Abstract:
A method for detecting a failure in a radio frequency (RF) device ( 200 ) that includes a transistor ( 210 ) having a first terminal at an RF input ( 220 ), a second terminal at an RF output ( 260 ), and a third terminal at an RF common ( 230 ). The method includes the steps of: detecting a first voltage at the first terminal; comparing the first voltage to a reference voltage; and determining whether the RF transistor ( 210 ) is operating in a normal state or a failed state as a function of whether the first voltage falls inside or outside of a predetermined threshold value of the reference voltage.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to radio frequency (RF) devices, and more specifically to a method for detecting a failure in a RF transistor. 
     BACKGROUND OF THE INVENTION 
     Radio frequency (RF) devices, e.g., RF transistors, and RF power amplifiers (“PAs”) containing RF transistors, are used in a wide variety of communications and other electronic applications, such as cellular handsets and base radio repeaters. RF power amplifiers are typically made up of one or more cascaded amplifier stages, each of which increases the level of the signal applied to the input of that stage by an amount known as the gain stage. Additionally each cascaded amplifier stage can be paralleled with another amplifier to increase the RF power output of the cascaded stage if needed. Ideally, the input to output transfer of each stage is linear, i.e., a perfect replica of the input signal increased in amplitude appears at the amplifier output. In reality, however, all power amplifiers have a degree of non-linearity in their transfer characteristics. This non-linearity is directly related to the level of signal distortion in the amplifier signal output, i.e., the more linear the amplifier, the less distortion is added to the signal. 
     RF device fault detection is needed so that the end user, e.g., the customer, can be notified that a particular field replaceable unit needs to be replaced. In the event of a paralleled RF device failure in a power amplifier, this notification is needed, for instance, to determine if the product operating with a failed RF device is producing distortion products in excess of requirements imposed by the Federal Communications Commission (“FCC”). If such products exceed FCC limits, the law is being violated and system performance for the user and other system operators will be degraded. 
       FIG. 1  illustrates a prior art power amplifier  100  configured for enabling the detection of a failed RF device that is included in the amplifier. Power amplifier  100  includes a transistor  110  that in this instance is a metal oxide semiconductor field effect transistor (MOSFET) but may also be some other transistor type such as a Bipolar Junction Transistor (BJT) or a Junction Field Effect Transistor (JFET). Power amplifier  100  also includes: a RF de-coupling device or Radio Frequency Choke (“RFC”)  136  connected to the gate  105  of transistor  110  and a RFC  138  connected to the drain of transistor  110  that are both preferably inductors; a resistor  132  connected between RFC  138  and RFC  136 ; a resistor  134  connected between a common (ground) at a terminal  130  and the junction of resistor  132  and RFC  136 ; a resistor  140  connected between a voltage source V+ at a terminal  150  and the junction of resistor  132  and RFC  138 ; a differential amplifier  142  connected across resistor  140  and having an output swing from substantially close to V+ to substantially close to ground; an RF matching circuit  112  connected to the gate  105  of transistor  110  for receiving an RF input signal (“RF IN ”) at a terminal  120 ; and a RF matching circuit  162  connected to the drain  107  of transistor  110  through which a matched and amplified RF output signal (“RF OUT ”) is presented at a terminal  160 . In addition, the source of transistor  110  is coupled to an RF common (ground) at terminal  130 . Amplifier  100  is illustrated with one transistor for ease of discussion. However, those of ordinary skill in the art will realize that amplifier  100  may, for instance, have cascaded stages and be implemented with additional transistors. 
     A well-known method for sensing whether transistor  110  has failed is to sense or detect the current being drawn by the drain of transistor  110 , for instance, through the generation of an “I-Monitor” signal at a terminal  170 . Specifically, a differential voltage is measured across resistor  140  that is proportional to the current being drawn through it and that is a function of the drain current of transistor  110 . This differential voltage is then buffered or amplified by the differential amplifier  142 , which delivers the single ended (i.e. referenced to ground) “I-Monitor” voltage signal. Accordingly, the “I-Monitor” signal is a single ended voltage representation of the current through resistor  140 . The “I-Monitor” signal can then be detected at terminal  170  and compared to a static reference to enable a determination to be made as to whether transistor  110  is operating in a normal state or whether it has failed. 
