Patent Publication Number: US-2017359033-A1

Title: Circuits and operating methods thereof for monitoring and protecting a device

Description:
BACKGROUND 
     Technical Field 
     The technology relates to circuits to safeguard a device, such as a gallium nitride (GaN) device, from operating conditions that can damage or destroy the device. 
     Discussion of the Related Art 
     GaN semiconductor material has received appreciable attention in recent years because of its desirable electronic and electro-optical properties. GaN has a wide, direct bandgap of about 3.4 eV. Because of its wide bandgap, GaN is more resistant to avalanche breakdown and has a higher intrinsic field strength compared to more common semiconductor materials, such as silicon and gallium arsenide. In addition, GaN is able to maintain its electrical performance at higher temperatures as compared to other semiconductors, such as silicon or gallium arsenide. GaN also has a higher carrier saturation velocity compared to silicon. Additionally, GaN has a Wurtzite crystal structure, is a hard material, has a high thermal conductivity, and has a much higher melting point than other conventional semiconductors such as silicon, germanium, and gallium arsenide. Accordingly, GaN is useful for high-speed, high-voltage, and high-power applications. For example, GaN materials may be used as active circuit components in semiconductor amplifiers for radio-frequency (RF) communications, radar, and microwave applications. 
     SUMMARY 
     According to one aspect, a system for providing an output signal to a load is provided. The system includes a first transistor having a drain terminal constructed to provide the output signal to the load and a gate terminal constructed to receive an input signal, a second transistor coupled in series with the first transistor and having a gate terminal, a current sensing circuit coupled to the first transistor and constructed to measure a magnitude of a current in the first transistor, a feedback circuit coupled to the current sensing circuit and constructed to generate a feedback signal indicative of whether the magnitude of the current in the first transistor is above a threshold, and a driver circuit coupled to the feedback circuit and the gate terminal of each of the first and second transistors. The driver circuit may be constructed to apply a voltage to the gate terminal of the first transistor and reduce the magnitude of the current in the first transistor by adjusting a gate voltage of the second transistor responsive to the feedback signal indicating that the magnitude of the current in the first transistor is above the threshold. 
     In one embodiment, the threshold is a configurable threshold. In one embodiment, the first transistor is a gallium nitride (GaN) transistor and the second transistor is a metal-oxide-semiconductor field-effect transistor (MOSFET). In one embodiment, the driver circuit is a GaN sequencer and is further constructed to apply the bias voltage to the gate terminal of the GaN transistor before turning on the MOSFET. 
     In one embodiment, the current sense circuit includes a current sense resistance coupled in series with the first transistor, a first level-shifter coupled to a first terminal of the current sense resistance, and a second level-shifter coupled to a second terminal of the current sense resistance. In one embodiment, the feedback circuit includes a difference detector constructed to compare a voltage signal indicative the magnitude of the current in the first transistor with a reference voltage that defines the threshold and generate the feedback signal based on the comparison. In one embodiment, the feedback circuit further includes a programmable voltage source constructed to generate the reference voltage and provide the reference voltage to the difference detector. 
     In one embodiment, the second transistor has a drain terminal coupled to a source terminal of the first transistor. In one embodiment, the system further includes a third transistor having a gate terminal coupled to the driver circuit and a drain terminal coupled to the drain terminal of the second transistor. In one embodiment, the driver circuit is constructed to reduce the magnitude of the current in the first transistor by adjusting a gate voltage of each of the second and third transistors. 
     According to at least one aspect, a circuit for protecting a gallium nitride (GaN) transistor is provided. The circuit includes a current sensing circuit to measure a magnitude of a current in the GaN transistor, a feedback circuit coupled to the current sensing circuit and constructed to generate a feedback signal indicative of whether the magnitude of the current in the GaN transistor is above a threshold, and a driver circuit constructed to couple to a gate terminal of the GaN transistor and a gate terminal of a transistor coupled in series with the GaN transistor. The driver circuit may be further constructed to receive the feedback signal, apply a bias voltage to the GaN transistor, and reduce the magnitude of the current in the GaN transistor by adjusting a gate voltage of the transistor coupled in series with the GaN transistor responsive to the feedback signal indicating that the magnitude of the current is above the threshold. 
