology are contemplated, including heterojunction bipolar transistors (HBTs) (bipolar technology) and pseudomorphic high electron mobility transistor (PHEMTs) (field effect transistor technology). 
     The controller  206  is coupled to the voltage variable amplifier (VVA)  210  by way of line “b”. Through line “b”, the controller  206  controls the gain or attenuation of the voltage variable amplifier (VVA)  210 . The voltage variable amplifier (VVA)  210  is used to change the drive level to the amplifier stage  202 . The voltage variable amplifier (VVA)  210  receives an RF input signal at a particular level. By having the controller  206  vary the gain or attenuation of the voltage variable amplifier (VVA)  210 , the power level of the RF signal at the input of the amplifier stage  202  can be varied. This can be used to change the “Q” point for the amplifier stage  202  to improve the impedance match between the output of the amplifier stage  202  and the load. This is particularly true for class AB type amplifiers. An attenuator, such as a pin diode attenuator, can also be used in place of the voltage variable amplifier (VVA)  210 . Additionally, the VVA can also be used to control the output power and overall gain of the amplifier. 
     The controller  206  is coupled to the amplifier stage  202  by way of line “c”. Through line “c”, the controller  206  receives a signal indicative of the temperature of the active device in the amplifier stage  202 . The controller  206  uses the device temperature information to reconfigure the amplifier stage  202  to either operate at a more benign operating condition or to shutdown when the device temperature exceeds a predetermined threshold. The benign operating condition or shutdown continues until the device temperature drops to an acceptable level. 
     In operation, if the load impedance increases, the controller  206  senses the mismatched condition by monitoring the forward and reverse powers received from the directional coupler  204 . The controller  206  then instructs the controlled bias source  208  to increase the drain or collector voltage to the amplifier stage  202  in incremental steps. Between adjacent steps, the controller  206  re-determines the impedance matching between the output of the amplifier stage  202  and the load. The controller  206  adjusts the controlled bias source  208  until the drain or collector voltage is reached that results in the desired impedance match between the output of the amplifier stage  202  and the load. The voltage changed can include the drain voltage if a type field effect transistor is used in the amplifier stage  202  or the collector voltage if a type bipolar transistor is used in the amplifier stage  202 . 
     If the load impedance decreases, the controller  206  senses the mismatched condition by monitoring the forward and reverse powers received from the directional coupler  204 . The controller  206  then instructs the controlled bias source  208  to decrease the drain or collector voltage to the amplifier stage  202  in decremental steps. Between adjacent steps, the controller  206  re-determines the impedance matching between the output of the amplifier stage  202  and the load. The controller  206  adjusts the controlled bias source  208  until the drain or collector voltage is reached that results in the desired impedance match between the output of the amplifier stage  202  and the load. Again, the bias voltage can include the drain voltage if a field effect transistor is used in the amplifier stage  202  or the collector voltage if a bipolar transistor is used in the amplifier stage  202 . 
     Alternatively, if the load impedance increases, the controller  206  senses the mismatched condition by monitoring the forward and reverse powers received from the directional coupler  204 . The controller  206  then increases the gain (or decreases the attenuation) of the voltage variable amplifier (VVA)  210  in incremental steps to raise the drive to the amplifier stage  202 . Between adjacent steps, the controller  206  re-determines the impedance matching between the output of the amplifier stage  202  and the load. The controller  206  continues to increase the gain (or decreases the attenuation) of the voltage variable amplifier (VVA)  210  until desired impedance match between the output of the amplifier stage  202  and the load is achieved. 
     If the load impedance decreases, the controller  206  senses the mismatched condition by monitoring the forward and reverse powers received from the directional coupler  204 . The controller  206  then decreases the gain (or increases the attenuation) of the voltage variable amplifier (VVA)  210  in decremental steps to lower the drive to the amplifier stage  202 . Between adjacent steps, the controller  206  re-determines the impedance matching between the output of the amplifier stage  202  and the load. The controller  206  continues to decrease the gain (or increase the attenuation) of the voltage variable amplifier (VVA)  210  until the desired impedance match between the output of the amplifier stage  202  and the load is achieved. 
