Abstract:
Aspects of embodiments according to the present invention are directed toward a circuitry and a method to accurately measure the junction temperature of power amplifier and uses the measurement to enable optimization of performance in the presence of a mismatched load via control of the power amplifier such that corrective action to mitigate effects of the mismatched load can be performed.

Description:
BACKGROUND 
     1. Field 
     Aspects of embodiments according to the present invention are directed toward the field of power amplifiers, and in particular, performance optimization of power amplifiers in RF systems. 
     2. Description of Related Art 
     In an RF system, e.g., a radar system, wireless communication system, etc., power amplifiers are designed to efficiently convert DC power to RF power. To achieve this goal effectively, the constituent transistors are presented with favorable impedances that optimally perform this energy conversion. In addition, the network that presents this favorable impedance also provides an efficient (low loss) power transformation to the follow on circuitry. It is desirable to maintain this low loss transformation, however this results in the performance of the power amplifier being highly sensitive to the load (e.g., impedance) presented by the follow on circuitry. A mismatch between the power amplifier and the load presented to it may in turn enhance or deteriorate DC-to-RF power conversion. Poor power conversion at the output stage results in power dissipated within the transistor of the output stage and correspondingly an increase in the device&#39;s temperature (e.g., junction temperature for a bipolar transistor or a field effect transistor) and a corresponding reduction in long-term reliability. Therefore, an optimized balance between RF power generation and long term reliability is desired. 
     In the related art, a circulator/isolator is typically utilized to protect the power amplifier from mismatch conditions. The circulator/isolator is used to control the impedance seen by the output stage of the power amplifier. However, the bandwidth of the circulator/isolator may limit utilization of the available area for the aperture. For example, in an active electronically scanned antenna (AESA), the circulator/isolator provides the duplexing function between transmit and receive, as well as to isolate the power amplifier from the active impedance of the AESA. While the circulator/isolator can mitigate the mismatch problem between the power amplifier and the load, as the operational bandwidth increases and/or the low end frequency gets lower, the size and weight of the circulator/isolator becomes a limiter in terms of how many active elements can be provided given space constraints. Some AESA designs partition the active area into a transmit and a receive aperture because the bandwidth exceeds that possible with typical circulator/isolator technology. However, such an approach reduces the available area for use for both the transmit and receive functions and does not alter the large impedance variation presented to the power amplifier as the beam is steered. 
     SUMMARY 
     The accompanying drawings, together with the specification, illustrate exemplary embodiments of the present invention, and, together with the description, serve to explain the principles of the present invention. 
     Exemplary embodiments of the present invention relate to a mechanism to accurately measure the temperature of a power amplifier such that corrective action can be taken to optimize the two opposing requirements of the power amplifier, namely, RF power generation and long term reliability. 
     According to an embodiment of the present invention, a circuit includes an amplifier circuit for driving an RF load, a sensor for sensing a temperature condition of the amplifier circuit, and a controller for initiating corrective action and/or feedback of the temperature condition to an external controller in accordance with a load condition of the RF load. 
     In an embodiment, the load condition may include an impedance mismatch condition. 
     In an embodiment, the amplifier circuit may be configured to drive the RF load with or without a circulator or an isolator coupled between the amplifier circuit and the RF load. 
     In an embodiment, the sensor may be in thermal contact with the amplifier circuit such that the temperature sensed by the sensor correlates to that of at least one transistor of the amplifier circuit. 
     In an embodiment, the temperature sensed by the sensor may correlate to a junction temperature of the at least one transistor. 
     In an embodiment, the amplifier circuit may include a plurality of transistor cells, each of the at least one transistor being included in a corresponding one of the transistor cells, and the sensor may be included in a corresponding one of the transistor cells. 
     In an embodiment, the circuit may further include a sense circuitry for measuring a state of the sensor. The state may be a voltage or current output of the sensor. 
     In an embodiment, the sensor may include a diode coupled to the sense circuitry. 
     In an embodiment, the sensor may include a transistor coupled to the sense circuitry. 