     In one embodiment, the driver circuit is a GaN sequencer and is further constructed to apply the bias voltage to the gate terminal of the GaN transistor before turning on the transistor coupled in series with the GaN transistor. In one embodiment, the current sense circuit includes a first programmable level-shifter constructed to couple to a first terminal of a current sense resistance and a second programmable level-shifter constructed to couple to a second terminal of the current sense resistance. 
     In one embodiment, the feedback circuit includes a difference detector constructed to compare a voltage indicative of the magnitude of the current in the first transistor with a reference voltage that defines the threshold and generate the feedback signal based on the comparison. In one embodiment, the feedback circuit further includes a programmable voltage source constructed to generate the reference voltage and provide the reference voltage to the difference detector. 
     According to at least one aspect, a method for protecting an amplifier that is providing an output signal to a load is provided. The method includes monitoring a magnitude of a current in a first transistor of the amplifier, determining whether the magnitude of the current in the first transistor is above a threshold by comparing a voltage signal indicative of the magnitude of the current in the first transistor with a reference voltage, and reducing the magnitude of the current in the first transistor by adjusting a gate voltage of a second transistor in the power amplifier coupled in series with the first transistor responsive to determining that the magnitude of the current in the first transistor is above the threshold. 
     In one embodiment, the act of determining whether the magnitude of the current in the first transistor is above the threshold includes determining that the magnitude of the current in the first transistor is below the threshold responsive to the voltage signal being less than the reference voltage and determining that the magnitude of the current in the first transistor is above the threshold responsive to the voltage signal being greater than the reference voltage. 
     In one embodiment, the act of reducing the magnitude of the current in the first transistor includes turning off the second transistor. In one embodiment, the act of reducing the magnitude of the current in the first transistor includes adjusting a gate voltage of the second transistor and a third transistor, the third transistor being coupled to the second transistor. In one embodiment, the act of reducing the magnitude of the current in the first transistor includes turning off the second transistor and turning on the third transistor. 
     According to at least one aspect, a system for providing an output signal to a load is provided. The system includes a first transistor having a drain terminal constructed to provide the output to the load and a gate terminal constructed to receive a first input signal, a first circuit coupled to the gate terminal of the first transistor and constructed to apply a bias voltage to the gate terminal of the first transistor, a second circuit coupled to the first transistor and constructed to measure a magnitude of a current in the first transistor, a third circuit coupled to the second circuit and constructed to generate a feedback signal indicative of whether the magnitude of the current in the first transistor is above a threshold, and a fourth circuit coupled to the third circuit and the gate terminal of the first transistor, the fourth circuit being constructed to receive a second input signal and generate the first input signal based on the feedback signal and the second input signal, the first input signal being less than the magnitude of the second input signal responsive to the feedback signal indicating that the magnitude of the current in the first transistor is above the threshold. 
     In one embodiment, the threshold is a configurable threshold. In one embodiment, the system further includes a second transistor coupled in series with the first transistor, the second transistor having a gate terminal coupled to the driver circuit. In one embodiment, the first transistor is a gallium nitride (GaN) transistor and the second transistor is a metal-oxide-semiconductor field-effect transistor (MOSFET). In one embodiment, the first circuit is a GaN sequencer and is further constructed to apply the bias voltage to the gate terminal of the GaN transistor before turning on the MOSFET. 
     In one embodiment, the second circuit includes a current sense resistance coupled in series with the first transistor, a first level-shifter coupled to a first terminal of the current sense resistance, and a second level-shifter coupled to a second terminal of the current sense resistance. In one embodiment, the third circuit includes a difference detector constructed to compare the magnitude of the current in the first transistor with a reference voltage that defines threshold and generate the feedback signal based on the comparison. In one embodiment, the third circuit further includes a programmable voltage source constructed to generate the reference voltage and provide the reference voltage to the difference detector. In one embodiment, the first input signal has approximately the same magnitude as the second input signal responsive to the feedback signal indicating that the magnitude of the current in the first transistor is below the threshold. 
     According to at least one aspect, a circuit for protecting a gallium nitride (GaN) transistor is provided. The circuit includes a first circuit constructed to couple to a gate terminal of the GaN transistor and apply a bias voltage to the gate terminal of the GaN transistor, a second circuit constructed to measure a magnitude of a current in the GaN transistor, a third circuit coupled to the current sensing circuit and constructed to generate a feedback signal indicative of whether the magnitude of the current in the GaN transistor is above a threshold, and a fourth circuit constructed to couple to a gate terminal of the GaN transistor. The fourth circuit may be constructed to receive an input signal, generate an output signal based on the feedback signal and the input signal, and provide the output signal to the gate terminal of the GaN transistor, the output signal being less than the magnitude of the input signal responsive to the feedback signal indicating that the magnitude of the current in the GaN transistor is above the threshold. 