     In an alternative embodiment, the amplifier stage  202  is configured so that the impedance match between the output of the amplifier and the load is substantially optimum when the input drive to the amplifier stage  202  is relatively low. Then, when the load impedance changes (i.e. either increasing or decreasing), the controller  206  causes the voltage variable amplifier (VVA)  210  to increase in gain (or decrease in attenuation) so that the input drive to the amplifier stage  202  is increased. This action improves the impedance match between the output of the amplifier stage  202  and the load when the load increases. 
     Also alternatively, if the load impedance increases, the controller  206  senses the mismatched condition by monitoring the forward and reverse powers received from the directional coupler  204 . The controller  206  then instructs the controlled bias source  208  to change the gate voltage of a field effect transistor for decreased channel conduction or decrease the base current of a bipolar transistor of the amplifier stage  202  in steps. Between adjacent steps, the controller  206  re-determines the impedance matching between the output of the amplifier stage  202  and the load. The controller  206  continues to instruct the controlled bias source  208  accordingly until the desired impedance match between the output of the amplifier stage  202  and the load is achieved. 
     If the load impedance decreases, the controller  206  senses the mismatched condition by monitoring the forward and reverse powers received from the directional coupler  204 . The controller  206  then instructs the controlled bias source  208  to change the gate voltage of a field effect transistor for increased channel conduction or increase the base current of a bipolar transistor of the amplifier state  202  in steps. Between adjacent steps, the controller  206  re-determines the impedance matching between the output of the amplifier stage  202  and the load. The controller  206  continues to instruct the controlled bias source  208  accordingly until the desired impedance match between the output of the amplifier stage  202  and the load is achieved. 
     The above three methods (i.e. changing the drain voltage, the drive level, and the gate voltage for field effect transistor amplifiers, or changing the collector voltage, the drive level, and the base current for bipolar amplifiers) of compensating for changes in the load impedance can be performed individually or in any combination. 
     FIG. 3 illustrates a block diagram of an exemplary multiple stage amplifier  300  in accordance with the invention. The amplifier  300  comprises two or more amplification stages. In the example shown, the amplifier  300  includes “n” amplification stages. For each amplification stage, the amplifier  300  includes a voltage variable amplifier (VVA) and a controlled bias source (CBS) coupled to the system power. The voltage variable amplifiers (VVA) can vary the drive to respective amplification stages, and the controlled bias sources (CBS) can vary the drain and/or gate voltages for respective field effect transistor amplification stages, or the collector voltage and/or base current for respective bipolar transistor amplification stages. The amplifier  300  further includes a directional coupler  302  at the output of the last stage (i.e. stage “n”). The directional coupler provides the controller  304  the forward and reverse power at the output of the amplifier stage “n”. The controller  304  can determine the VSWR or quality of the impedance match between the output of the output stage (i.e. stage “n”) and the load (not shown). 
     The controller  304  monitors the impedance match between the output of the output stage (i.e. stage “n”) and the load, and tunes each of the amplification stages (i.e. stages  1  through “n”) to achieve the desired impedance matching between output amplification stage (i.e. stage “n”) and the load. The controller  304  tunes each of the amplification stages by changing the respective drain voltages (field effect transistors) or collector voltages (bipolar transistors) with the use of the respective controlled bias sources (CBS′), and/or by changing respective drive inputs with the use of respective voltage variable amplifiers (VVAs), and/or by changing respective gate voltages (field effect transistors) or base currents (bipolar transistors), while monitoring the impedance match between the output stage (i.e. stage “n”) and the load. Thus, the controller  304  can tune all or some of the stages of the amplifier  300  to achieve the desired performance for the amplifier  300 . The amplification stages can be all be implemented with field effect transistors or bipolars, or a mixture of bipolars and field effect transistors. 
     FIG. 4 illustrates a block diagram of yet another exemplary multiple stage amplifier  400  in accordance with the invention. The multiple stage amplifier  400  is similar to multiple stage amplifier  300  previously discussed, except that a directional coupler (i.e. directional couplers  402 - 1 ,  402 - 2  and  403 - 3 ) is at the output of each of the amplification stages in order for a controller  404  to monitor the impedance match between amplification stages. The controller  404  tunes each of the amplification stages by changing the respective drain voltages (field effect transistors) or collector voltages (bipolar transistors) with the use of the respective controlled bias sources (CBS′), and/or by changing respective drive inputs with the use of respective voltage variable amplifiers (VVAs), and/or by changing respective gate voltages (field effect transistors) or base currents (bipolar transistors), while monitoring the impedance matches between stages and between the output stage and the load. Thus, the controller  404  can tune all or some of the stages of the amplifier  400  to achieve the desired performance for the amplifier  400 . The amplification stages can be all be implemented with field effect transistors or bipolars, or a mixture of bipolars and field effect transistors. Although in the exemplary multiple stage amplifier  400  includes is a directional coupler between each of the stages, it shall be understood that there need not be a directional coupler between every cascaded stages. 