     In an embodiment, the controller may be configured to adjust at least one operating condition of the amplifier circuit to compensate for a mismatch condition of the RF load. 
     In an embodiment, the at least one operating condition may include a bias condition of the amplifier circuit. 
     In an embodiment, the at least one operating condition may include a drain voltage of a transistor included in the amplifier circuit. 
     In an embodiment, the at least one operating condition may include a drive level provided to the amplifier. 
     In an embodiment, the controller may be configured to retune an output stage of the amplifier circuit. 
     According to another embodiment of the present invention, a method of operating an amplifier circuit includes operating the amplifier circuit to drive an RF load coupled to the amplifier circuit, sensing a temperature of the amplifier circuit, determining a mismatch condition between the RF load and the amplifier circuit in accordance with the temperature of the amplifier circuit, and compensating for the mismatch between the amplifier circuit and the RF load. This information is stored along with system parameters such as beam position for recall and application. 
     In an embodiment, the method may further include driving the RF load without a circulator or an isolator coupled between the amplifier circuit and the RF load. 
     In an embodiment, the compensating for the mismatch condition may include adjusting at least one operating condition of the amplifier circuit to compensate for the mismatch condition. 
     In an embodiment, the adjusting of the at least one operating condition may include adjusting a bias condition of the amplifier circuit. 
     In an embodiment, the adjusting of the at least one operating condition may include adjusting a drain voltage of a transistor included in the amplifier circuit. 
     In an embodiment, the adjusting of the at least one operating condition may include adjusting a drive level to the amplifier. 
     In an embodiment, the compensating for the mismatch condition may include retuning an output stage of the amplifier circuit in accordance with the temperature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit layout illustrating a representative placement of a thermal sensor in a power amplifier circuit layout. 
         FIG. 2  is a graph illustrating the correlation between the temperature sensed by the thermal sensor of  FIG. 1  and the junction temperature of a transistor in the power amplifier. 
         FIG. 3A  illustrates a conceptual block diagram in which a thermal sensor is implemented as a diode, according to an embodiment of the present invention. 
         FIG. 3B  illustrates a conceptual block diagram in which a thermal sensor is implemented as a transistor, according to an embodiment of the present invention. 
         FIG. 4  is a circuit diagram illustrating a sense circuitry and a current source of  FIG. 3A  in more detail according to an embodiment of the present invention when the thermal sensor  202  is configured as a diode. 
         FIG. 5  is a circuit diagram of a sense circuitry according to an embodiment of the present invention when the thermal sensor is configured as a transistor. 
         FIG. 6  is a circuit diagram of an exemplary tuning circuit in which circuit elements are tunable to compensate for a mismatch condition between the power amplifier and a load. 
         FIGS. 7 and 8  are diagrams illustrating the concept of mismatch mitigation. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, only certain exemplary embodiments of the present invention have been shown and described, simply by way of illustration. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature and not restrictive. In addition, when a circuit element is referred to as being “connected to” or “coupled to” another element, it can be directly connected to the element or be indirectly connected to the element with one or more intervening elements interposed therebetween. Hereinafter, like reference numerals refer to like elements. 
     Hereinafter, the embodiments of the present invention will be described in detail with reference to the accompanying drawings. To achieve an optimized balance between RF power generation and DC-to-RF power conversion, embodiments of the present invention disclose a mechanism to accurately measure the junction temperature of a transistor in a power amplifier to allow corrective action to be taken to optimize two opposing requirements (e.g., RF power generation and long term reliability). According to the embodiments of the present invention, an additional transistor cell is located to be in intimate thermal contact with the power amplifier in order to accurately measure the junction temperature of the transistor. 
       FIG. 1  is a drawing illustrating a representative placement of a thermal sensor  102  in a power amplifier layout  100 . However, the present invention is not limited to the placement of the thermal sensor  102  shown in  FIG. 1 . The thermal sensor  102  may be positioned in other suitable locations. 