     The above method of detecting RF device failure is adequate if the RF device is operating in “class A” because the drain current does not change versus RF drive, thereby enabling the use of a static reference. However, operating in “class A” is usually prohibitive from a utility power and thermal perspective for high power PA&#39;s, i.e., amplifiers having an average power out of greater than a few watts. Therefore, most high power PAs operate in “class AB” where the drain or collector currents are proportional to drive. In a class AB operation (as well as with all other classes of operation, e.g., Class B, Class C, etc.) the “I-Monitor” voltage signal will change in approximate proportion to the RF drive since the current through resistor  140  is changing. Thus, with respect to RF devices operating in these latter classes of operations, the current draw in the RF device cannot typically be gauged against a static reference. This means that a static reference cannot typically be used to determine if a failure has occurred. 
     Therefore for all practical purposes, short of including circuitry in the PA device that detects power out and developing a massive look up table for each RF device (operating in other than Class A) that lists the current drawn as a function of power out, the “health” of the RF device may only be detected using the prior art method during a non-RF drive situation when the RF device is dekeyed so that its quiescent drain current (“Idq”) can be detected and compared to a known value. Ideally, the PA should be frequently dekeyed so that this Idq measurement/comparison can be made. However some PAs operate in a continuously keyed mode, so the Idq measurement/comparison can only occur during maintenance windows, which are typically scheduled a substantial time apart from each other so as not to affect system availability. Thus, if an RF device failed during the time span between maintenance windows, the PA could potentially be operating in an illegal manner for quite some time before an RF device failure is detected. 
     In addition, this method can be troublesome to implement because resistor  140  may have to be adjusted to a larger value than desired to get a “usable” sensitivity out of differential amplifier  142 . This is because operational amplifiers, such as differential amplifier  142 , generally have too high of an input offset voltage error to accurately detect the bias level of a transistor. However, raising the value of resistor  140  to increase the sensitivity of differential amplifier  142  would have the adverse effect of causing a lower DC voltage to be coupled to the drain of transistor  110 . This would decrease the maximum AC compliance as well as the efficiency of power amplifier  100 , which is undesirable. A further shortcoming of detecting the bias level of transistor  110 , besides not being very practical, is that it cannot be used with Class C or B designs since these designs do not draw a bias current to begin with. 
     Thus, there exists a need for a more effective and efficient method for detecting in real-time whether an RF device has failed and that can be used with an amplifier, operating in any class, without having to dekey the amplifier. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       A preferred embodiment of the invention is now described, by way of example only, with reference to the accompanying figures in which: 
         FIG. 1  illustrates a circuit diagram of a prior art power amplifier configured for enabling the detection of the status of an RF device included in the amplifier; 
         FIG. 2  illustrates a circuit diagram of a power amplifier configured for enabling the status of an RF device in the amplifier to be detected in accordance with the present invention; 
         FIG. 3  illustrates a simple block diagram of the circuit illustrated in  FIG. 2 ; 
         FIG. 4  illustrates a simple block diagram of an embodiment of the present invention when used to detect the failure of a non-paralleled RF stage amplifier device. 
         FIG. 5  illustrates a simple block diagram of another embodiment of the present invention when used to detect the failure of a paralleled RF stage amplifier device. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     It will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to each other. Further, where considered appropriate, reference numerals have been repeated among the figures to indicate corresponding elements. 
     This method of failure detection shown in  FIG. 2  leverages off of the two ways a transistor will typically fail. One is a gate short and the second is a drain short. By looking for evidence of these two failure mechanisms one can detect a substantial majority of all device failures. 