     In one embodiment, the first circuit is a GaN sequencer and is further constructed to apply the bias voltage to the gate terminal of the GaN transistor before turning on a transistor coupled in series with the GaN. In one embodiment, the second circuit includes a first programmable level-shifter constructed to couple to a first terminal of a current sense resistance and a second programmable level-shifter constructed to couple to a second terminal of the current sense resistance. 
     In one embodiment, the third circuit includes a difference detector constructed to compare the magnitude of the current in the first transistor with a reference voltage that defines the threshold and generate the feedback signal based on the comparison. In one embodiment, the third circuit further includes a programmable voltage source constructed to generate the reference voltage and provide the reference voltage to the difference detector. In one embodiment, the input signal has approximately the same magnitude as the output signal responsive to the feedback signal indicating that the magnitude of the current in the GaN transistor below the threshold. 
     According to at least one aspect, a method for protecting an amplifier that is providing an output signal to a load is provided. The method includes monitoring a magnitude of a current in a first transistor of the amplifier, determining whether the magnitude of the current in the first transistor is above a threshold by comparing a voltage signal indicative of the magnitude of the current in the first transistor with a reference voltage, and reducing the magnitude of the current in the first transistor by attenuating an input signal to the first transistor responsive to determining that the magnitude of the current in the first transistor is above the threshold. 
     In one embodiment, the act of determining whether the magnitude of the current in the first transistor is above the threshold includes determining that the magnitude of the current in the first transistor is below the threshold responsive to the voltage signal being less than the reference voltage and determining that the magnitude of the current in the first transistor is above the threshold responsive to the voltage signal being greater than the reference voltage. In one embodiment, the method further includes maintaining the magnitude of the input signal responsive to determining that the magnitude of the current in the first transistor is below the threshold. 
     The foregoing apparatus and method embodiments may be included in any suitable combination with aspects, features, and acts described above or in further detail below. These and other aspects, embodiments, and features of the present teachings can be more fully understood from the following description in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The skilled artisan will understand that the figures, described herein, are for illustration purposes only. It is to be understood that in some instances various aspects of the embodiments may be shown exaggerated or enlarged to facilitate an understanding of the embodiments. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the teachings. In the drawings, like reference characters generally refer to like features, functionally similar and/or structurally similar elements throughout the various figures. A depicted device or circuit may be integrated within a larger circuit. 
       When referring to the drawings in the following detailed description, spatial references “top,” “bottom,” “upper,” “lower,” “vertical,” “horizontal,” and the like may be used. Such references are used for teaching purposes, and are not intended as absolute references for embodied devices. The terms “on” and “over” are used for ease of explanation relative to the illustrations, and are not intended as absolute directional references. An embodied device may be oriented spatially in any suitable manner that may be different from the orientations shown in the drawings. The drawings are not intended to limit the scope of the present teachings in any way. 
         FIGS. 1A-1D  each show an example circuit to protect a device from dangerous operating conditions, according to some embodiments; 
         FIG. 2A  shows an example amplifier system implementing the protection scheme shown in  FIG. 1A , according to some embodiments; 
         FIGS. 2B and 2C  each show an example amplifier system implementing the protection scheme shown in  FIG. 1B , according to some embodiments; and 
         FIG. 3  shows an example method of operation of a circuit to protect a device from dangerous operating conditions, according to some embodiments. 
     
    
    
     Features and advantages of the illustrated embodiments will become more apparent from the detailed description set forth below when taken in conjunction with the drawings. 
     DETAILED DESCRIPTION 
     As described above, transistors comprising gallium nitride (GaN) material are useful for high-speed, high-voltage, and high-power applications because of the favorable material properties of GaN. Some applications relating to RF communications, radar, and microwaves can place demanding performance requirements on devices that include GaN transistors. For example, some applications may require high-power transistors capable of amplifying signals to power levels between approximately 50 Watts and approximately 200 Watts. 
     The favorable properties of GaN transistors also come with new limitations relative to silicon based transistors. For example, the gate-to-source breakdown voltage of a GaN transistor may decrease as the temperature of the GaN transistor increases. The temperature of the GaN transistor may rise because of increases in the magnitude of the current in the GaN transistor caused by operating condition changes. The lower gate-to-source breakdown voltage increases the gate-to-source leakage current in the GaN transistor and may lead to the complete failure of the GaN transistor. 