     In  FIG. 1 , the thermal sensor  102 , which is in a transistor cell, is located in intimate thermal contact with the power amplifier FET  104 . In some embodiments, the thermal sensor  102  is an additional transistor cell adjacent to a transistor of the power amplifier FET  104 . The close proximity between the thermal sensor  102  and the power amplifier FET  104  provides a mechanism to accurately measure the junction temperature of the transistor in the power amplifier  104 . With the junction temperature accurately measured, it is possible to assess potential reliability degradation of the transistor in the power amplifier  104  based on the junction temperature. For example, in an AESA application, this information enables control of the array to limit or reduce reliability degradation while allowing operation, albeit at reduced performance. Exemplary control operations that can be performed include controlling bias (e.g., reducing drain voltage) and retuning the output stage of the power amplifier. In case of potential abnormal failures, the bias of the power amplifier can be turned off so that the fault can be corrected or avoided without amplifier failures. In addition, the mechanism to determine the potential reliability degradation of the power amplifier enables high power transmit operation without circulator/isolator such that significant packaging density improvements can be achieved, as well as increasing bandwidth utilization of radiating elements. 
     According to embodiments of the present invention, the above described mechanism to accurately measure the junction temperature of the power amplifier facilitates accurate understanding of the relationship between MMIC life data to an actual measured junction temperature, rather than an absolute thermal model referenced to a baseplate temperature measurement. However, the embodiments still rely on a relative thermal model between the thermal sensor and RF cells of the power amplifier. When the temperature measured by the thermal sensor is above a predetermined threshold value, the drain voltage of the transistor may be reduced to minimize or reduce junction temperature, thereby enhancing reliability. Accordingly, the circulator/isolator may be eliminated from the system. Therefore, it affords more flexibility in efficient packaging of AESAs as well as using the radiating aperture over a wider bandwidth than would otherwise be permitted where circulator/isolators are employed. In addition, the embodiments provide a mechanism whereby mismatch can be minimized or reduced on an element by element basis. Some embodiments may include control mechanism to optimize performance, wherein tunable elements are provided in the design. 
       FIG. 2  is a graph  300  illustrating the correlation between the temperature sensed by the thermal sensor  102  and the junction temperature of a transistor in the power amplifier  104 . 
     The graph  300  presents an analysis of the sensor temperature relative to the peak channel temperature of the power amplifier  104  under mismatched conditions. Due to the intimate thermal contact between the thermal sensor  102  and the transistor of the power amplifier  104 , the temperature of the thermal sensor  102  is strongly correlated to that of the transistor (e.g., an RF transistor) of the power amplifier  104 . As the current-voltage characteristics of a transistor are a function of temperature, the thermal sensor  102  can serve as an effective electrical probe of the RF transistor&#39;s condition and effectiveness in power conversion under different operating conditions. In  FIG. 2 , the curve  302  represents the junction (channel) temperature of the transistor in the power amplifier  104 , the curve  304  represents the temperature of a thermal spreader, and the curve  306  represents the temperature of the thermal sensor  102 . As shown in  FIG. 2 , the temperature of the thermal sensor  102  correlates to the temperature of the junction temperature of the transistor in the power amplifier  104  under different mismatched conditions. 
       FIG. 3A  illustrates a conceptual block diagram  400   a  in which a thermal sensor  102   a  is implemented as a diode, according to an embodiment of the present invention. 
     In  FIG. 3A , the thermal sensor  102   a  is a transistor configured in a diode configuration. The gate electrode of the transistor (thermal sensor  102   a ) is coupled to a current source  200  by a switch  202 . The switch  202  may provide noise isolation when the temperature sensing function is performed. However, the switch  202  may be removed in other embodiments of the present invention. That is, the current source  200  may be connected to the gate electrode of the transistor  202  without the switch  202 . A sense circuitry  204   a  is coupled to the gate electrode of the thermal sensor  102   a , and a control logic  206  receives an output signal (e.g., a voltage signal or a current signal) from the sense circuitry  204   a . By measuring the voltage at the gate of the thermal sensor  102   a  with the sense circuitry  204   a , the control logic  206  can determine the temperature of the thermal sensor  102   a  based on its temperature-voltage characteristics. Because the temperature of the thermal sensor  102   a  strongly correlates to the channel temperature of the power amplifier  104  (shown in  FIG. 1 ) due their intimate thermal contact, the control logic  206  can select the appropriate corrective action, if needed, to be performed on the power amplifier  104  based on the temperature of the thermal sensor  102   a , which correlates to the junction temperature of the power amplifier  104 . In some embodiments, the control logic  206  can provide feedback information based on the measured temperature to other external circuits (not shown) that can perform corrective actions, if needed, to the power amplifier  104 . 