       FIG. 2  illustrates a circuit diagram of a power amplifier  200  configured for enabling the status of an RF device in the amplifier to be detected in accordance with the present invention. Power amplifier  200  preferably includes a MOSFET transistor  210 , a RF de-coupling device (RFC)  236  connected to the gate  205  of transistor  210  and a RFC  238  connected to the drain  207  of transistor  210  that are both preferably inductors, a resistor  232  connected between RFC  238  and RFC  236 ; a resistor  234  connected between a common (ground) at a terminal  230  and the junction of resistor  232  and RFC  236 , a fuse  240  connected between a voltage source V+ at a terminal  250  and the junction of resistor  232  and RFC  238 , an RF matching circuit  212  connected to the gate  205  of transistor  210  for receiving an RF input signal (RF IN ) at a terminal  220 , and a RF matching circuit  262  connected to the drain  207  of transistor  210  through which a matched and amplified RF output signal (RF OUT ) is presented at terminal  260 . In addition, the source of transistor  210  is coupled to an RF common (ground) at terminal  230 . Amplifier  200  is illustrated with one transistor for ease of discussion. However, those of ordinary skill in the art will realize that amplifier  200  may also be implemented with additional transistors or with a different type of transistor, e.g., a BJT or a JFET. 
     By reference to power amplifier  200 , the status of whether transistor  210  is operating in a normal state or a failed state is determined according to the present invention as follows. First, a voltage signal V-Monitor at the gate of transistor  110  is detected at terminal  270 . The V-Monitor signal at terminal  270  can then be compared to a reference voltage, using a number of ways known in the art, for determining whether transistor  210  is operating in a normal state or in a failed state. Transistor  210  is operating in a normal state when the V-Monitor voltage signal is substantially equal to the voltage defined, “in this case”, by the equation V+*(R 234 /(R 234 +R 232 ). The direct current (DC) impedance the RF MOSFET  210  can be negated because in an unfailed state its impedance is many orders of magnitude greater than the resistance of R 234 . It should be noted that in the preferred embodiment, the resistance of R 234  should be designed such that it&#39;s impedance is less than two orders of magnitude the nominal DC impedance of the RF device. 
     Transistor  210  is operating in a failed state when there is a gate failure, a drain failure or a contemporaneous gate and drain failure. During a gate failure, the impedance seen at the gate of transistor  210  is substantially reduced, which lowers the V-Monitor voltage signal. During a drain short failure, fuse  240  will open, which lowers the V-Monitor voltage signal. 
     Whether transistor  210  is operating in a normal state or a failed state is, thus, determined by detecting the V-Monitor signal at terminal  270  and comparing it to a voltage reference, using any of a number of methods known in the art. The reference voltage should be set such that when V-monitor is compared to the reference voltage when transistor  210  is operating in a normal state, a status signal is generated that indicates that transistor  210  is operating in a normal state. Conversely, the reference voltage should be set such that when V-monitor is compared to the reference voltage when transistor  210  is operating in a failed state, a status signal is generated that indicates that transistor  210  is operating in a failed state. The reference voltage may be generated by a reference generator such as a resistive divider, a Zener regulator, a precision regulator IC, etc., in an analog approach. These devices, however, must derive there source voltage prior to fuse  240  (unprotected side). In a digital approach, the reference voltage may be stored in a memory device such as an EEPROM. 
     Once the status signal is generated, it can be used in a number of different ways. For instance, a microprocessor can monitor the status signal. If a failure is indicated on the status line the microprocessor can execute a protocol that will shut down the RF amplifier in a graceful fashion. Specifically, if any calls are being processed through the amplifier it will (if opted) wait until those calls are finished and not allow any new calls to be routed to the amplifier. Once all current calls have terminated, the amplifier will be shut down and a failure report can be generated to, for instance, a command center and service shop. In addition, the microprocessor could generate an error report to be delivered to the PA&#39;s non-volatile memory, which makes known which RF device failed. This would aid in a more rapid repair of the amplifier at the repair center since technicians would know the specific problem. In addition, a total analog solution could be implemented using the present invention, wherein no microprocessor is needed. In such a case, once a failure is noted in logic, the amplifier is immediately shut down in real time. All calls in progress would be immediately dropped. These are just two examples of implementation of the present invention, but more could be designed. Moreover, FIG.  4  and  FIG. 5  illustrate two other implementations of the present invention. 