     The inventors have appreciated that the failure of GaN transistors from excess heat caused by overcurrent conditions can be avoided by quickly reducing the magnitude of the current in the GaN transistor. The inventors have conceived and developed various circuits and operating methods thereof to monitor the magnitude of the current in the GaN transistor (or other device) and rapidly reduce the magnitude of the current in the GaN transistor when the GaN transistor is heating up. In some embodiments, these circuits maintain the temperature of a GaN transistor within an appropriate range by monitoring the magnitude of the current in the GaN transistor and reducing the magnitude of the current in the GaN transistor and/or shutting down the GaN transistor when the magnitude of the current is outside an appropriate range (and/or above a threshold). The circuitry to monitor and/or control the magnitude of the current in the GaN transistor may utilize complementary metal-oxide-semiconductor (CMOS) devices to advantageously react quickly (e.g., within three microseconds) to shut down the GaN transistor once an unsafe operation condition is detected. It should be appreciated that the circuits and associated methods disclosed herein may be readily applied to protect devices other than GaN transistors. 
     An example circuit for protecting a device, such as a GaN transistor, is depicted in  FIG. 1A , according to some embodiments. The circuit  100 A includes various components to monitor a magnitude of the current in a protected device  108  and reduce the magnitude of the current in the protected device  108  when the protected device  108  is operating outside a safe range (e.g., above a current threshold). By reducing the magnitude of the current in the protected device  108  during operating conditions that are hazardous to the protected device  108 , the lifespan of the protected device  108  may be increased. 
     The circuit  100 A includes a current sensing circuit  102  coupled between the supply voltage Vcc and the protected device  108 . The current sensing circuit  102  is constructed to measure an amount of the current in a protected device  108 . The current sensing circuit  102  may measure the current in the protected device  108  by any of a variety of methods. For example, the current sensing circuit  102  may include a current sense resistance coupled in series with the protected device  108  and measure a voltage drop across the current sense resistance to determine the magnitude of the current in the protected device  108 . 
     A feedback circuit  104  is coupled to the current sensing circuit  102  and is constructed to receive a signal indicative of the magnitude of the current in the protected device  108  and determine whether the protected device  108  is operating within an appropriate range and/or above a threshold based on the magnitude of the current in the protected device  108 . For example, the feedback circuit  104  may be constructed to compare the magnitude of the current in the protected device  108  with a current threshold and generate a feedback signal indicative of whether the magnitude of the current in the protected device  108  is above (or below) the threshold. It should be appreciated that the threshold comparison may be substituted for a range. For example, the feedback circuit  104  may determine whether the magnitude of the current in the protected device  108  is within either a safe range or a hazardous range. 
     The feedback signal generated by the feedback circuit  104  is provided to a protection circuit  106 . The protection circuit  106  reduces the magnitude of the current in the protected device  108  and/or shuts down the protected device  108  responsive to the feedback signal indicating that the protected device  108  is operating outside of a safe range (e.g., above a current threshold). Otherwise, the protection circuit  106  allows the protected device  108  to continue operating without interruption. 
     The protection circuit  106  may reduce the current in the protected device  108  and/or shut down the protected device  108  by any of a variety of methods. The particular method employed may vary depending upon the location of the protection circuit  106  relative to the protected device  108 . As shown in  FIG. 1A  for example, the protection circuit  106  is coupled to the protected device  108  and a device providing the input signal  110  (not illustrated) for the protected device  108 . In this example, the protection circuit  106  may be constructed to reduce the magnitude of the current in the protected device  108  by attenuating the input signal  110  before providing the input signal  110  to the protected device  108 . The protection circuit  106  may completely attenuate a magnitude of the voltage of the input signal  110  to zero responsive to any deviation from the safe range or gradually attenuate the magnitude of the voltage of the input signal  110  proportionally to the amount of deviation from the safe range. For example, the feedback signal from the feedback circuit  104  may indicate that the protected device  108  is only 5% over a threshold and the protection circuit  106  may reduce a magnitude of the input signal  110  by 20%. In this example, the protection circuit  106  may apply larger amounts of attenuation to the input signal  110  for larger deviations from the threshold. The feedback circuit  104  may also continually monitor feedback signal and further reduce the magnitude of the input signal  110  if the magnitude of the current in the protected device  108  does not fall below the threshold within a predetermined period of time. 