       FIG. 4  is a circuit diagram illustrating the sense circuitry  204   a  and the current source of  FIG. 3A  in more detail according to an embodiment of the present invention when the thermal sensor  102   a  is configured as a diode. 
     In  FIG. 4 , the sense circuitry  204   a  includes a transistor T 1  and a transistor T 2 . The first transistor T 1  has a first electrode coupled to a voltage source V 1  via a resistor R 1 , and a second electrode coupled to a voltage source VSS via a resistor R 2 . The transistor T 2  has a first electrode coupled to the voltage source V 1  via a resistor R 3 , and a second electrode coupled to the voltage source VSS via a resistor R 4 . The second electrode of the transistor T 2  is also coupled to the control logic  206 . The first electrode of the transistor T 1  is coupled to a control electrode of the transistor T 2  via a resistor R 5 . While the sense circuitry  204   a  in  FIG. 4  is implemented with bi-polar transistors, the present invention is not limited thereto. For example, the sense circuitry  204   a  may be implemented with MOS type transistors or other suitable transistors. In  FIG. 4 , the current source  200  includes a transistor T 3  and a transistor T 4 . A first electrode of the transistor T 3  is coupled to a voltage source V 2 , and a second electrode of the transistor T 3  is coupled to a ground voltage source via a resistor R 6 . The voltage source VSS and the ground voltage source may be the same. The voltage sources V 1  and V 2  may have the same voltage. A control electrode of the transistor T 3  is coupled to the second electrode thereof and a control electrode of the transistor T 4 . A first electrode of the transistor T 4  is coupled to the voltage source V 2 , and a second electrode of the transistor T 4  is coupled to a gate electrode of the transistor T 5  constituting the thermal sensor  102   a . The gate electrode of the transistor T 5  is also coupled to a control electrode of the transistor T 1  via a resistor R 7 . 
       FIG. 3B  illustrates a conceptual circuit diagram  400   b  in which a thermal sensor  102   b  is implemented as a transistor. 
     In  FIG. 3B , the thermal sensor  102   b  is a transistor having a gate electrode coupled to a gate bias voltage source  201  and a drain electrode coupled to a current source  200  by a switch  202 . The switch  200  may provide noise isolation when the temperature sensing function is not performed. In some embodiments, the thermal sensor  102   b  may be connected to the current source  200  without the switch  202 . A sense circuitry  204   b  is coupled to the drain electrode of the thermal sensor  102   b , and the control logic  206  receives an output signal (e.g., a voltage signal or a current signal) from the sense circuitry  204   b . By measuring the voltage at the drain electrode of the thermal sensor  102   b  with the sense circuitry  204   b , the control logic  206  can determine the temperature of the thermal sensor  102   b  based on its temperature-voltage characteristics. Because the temperature of the thermal sensor  102   b  strongly correlates to the channel temperature of the power amplifier  104  (e.g., as shown in  FIG. 1 ) due their intimate thermal contact, the control logic  206  can select the appropriate corrective action, if needed, to be performed on the power amplifier  104  based on the temperature of the thermal sensor  102   b , which correlates to the junction temperature of the power amplifier  104 . In some embodiments, the control logic  206  can provide feedback information based on the measured temperature to other external circuits (not shown) that can perform corrective actions, if needed, to the power amplifier  104 . 
       FIG. 5  is a circuit diagram of the sense circuitry  204   b  in more detail according to an embodiment of the present invention when the thermal sensor is configured as a transistor. 