     In the exemplary embodiments, the tuning of the amplification stages can be performed in a manner that the operating class of the active device can be changed. For example, if greater output linearity of an amplification stage is desired, its corresponding drain or collector voltage, input drive, and/or gate voltage or bias current may be changed to reconfigure the amplification stage from operating in a class “A” condition to a class “AB” condition, a class “B”, or a class “C”. More generally, the operating condition of the amplifier can be changed so that the amplifier is reconfigured from operating in a class to operating in another class. In this manner, the amplification stage or amplifier can be re-configured to operate in a more desired manner. 
     The above exemplary embodiments of the invention have been generally directed at dynamic controlling of the operating conditions of an amplifier to achieve a desired result in the midst of changes in various environment parameters, including load impedance, temperature, and input drive variations. There are other modes of operations, apart from the dynamic control operation, for the above amplifiers. These are factory-set operating condition amplifiers, user-selectable operating condition amplifiers, and pre-programmed operating condition amplifiers. 
     In factory-set operating condition amplifiers, a computer or other processor-based system is used in a similar manner as controller  206  to determine a desired operating condition for the amplifier given a known set of conditions. More specifically, in the factory, a computer or other processor-based system is used to set a desired operating condition by varying the bias and input drive to the amplifier in accordance with the invention as described above. Once the desired operating condition for the amplifier has been determined, data representing the desired operating condition can be rewritten into a non-volatile memory (e.g. a flash EEPROM) to be accessed by a controller upon powering the amplifier. The controller then uses the data to set the amplifier at the desired operating condition. 
     In a user selectable operating condition amplifier, the amplifier will be used in several distinct applications where the desired operating condition for the amplifier differs. In a factory setting, a computer or other processor based system is used to set the desired operating condition for each of the applications for the amplifier. Data representing the various operating conditions is written into a non-volatile memory to be accessed by a controller for varying the operating condition of the amplifier. A user can select a desired application for the amplifier by activating a switch or the like, and the controller responds accordingly by accessing the data relating to the corresponding operating condition from the non-volatile memory. The controller then uses the data to set the amplifier at the corresponding operating condition. 
     In a pre-programmed operating condition amplifier, the desired operating condition for an amplifier may depend on many environment variables, including temperature, load impedance, and input drive, to name a few. In a factory setting, a computer or other processor based system is used to determine the desired operating conditions for respective sets of environment parameters. These various operating conditions and corresponding environment variable sets can be written into a non-volatile memory in a look-up table fashion to be accessed by a controller to set the operating condition of the amplifier. During operation of the amplifier, a controller monitors the environment parameters of the amplifier, and then searches the look-up table in the memory to access the corresponding operating condition for the amplifier. The controller then sets the amplifier to the selected operating condition. 
     FIG. 5 illustrates an exemplary amplifier  500  that is useful for the various mode of operations, including the dynamic control, factory-set, user-selectable, and the pre-programmed amplifiers. Amplifier  500  is similar to amplifier  200  (FIG. 2) in that it comprises an amplification stage  502 , an output directional coupler  504 , a controller  506 , a controlled bias source  508 , and a voltage variable amplifier (VVA)  510 . The amplifier  500  further includes a non-volatile memory  512  (e.g. a flash EEPROM) for storing data relating to one or more operating condition settings for the amplifier  500 . The data may be accessed by the controller  506  for setting the amplification stage  502  to the selected operating condition. The amplifier  500  may also include a directional coupler  514  at its input to allow the controller  506  to determine the input matching. 
     Amplifier  500  lends itself to mass production since a plurality of this type of amplifiers can be configured into a bank, and the operating conditions for each of the amplifiers can be automatically set and written into the non-volatile memory  512  by a dedicated computer or processor-based system. Amplifier  500  also lends itself to servicing in the field with the use of a portable computer or other processor-based system that can update the operating condition(s) data stored in the non-volatile memory  512 . 
     In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto departing from the broader spirit and scope of the invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.