     In some embodiments, the protection circuit  106  may be integrated with the protected device  108  and/or circuitry constructed to control the protected device  108 . Such an example circuit is illustrated by circuit  100 B in  FIG. 1B . In circuit  100 B, the protection circuit  106  may directly control the protected device  108  and/or devices proximate the protected device  108 . For example, the protected device  108  may be a GaN transistor in an amplifier and the protection circuit  106  may control the GaN transistor in addition to a metal-oxide-semiconductor field-effect transistor (MOSFET) coupled in series with the GaN transistor. In this example, the protection circuit  106  may reduce the magnitude of the current in the GaN transistor by turning off the MOSFET that is coupled in series with the GaN transistor. 
     It should be appreciated that the protection circuit  106  may be placed in other locations relative to the protected device  108 . As shown in  FIG. 1C , according to some embodiments, the protection circuit  106  may be coupled between the current sensing circuit  102  and the protected device  108 . In these embodiments, the protection circuit  106  may reduce the magnitude of the current in the protected device  108  by turning off a transistor coupled in series with the protected device  108 . By turning off a transistor coupled in series with the protected device  108 , the total impedance between the supply voltage Vcc and ground is significantly increased and the magnitude of the current in the protected device  108  is reduced. 
     As shown in  FIG. 1D , the protection circuit  106  may also be coupled between the protected device  108  and ground instead of between the current sensing circuit  102  and the protected device  108 . The protection circuit  106  in  FIG. 1D  may operate similarly to the protection circuit  106  described above in  FIG. 1C . For example, the protection circuit  106  may reduce the magnitude of the current in the protected device  108  by turning off a transistor coupled in series with the protected device  108 . 
     In some embodiments, the feedback circuit  104  and/or the protection circuit  106  shown in  FIGS. 1A-1D  may be advantageously constructed from devices with a quick reaction time to increase the speed at which the circuits  100 A- 100 D can react to changes in the magnitude of current. CMOS devices, for example, may have a propagation delay that is very small (e.g.,  10  nanoseconds). Thereby, constructing the feedback circuit  104  and/or the protection circuit  106  from CMOS devices may yield fast reaction times (e.g., less than 1 microsecond) between the magnitude of current in the protected device  108  rising above a threshold and the magnitude of current in the protected device  108  being reduced below the threshold. It should be appreciated that the feedback circuit  104  and/or the protection circuit  106  may be constructed from devices other than CMOS devices. 
     As discussed above, various protection schemes may be employed to safeguard a particular device, such as a GaN transistor, from dangerous operating conditions. These protection schemes may be implemented in any of a variety of systems including, for example, amplifier systems.  FIG. 2A  illustrates such an example amplifier system  200 A that implements the protection scheme shown in  FIG. 1A  above. 
     The amplifier system  200 A includes an amplifier  226  constructed to receive and amplify the input signal  208  to generate the output signal  210 . The amplifier  226  includes a first transistor  204  that performs the signal amplification and is also designated as the protected device  108 . The amplifier system  200 A protects the first transistor  204  by monitoring a magnitude of the current in the first transistor  204  and attenuating the input signal  208  when the first transistor  204  is operating in dangerous conditions. The first transistor  204  receives the input signal  208  at a gate terminal and provides the output signal  210  at a drain terminal. As shown, a source terminal of the first transistor  204  is connected to ground. A driver circuit  202  applies a bias voltage to the gate terminal first transistor  204  and also controls the operation of a second transistor  206  coupled in series with the first transistor  204 . The second transistor  206  has a drain terminal coupled to the current sense resistor  212  and a source terminal coupled to the drain terminal of the first transistor  204 . 
     It should be appreciated that the bias voltage from the driver circuit  202  and the input signal  208  may be combined by a combiner circuit (not illustrated) coupled between the driver circuit  202  and the gate terminal of the first transistor  204 . For example, the combiner circuit may include an inductor coupled between the driver circuit  202  and the gate terminal of the first transistor  204  to pass the bias voltage and block higher frequencies in addition to a capacitor coupled between the gate terminal of the first transistor  204  and the protection circuit  106  to pass the input signal  208  and block lower frequencies. 