       FIG. 3  illustrates a simple block diagram of the power amplifier  200  illustrated in FIG.  2 . This block diagram of power amplifier  200  shows the terminal  220  for receiving RF IN , the terminal  230  that couples the source of transistor  210  ( FIG. 2 ) to ground, the terminal  250  connected to a voltage source V+, the terminal  260  at which a matched and amplified RF OUT  is presented, and a terminal  270  at which the transistor  210  V-Monitor signal is generated. 
       FIG. 4  illustrates a simple block diagram of an embodiment of the present invention when used to detect the failure of a RF device in a non-paralleled stage power amplifier network  400 . Power amplifier network  400  includes a power amplifier  200  that is identical to the amplifier circuit described in FIG.  2  and the corresponding block diagram of FIG.  3 . Power amplifier network  400  also includes a reference voltage source  372  and preferably an op-amp comparator  374  for detecting the V-Monitor signal at terminal  270  and comparing it to the reference  372  for generating a status signal  376  that indicates whether the RF device in amplifier  200  is operating in a normal state or a failed state. 
     FIG.  5 . illustrates a simple block diagram of an embodiment of the present invention when used to detect the failure of a RF device in a paralleled stage power amplifier network  500 . Power amplifier network  500  includes two power amplifiers  200  that are each identical to the amplifier circuit described in FIG.  2  and the corresponding block diagram of FIG.  3 . Both amplifiers are connected such as to have common terminals  220  for receiving RF IN , common terminals  230  that couple the source of internal transistors (not shown) to ground, common terminals  250  connected to a voltage source V+, and common terminals  260  at which a matched and amplified RF OUT  is presented. 
     PA network  500  also includes a logic element  580  coupled to the power amplifiers  200  for detecting their respective V-Monitor voltage signals for generating a status signal  582  that indicates whether the RF device in either or both amplifiers  200  is operating in a normal state or a failed state. The status signal  582  can then be used, for instance, to excite an indicator to feed to a processing unit to perform a more useful function. 
     The logic element  580  shown in  FIG. 5  should be chosen such that the nominal biasing voltage V-Monitor will constitute a “high state” to the input of this logic element. If V-Monitor is higher than the allowable limit for the logic element then the bias voltage can be appropriately divided down to meet high-level limit restrictions. In the event of a RF device failure the V-Monitor signal will fall to a level that constitutes an input low to the logic element. In addition, logic element  580  should be chosen such that if all “high state” inputs are delivered to said logic element, then the output of the said logic device will reflect a different logic state than if any one or more of the input signals go to a “low state”. 
     Referring again to  FIG. 5 , an op-amp comparator as was used in the circuit illustrated in  FIG. 4  could alternatively be coupled to each power amplifier  200  for detecting the respective V-Monitor signals at terminal  270 . In this embodiment, the output status signal of each op-amp could then be combined via a logic element  580  as described above. 
     There are several advantages of the present invention which provide significant improvement over the prior art method discussed by FIG.  1 . One, the present invention will detect a device failure in a “class AB” PA during excitation. 
     Another advantage of the present invention over the prior art is that even if operational amplifier technology is improved, the prior art method cannot be used for Class C or B designs, since these designs do not draw a bias current. However, the present invention could be used to detect a RF device failure in a Class C or B design since it looks at the first terminal voltage (V-Monitor) of the RF device. 
     Yet another advantage of the present invention is that no maintenance period is required for continually keyed PA&#39;s. 
     A further advantage of the present invention is that it allows the use of single ended detection circuits as opposed to differential detection. 
     Another advantage of the present invention is that no bulky metering shunts are required for RF device failure detection. 
     Yet another advantage of the present invention is that it is easier to integrate a circuit that uses the present invention into a single RF integrated circuit. 
     A final advantage of the present invention is that it provides “real time” failure detection. 
     While the invention has been described in conjunction with specific embodiments thereof, additional advantages and modifications will readily occur to those skilled in the art. The invention, in its broader aspects, is therefore not limited to the specific details, representative apparatus, and illustrative examples shown and described. Various alterations, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Thus, it should be understood that the invention is not limited by the foregoing description, but embraces all such alterations, modifications and variations in accordance with the spirit and scope of the appended claims.