     In  FIG. 5 , the circuitries of the sense circuitry  204   b  and the current source  200  in  FIG. 5  are substantially the same as those of the sense circuitry  204   a  and the current source  200  in  FIG. 4 . Therefore, redundant description thereof will not be repeated. In  FIG. 5 , the thermal sensor  102   b  is configured as a transistor T 5  that has a first electrode coupled to the current source  200  and the sense circuitry  204   b , and a second electrode coupled to the ground voltage source. A capacitor C is coupled between the first electrode and the second electrode of the transistor T 5 . The gate electrode of the transistor T 5  is applied with a suitable bias voltage. 
       FIG. 6  is a circuit diagram of an exemplary tunable circuit  500  in which circuit elements are tunable to compensate for a mismatch condition between the power amplifier and a load. This circuit  500  is a simple reactive tuning network that can be used to improve the impedance presented to the power amplifier (coupled to port A) when the follow on circuitry (coupled to part B) results in a non-ideal impedance. The simple network makes use of variable capacitances C 1  and C 2  to realize the reconfigurable network. 
       FIGS. 7 and 8  are diagrams for illustrating the concept of mismatch mitigation. 
       FIG. 7  shows contours of RF power out and power added efficiency (PAE) for an exemplary power amplifier as a function of load presented to the power amplifier. As PAE decreases, the power amplifier junction temperature will increase which in turn will be sensed by the thermal sensor.  FIG. 8  illustrates the use of the temperature sensor in a situation wherein the load magnitude is constant but the load phase varies. This description is to be regarded as illustrative in nature and not restrictive because the concept is applicable for varying load magnitude and/or phase. The data points shown as circles represent data points obtained without the use of the information provided by the thermal sensor. The data points shown as triangles represent data points obtained by using the feedback from the thermal sensor to adjust the drain voltage. 
     According to the exemplary embodiments of the present invention, the temperature of the thermal sensor  102  can be used as a basis for adjusting the voltage (e.g., bias voltage) supplied to the power amplifier  104 , thus adjusting the power dissipation and thereby optimizing channel temperature and power generation. In some applications, such as active electronically scanned antennas (AESAs), this temperature may be sensed and recorded as a function of frequency and scan angle during calibration. The suitable voltages to control channel temperature can then be stored in a calibration file. During field operation, a periodic built-in test sequence can be implemented to support prognostication of performance and reliability over the life of the power amplifier and/or AESA. In some cases, only the amplifiers at the edges of the AESA may be adversely affected at a particular frequency or scan angle. In this case, the corrective action may be to turn those few amplifiers off and leave the voltage on for the majority of the amplifiers in the AESA. 
     In the related art, an isolator or circulator may be utilized after the power amplifier in RF systems as a way to control the impedance seen by the power amplifier. In a comparative example, in an AESA, the circulator provides the duplexing function between transmit and receive as well as to isolate the power amplifier from the active impedance of the AESA. Utilization of the circulator or isolator is usually the typical solution used in the related art. As the operational bandwidth increases and/or the low end frequency gets lower, the size and weight of the circulator/isolator becomes a limiter in terms of how many active elements can be provided given space constraints. If the bandwidth ratio is too large, the current state of the art circulator/isolator would not support the requirement, e.g., frequency ratios (f high /f low ) in excess of 4:1. However, according to embodiments of the present invention, the power amplifier is coupled to the radiating elements without a circulator/isolator while providing a mechanism to optimize power transfer and avoid degradation of the power amplifier. In some embodiments, the duplexer function can be implemented with a solid state switch. Since there is no isolation when a switch is utilized, the embodiments of the present invention described herein can be utilized to apply the appropriate corrective action to prevent or reduce reliability degradation to the power amplifier. 
     Further, because the embodiments of the present invention provide insight as to the mismatch between the power amplifier and the external load, they can provide data whereby the mismatch may be minimized or reduced on an element by element basis. This improvement may be a fixed implementation in the circuit design. For the case of designs incorporating tunable elements, the embodiments of the present invention may provide the mechanism to optimize the selection of the tuning elements. This information is stored along with system parameters such as beam position for recall and application. 
     While the present invention has been described in connection with certain exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, and equivalents thereof.