     As shown in  FIG. 2A , the current sensing circuit  102  includes a current sense resistance  212  coupled between the supply voltage Vcc and the drain terminal of the second transistor  206 . The current sense resistance  212  may be selected such that the voltage drop across the current sense resistance  212  is small to minimize the excess power dissipation caused by an additional impedance coupled between the supply voltage Vcc and ground. For example, the supply voltage Vcc may be between 30 and 50 Volts and the voltage drop across the current sense resistance  212  may be 1 Volt. 
     The current sense circuit  102  shown in  FIG. 2A  includes a first programmable level shifter coupled to a first terminal of the current sense resistance  212  and a second programmable level shifter coupled to a second terminal of the current sense resistance  212 . The first and second programmable level shifters  214  and  216 , respectively, may reduce a magnitude of the voltage at the ends of the current sense resistance  212  to a suitable range for the feedback circuit  104 . For example, the programmable level shifters  214  and  216  may reduce a magnitude of the voltages to between 25 and 50 millivolts (mV). The impedance of the programmable level shifts  214  and  216  may be high to reduce the power losses. For example, the supply voltage Vcc may be between 30 and 50 Volts and the impedance of the programmable level shifters  214  and  216  may be at least 100,000 Ohms. It should be appreciated that various alternations may be made to the programmable level shifters  214  and  216  without departing from the scope of the present disclosure. For example, the variable impedances in the programmable level shifters  214  and  216  may be replaced with and/or programmable current sources. In another example, the programmable level shifters  214  and  216  may be replaced by fixed level shifters by replacing the variable impendence coupled to ground with a fixed impendence and/or a fixed current source. Further, the current sensing circuit  102  may include an amplification stage coupled to the programmable level shifters  214  and  216  to amplify the signal from the programmable level shifters  214  and  216 . 
     The voltage across the two outputs of the programmable level shifters  214  and  216  is indicative of the magnitude of the current in the first transistor  204  and may be provided to the feedback circuit  104 . As shown in  FIG. 2A , the feedback circuit  104  includes a difference detector  220  coupled to the programmable level shifters  214  and  216  and a programmable reference generator  218  that generates a voltage reference indicative of a maximum current threshold. The difference detector  220  is constructed to compare the voltage reference from the programmable reference generator  218  with the voltage from programmable level shifters  214  and  216 . Thereby, the difference detector  220  compares the magnitude of the current in the first transistor  204  with a threshold set by the programmable reference generator  218 . The programmability of the programmable reference generator  218  may provide flexibility allowing, for example, the re-use of the same circuitry for different protected devices  108  with different sensitivities to current. 
     The feedback signal  222  may be indicative of whether the magnitude of the current in the first transistor  204  is above (or below) the threshold set by the programmable reference generator  218 . For example, the difference detector  220  may indicate via the feedback signal  222  that the magnitude of the current in the first transistor  204  is above the threshold responsive to the voltage from the programmable level shifters  214  and  216  being greater than the reference voltage from the programmable reference generator  218 . Conversely, the difference detector  220  may indicate via the feedback signal  222  that the magnitude of the current in the first transistor  204  is below the threshold responsive to the voltage from the programmable level shifters  214  and  216  being less than the voltage from the programmable reference generator  218 . 
     In some embodiments, the feedback signal  222  generated by the difference detector  220  may be an analog signal. For example, the difference detector  220  may be implemented as a difference amplifier and the feedback signal  222  provided by the difference amplifier may be indicative of a difference between the voltage from the programmable level shifters  214  and  216  and the voltage from the programmable reference generator  218 . 
     It should be appreciated that the feedback circuit  104  may be constructed to compare the magnitude of the current in the first transistor  204  to multiple thresholds and/or ranges depending upon the particular implementation. For example, the difference detector  220  may receive multiple reference voltages and compare the voltage from the programmable level shifters  214  and  216  to each of the reference voltages. In this example, the difference detector  220  may identify the closest reference voltage that is above the voltage from the programmable level shifters  214  and  216  and the closest reference voltage that is below the voltage from the programmable level shifters  214  and  216  to identify a range that the magnitude of the current falls within. Based on which range that the magnitude of the current falls within, the protection circuit  106  may take appropriate action to reduce the magnitude of the current in the first transistor  204  or allow the first transistor  204  to continue operating without interruption. 
     As shown in  FIG. 2A , the protection circuit  106  receives the input signal  208  and attenuates the input signal  208  based on the feedback signal  222  before providing the input signal  208  to the gate terminal of the first transistor  204 . The attenuation of the input signal  208  in the protection circuit  106  may be performed by an attenuator  224 . The attenuator  224  may attenuate the input signal  208  to (or near) zero responsive to the feedback signal  222  indicating that the magnitude of the current in the first transistor  204  is above the threshold. Alternatively, the attenuator  224  may apply attenuation to the input signal  208  gradually as the current in the first transistor  204  exceeds a threshold and/or particular thresholds in a set of thresholds. For example, the feedback signal  222  may indicate that the magnitude of the current in the first transistor  204  is between the two lowest thresholds (e.g., in a first range) and attenuator  224  may slightly attenuate the input signal  208 . The attenuator  224  may apply larger amounts of attenuation when the feedback signal  222  indicates that the magnitude of the current in the first transistor  204  is above the two lowest thresholds (e.g., in a second range). In another example, the feedback signal  222  may be an analog signal that indicates the deviation between the magnitude of current in the first transistor  204  and a threshold. In this example, the magnitude of attenuation applied by attenuator  224  may be proportional to the magnitude of the deviation indicated by the feedback signal  222 . The attenuator  224  may also modify the amount of attenuation based on the response exhibited by the first transistor  204 . For example, the attenuator  224  may first apply a small level of attenuation to the input signal  208  responsive to the magnitude of the current being above the lowest threshold and subsequently increase the amount of attenuation applied to the input signal  208  responsive to the magnitude of the current remaining above the lowest threshold. 
     As discussed above with reference to  FIG. 1B , the functionality of the protection circuit  106  may be integrated with the protected device  108  and/or integrated with the circuitry that controls operation of the protected device  108 .  FIG. 2B  illustrates such an example amplifier system  200 B with the functionality of the protection circuit integrated into the amplifier  226 . Integrating the functionality of the protection circuit into the amplifier may advantageously simplify the construction of the amplifier system  200 B by leveraging existing devices to perform various functions performed by the protection circuit. Thereby, the cost and complexity of manufacturing the amplifier system may be reduced. 
     As shown in  FIG. 2B , the feedback signal  222  from the feedback circuit  104  is provided to the driver circuit  202  of the amplifier  226 . In this implementation, the driver circuit  202  may be constructed to control operation of the second transistor  206  based on the feedback signal  222 . For example, the feedback signal  222  may indicate that the magnitude of the current in the first transistor  204  is above the threshold and the driver circuit  202  may turn off the second transistor  206  to reduce the magnitude of the current in the first transistor. If the feedback signal  222  indicates that the magnitude of the current in the first transistor  204  is below the threshold, the driver circuit  202  may turn on the second transistor  206  (or keep the second transistor  206  on). Thereby, the functionality of the protection circuit is integrated into existing components of the amplifier  226  (e.g., the driver circuit  202  and the second transistor  206 ). 
       FIG. 2C  illustrates another example amplifier system  200 C with the functionality of the protection circuit integrated into the amplifier  226 . As shown in  FIG. 2C , the drain terminal of the second transistor  206  is coupled to the source terminal of the first transistor  204  instead of the drain terminal of the first transistor  204  as shown in  FIGS. 2A and 2B . The amplifier  226  shown in  FIG. 2C  further includes a third transistor  228  coupled in series with the second transistor  206 . The third transistor  228  has a source terminal coupled to a second supply voltage Vcc 2 , a gate terminal coupled to the driver circuit  202 , and a drain terminal coupled to the drain terminal of the second transistor  206 . In this implementation, the driver circuit  202  is constructed to reduce the magnitude of the current in the first transistor  204  by simultaneously turning off the second transistor  206  and turning on the third transistor  228  to increase the voltage level at the source terminal of the first transistor  204 . For example, turning off the second transistor  206  and turning on the third transistor  228  raises the voltage level at the source terminal of the first transistor  204  from ground to Vcc 2 . 
     By increasing the voltage at the source terminal of the first transistor  204 , both the voltage drop across the first transistor  204  and the magnitude of the current in the first transistor  204  are reduced. The driver circuit  202  may turn on the second transistor  206  and turn off the third transistor  228  during normal operation when the magnitude of the current in the first transistor  204  is below the threshold. 
     As shown in  FIG. 2C , the third transistor  228  may be a p-type transistor while the second transistor  206  may be an n-type transistor. Implementing the second and third transistors  206  and  228 , respectively, as different types may allow the driver circuit to apply a similar (or same) control signal to both transistors and achieve the desired modes of operation. For example, a first control signal may be sent to the second and third transistors  206  and  228 , respectively, to turn off the second transistor  206  and turn on the third transistor  228  to increase the voltage applied to the source terminal of the first transistor  204 . 
     Conversely, a second control signal may be sent to the second and third transistors  206  and  228 , respectively, to turn on the second transistor  206  and turn off the third transistor  228  to ground the source terminal of the first transistor  204 . 
     In some embodiments, the amplifier  226  shown in each of  FIGS. 2A-2C  includes a GaN transistor. For example, the first transistor  204  may be a GaN transistor and the second transistor  206  may be formed from a different type of semiconductor, such as a silicon metal-oxide-semiconductor field-effect transistor (MOSFET). In these embodiments, the driver circuit  202  may be a GaN sequencer that is constructed to apply the bias voltage to the GaN transistor before turning on the MOSFET to avoid damage to the GaN transistor. In the implementations shown in  FIG. 2B and 2C , the GaN sequencer may control the second transistor  206  to both properly sequence the GaN transistor and also reduce the current in the GaN transistor responsive to the magnitude of the current in the GaN transistor exceeding a threshold. 
     As discussed above, various circuits may be designed to protect a device, such as a GaN transistor, from damage caused by hazardous operating conditions.  FIG. 3  illustrates an example method of operation  300  for these circuits according to some embodiments. The method  300  protects a device by monitoring a magnitude of the current in a protected device and determining whether the magnitude of the current in the protected device is above a threshold. If the magnitude of the current in the protected device is above the threshold, the magnitude of the current in the protected device is reduced. Otherwise, the protected device is allowed to continue operating without interruption. 
     In act  302 , the circuit monitors the magnitude of the current in the protected device. The protected device may include a GaN transistor or other device that is sensitive to heat and/or current. The circuit may monitor the current in the protected device by any of a variety of methods. For example, the circuit may include a current sense resistance coupled in series with the protected device and the circuit may measure a voltage drop across the current sense resistance to determine a magnitude of the current in the protected device. 
     In act  304 , the circuit determines whether the magnitude of the current in the protected device is above a threshold. The circuit may include, for example, a difference detector that is constructed to compare a voltage reference that is indicative of the threshold with a voltage signal indicative of the magnitude of the current in the protected device. In this example, the circuit may indicate that the magnitude of the current in the protected device is above the threshold responsive to the voltage signal indicative of the magnitude of the current in the protected device being larger than the voltage reference. Conversely, the circuit may indicate that the magnitude of the current in the protected device is below the threshold responsive to the voltage signal indicative of the magnitude of the current in the protected device being less than the voltage reference. If the magnitude of the current in the protected device is above the threshold, the circuit proceeds to act  306  to reduce the current in the protected device. Otherwise, the circuit returns to act  302  and continues to monitor the magnitude of the current in the protected device. 
     The circuit may employ any of a variety of methods in act  306  to reduce the magnitude of the current in the protected device. For example, the circuit may turn off a transistor coupled in series with the protected device and/or attenuate an input signal to the protected device. In examples where the protected device is a transistor, the circuit may also raise a voltage level at a source terminal of the transistor to both reduce a voltage drop across the transistor and reduce a magnitude of the current in the transistor. 
     It should be appreciated that the circuit may compare the magnitude of the current in the protected device with multiple thresholds or ranges in act  304 . For example, the circuit may determine whether the magnitude of the current in the protected device is within a first range that is safe for the protected device or a second higher range that is unsafe for the protected device. In this example, the circuit may proceed to act  306  responsive to the protected device operating in the second higher range. Otherwise, the circuit may return to act  302  and continue monitoring the protected device. 
     The terms “approximately” and “about” may be used to mean within ±20% of a target dimension in some embodiments, within ±10% of a target dimension in some embodiments, within ±5% of a target dimension in some embodiments, and yet within ±2% of a target dimension in some embodiments. The terms “approximately” and “about” may include the target dimension. 
     The technology described herein may be embodied as a method, of which at least some acts have been described. The acts performed as part of the method may be ordered in any suitable way. Accordingly, embodiments may be constructed in which acts are performed in an order different than described, which may include performing some acts simultaneously, even though described as sequential acts in illustrative embodiments. Additionally, a method may include more acts than those described, in some embodiments, and fewer acts than those described in other embodiments. 
     Having thus described at least one illustrative embodiment of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto.