# Patent

## Title
Method and apparatus for protecting devices in an RF power amplifier

## Abstract

A method and apparatus are provided for use with a power amplifier for protecting active devices on the power amplifier. A peak detector is used by control circuitry to detect the presence of a peak voltage that exceeds a threshold voltage. In response to the detection of a peak voltage, the gain of the power amplifier is reduced.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation of commonly owned U.S. patent application Ser. No. 10/673,750, filed on Sep. 29, 2003 now U.S. Pat. No. 7,145,396, entitled “METHOD AND APPARATUS FOR PROTECTING DEVICES IN AN RF POWER AMPLIFIER”, which is a continuation of commonly owned U.S. patent application Ser. No. 09/932,817, filed on Aug. 17, 2001, entitled “METHOD AND APPARATUS FOR PROTECTING DEVICES IN AN RF POWER AMPLIFIER” (U.S. Pat. No. 6,828,859), which are incorporated by reference herein. The following U.S. patent application is expressly incorporated herein by reference: Ser. No. 09/842,456, entitled “RF POWER DETECTOR” by Timothy J. Dupuis et al, filed on Apr. 26, 2001, which is a continuation-in-part of U.S. application Ser. No. 09/660,123, filed on Sep. 12, 2000, entitled “POWER AMPLIFIER CIRCUITRY AND METHOD”.


FIELD OF THE INVENTION
This invention relates to the field of power amplifiers. More particularly, this invention relates to circuitry for protecting devices in an RF power amplifier.
BACKGROUND OF THE INVENTION
In some applications utilizing a power amplifier, it is desirable to limit peak voltages to which active devices of the power amplifier are subjected. For example, in CMOS devices, the transistor breakdown voltage may be only slightly greater than the supply voltage. In RF power amplifiers, high peak voltages can be caused by load mismatches, temperature extremes, and device variations, for example. High peak voltages are capable of causing breakdown of the active devices, which can lead to reliability problems.
It can therefore be seen that there is a need for amplifier designs where peak voltages applied to active devices of the amplifier are limited so that the peak voltages are below the transistor breakdown voltages of the devices being used to implement the design.
SUMMARY OF THE INVENTION
An apparatus of the present invention provides a circuit for protecting devices in an RF power amplifier comprising: a peak detector coupled to an output of the power amplifier for detecting peak voltages at the output of the power amplifier; and control circuitry coupled to the peak detector and to the power amplifier for controlling the gain of the power amplifier, wherein the control circuitry decreases the gain of the power amplifier when the peak detector detects a voltage above a voltage threshold.
One embodiment includes a circuit comprising: an RF power amplifier having an input and an output; a peak detector coupled to the power amplifier for detecting a peak voltage at a node of the power amplifier; and power control circuitry coupled to the peak detector and to the power amplifier for controlling the gain of the power amplifier, wherein the power control circuitry limits the power at the output of the power amplifier when the peak detector detects a peak voltage greater than a threshold voltage.
Another embodiment of the invention provides a method of protecting devices in an RF power amplifier comprising the steps of: detecting a peak voltage at a first node of the power amplifier; determining whether the detected peak voltage is higher than a threshold voltage; and if it is determined that the detected peak voltage is higher than the threshold voltage, decreasing the gain of the power amplifier.
Another embodiment of the invention provides a method of controlling an RF power amplifier comprising the steps of: detecting the output power of the RF power amplifier; detecting a peak voltage at a first node of the power amplifier; increasing the gain of the power amplifier if the detected output power is less than a desired output power level and if the detected peak voltage does not exceed a threshold voltage; and decreasing the gain of the power amplifier if the detected output power is greater than the desired output power level or if the detected peak voltage exceeds a threshold voltage.
Other objects, features, and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description that follows below.



BRIEF DESCRIPTION OF THE DRAWINGS
The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements and in which:
FIG. 1 is a block diagram of a protection circuit of the present invention used with a power amplifier.
FIG. 2 is a flowchart illustrating the operation of the power control circuitry shown in FIG. 1.
FIG. 3 is a block diagram of a circuit similar to the circuit shown in FIG. 1 illustrating one implementation of a peak detector.
FIGS. 4 and 5 are timing diagrams illustrating the use of the invention in applications where power is ramped.



DETAILED DESCRIPTION
In order to provide a context for understanding this description, the following illustrates an example of a typical application of the present invention. A power amplifier using the protection techniques of the present invention may be used with a wireless transmission system such as a wireless telephone or other device. In a wireless device such as a cellular telephone, the wireless device may include a transceiver, an antenna duplexer, and an antenna. Connected between the transceiver and the antenna duplexer is an RF power amplifier for amplifying signals for transmission via the antenna. This is one example of an application of a power amplifier utilizing the present invention. Of course, the invention may be used in any other application requiring a power amplifier. In the case of a wireless telephone application, the invention may be applied to GSM, CDMA, PCS, DCS, etc., or other wireless systems.
FIG. 1 is a block diagram of a protection circuit of the present invention used with a power amplifier. FIG. 1 shows a circuit 100 including a power amplifier 102 and an antenna 104 coupled to the output 106 of the power amplifier 102. A transformation network 108 is connected between the antenna 104 and the output 106 of the power amplifier 102. The input 110 of the power amplifier 102 is connected to an RF input signal RFI.
During operation, the power amplifier 102 amplifies the input signal RFI to achieve a desired output power at the antenna 104. A power detector, such as directional coupler 112, is used to detect the output power. The directional coupler 112 generates an power control signal 114 which is provided to power control circuitry 116. The power control circuitry 116 has a first input 118 for receiving an input signal relating to the requested power (i.e., the desired power level provided to the antenna 104). The desired power level may depend on various factors such as the physical distance between a cellular phone and a base station (in a cellular phone environment). The power control circuitry 116 generates a control signal 120 which is provided to the power amplifier 102 to control the gain of the power amplifier 102.
As mentioned above, it is desirable to limit the peak voltages applied to active devices of a power amplifier resulting from a load mismatches, temperature extremes, device variations, etc. The present invention utilizes a peak detector 122 to detect the presence of peak voltages at any critical nodes of the power amplifier 102 and create a peak detection signal. An example of one suitable peak detector is described below. In the example shown in FIG. 1, the input 124 of the peak detector 122 is connected to the output 106 of the power amplifier 102. In other examples, the peak detector could be connected to other critical nodes of the power amplifier 102. In addition, the peak detector 122 (or multiple peak detectors) may be connected to multiple nodes of the power amplifier 102 to detect peak voltages at multiple nodes. The output 126 of the peak detector 122 is provided as an input to the power control circuitry 116. The power control circuitry 116 uses the output 126 from the peak detector 122 to control the power amplifier 102 in such a way that dangerous peak voltages are avoided or minimized.
FIG. 2 is a flowchart illustrating the operation of the power control circuitry shown in FIG. 1. As mentioned above, the power control circuitry 116 generates a control signal 120 based on three inputs. These inputs include the desired output power level of the power amplifier 102 (input 118), the actual detected power output level (power control signal 114), and the output of the peak detector 122 (output 126). At step 2-10, the peak voltage is detected by the peak detector 122. Next, at step 2-12, the output power is detected by the directional coupler 112. Note that the order of the steps illustrated in FIG. 2 is not essential to the invention. At step 2-14, it is determined whether the detected output power (step 2-12) is less than or equal to the requested power (as determined by the signal at input 118). If the detected output power is less than or equal to the requested power, the process proceeds to step 2-16 where it is determined whether the detected peak voltage (step 2-10) is less than the a threshold voltage (i.e., a maximum allowed voltage). The maximum allowed voltage can relate to a voltage level that does not adversely affect the active devices of the power amplifier 102, but at the same time is adequate to deliver a suitable output power level to the antenna 104. If the detected peak voltage is less than the maximum allowed voltage, the process proceeds to step 2-18 where the gain of the power amplifier 102 is increased. While FIG. 2 shows the process ending at that point, during use, the process will repeat. If, at step 2-14, it is determined that the detected output power is greater than the requested power, then the process proceeds to step 2-20 where the gain of the power amplifier 102 is decreased. Similarly, if it is determined at step 2-16 that the detected peak voltage is greater than or equal to the maximum allowed voltage, the process proceeds to step 2-20 where the gain of the power amplifier 102 is decreased.
In general, the power control circuitry 116 will adjust the gain of the power amplifier 102 until the output signal power matches the requested power. In the example described, the power control circuitry 116 increases the gain of the power amplifier 102 when the detected output power is less than the desired output power and decreases the gain when the detected output power is greater than the desired output power. However, even if the detected output power is less than the desired output power, the power control circuitry 116 will decrease the gain of the power amplifier 102 (and thereby limiting the power at the output) if the peak detector 122 has detected a peak voltage. In this way, the active devices of the power amplifier 102 are protected from high voltages, which could lead to device breakdown and overall reliability problems. The power control circuitry 116 may implement the algorithm described using analog or digital signal processing using many different techniques well known in control theory.
In some implementations, for example, if the invention is implemented using CMOS, the peak detection circuitry can be difficult to design and build with a desired accuracy. FIG. 3 is a block diagram of a circuit similar to the circuit shown in FIG. 1 illustrating one implementation of a peak detector. The circuitry illustrated in the block diagram of FIG. 3 may be used with a non-linear power amplifier and utilizes two simple peak detector circuits. FIG. 3 shows a circuit 300 which includes a power amplifier 102 and an antenna 104 coupled to the output 106 of the power amplifier 102. A transformation network 108 is connected between the antenna 104 and the output 106 of the power amplifier 102. The input 110 of the power amplifier 102 is connected to an RF input signal RFI. A directional coupler 112 generates a power control signal 114 which is provided to power control circuitry 116. The power control circuitry 116 is coupled to the power amplifier 102 and to peak detector 322.
Peak detector 322 is implemented using a first peak detector 324 and a second peak detector 326. The first peak detector 324 has an input 328 which is coupled to the output 106 of the power amplifier 102. In the example shown in FIG. 3, the input 328 is connected to a divider circuit formed by capacitors C1 and C2 connected between the power amplifier output 106 and ground. Of course, other implementations are possible. The peak detector 326 has an input 330, which is coupled to a reference tone. The reference tone may be comprised of a signal having a known amplitude at the carrier frequency. In another example, the reference tone may be comprised of a constant amplitude modulated signal (e.g., the RF input in a GSM system which has a constant amplitude and consists of GMSK modulation). The reference tone may be provided from an existing signal in the device. For example, the reference tone could come from the transmit signal of the power amplifier 102 prior to final stage amplification. In this implementation, the peak detectors 324 and 326 are matched so there are no absolute accuracy requirements on the peak detectors. The output 126 of the peak detector 322 is generated by subtracting the output of the second peak detector 326 from the output of the first peak detector 324. The output 126 provides a peak feedback signal (PFB) represented by the following the equation:




  
    
      
        
          PFB
          =
          
            [
            
              
                
                  
                    C
                    2
                  
                  
                    
                      C
                      1
                    
                    +
                    
                      C
                      2
                    
                  
                
                ⁢
                
                  (
                  peak_RFO
                  )
                
              
              -
              
                (
                peak_tone
                )
              
            
            ]
          
        
        ,
      
    
    
      
        (
        1
        )
      
    
  




where “peak_RFO” is the peak voltage of the output of the power amplifier 102 as detected by the first peak detector 324 and “peak_tone” is the peak voltage of the reference tone as detected by the second peak detector 326. If the peak feedback signal FBS at output 126 is positive, then the peak voltage detected by the peak detector 322 is too high. Otherwise, the peak voltage is satisfactory.

Note that the peak feedback signal at output 126 may be generated using various types of peak detectors. In one example, the peak detectors may be comprised of conventional peak detectors that simply detect the peak voltage of a signal. In another example, where the signal detected is of a known type (such as a sine wave, square wave, etc.), each peak detector may be provided by the combination of an RMS detector followed by a correction circuit. Other examples may include other types of circuits that can detect some function of the waveform that relates to the peak voltage of the waveform.
In the case where the invention is used with applications where power is ramped up from zero in a controlled manner, the invention will not allow peak voltages on the output of the power amplifier to get higher than the maximum allowed voltage. This protects the active devices in the power amplifier, while limiting the ability of the power amplifier to deliver power to the load. FIGS. 4 and 5 are timing diagrams illustrating the use of the invention in applications where power is ramped. FIG. 4 shows an example where no peak voltages are detected which exceeds the maximum allowed peak voltage. FIG. 4 shows a first plot 410 which represents the requested power signal at input 118 shown in the Figures. As shown, the plot 410 starts at zero and ramps up until it reaches a desired level (e.g., 1 W). A second plot 412 is shown which represents the power measured by the directional coupler. Since no excessive peak voltages were detected in this example, the measured power ramps up and down along with the requested power signal.
FIG. 5 shows an example where a peak voltage is detected that exceeds the maximum allowed peak voltage. The arrow 508 shown in FIG. 5 illustrates the point at which a peak voltage is detected by the peak detector. When the peak voltage is detected, the power control circuitry protects the active devices in the power amplifier by reducing the gain and thus the output power of the power amplifier. In the example shown, the power measured at the output of the power amplifier (plot 512) is reduced to 0.75 Watts.
In the preceding detailed description, the invention is described with reference to specific exemplary embodiments thereof. Various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.

## Claims

1. A circuit for protecting devices in an RF power amplifier comprising:
a first detector coupled to the output of the RF power amplifier for detecting the output power of the RF power amplifier;
a second detector coupled to one or more nodes in the RF power amplifier for detecting voltages at the one or more nodes, wherein the second detector is comprised of a first voltage detector coupled to the one or more nodes and a second voltage detector coupled to a reference tone;
control circuitry coupled to the first and second detectors and to the power amplifier for controlling the gain of the power amplifier based on the output power of the RF power amplifier detected by the first detector and the detected voltages of the one or more nodes detected by the second detector.

2. The circuit of claim 1, wherein the second detector is a peak detector.

3. The circuit of claim 2, wherein:
the first voltage detector has an input coupled to the one or more nodes; and
a the second voltage detector has an input coupled to the reference tone.

4. The circuit of claim 3, wherein outputs of the first and second voltage detectors are combined to provide a peak detection signal to the control circuitry.

5. The circuit of claim 4, wherein the first and second voltage detectors are matched.

6. The circuit of claim 4, wherein the outputs of the first and second voltage detectors are combined by subtracting the output of the second voltage detector from the output of the first voltage detector.

7. The circuit of claim 4, further comprising a divider circuit coupled between the first voltage detector and the one or more nodes.

8. The circuit of claim 7, wherein the divider is comprised of first and second capacitors coupled between the one or more nodes.

9. The circuit of claim 1, wherein the control circuitry decreases the gain of the power amplifier when the second detector detects a voltage above a voltage threshold.

10. The circuit of claim 1, wherein the control circuitry protects devices in the RF power amplifier by decreasing the gain of the power amplifier when the second detector detects a voltage above a voltage threshold at the one or more nodes in the RF power amplifier.

11. A method of protecting devices in an RF power amplifier comprising:
detecting the output power of the RF power amplifier;
detecting the voltage at a first node of the power amplifier by providing a first detector coupled to the first node and a second detector coupled to a reference tone;
determining whether the detected voltage at the first node is higher than a threshold voltage; and
if it is determined that the detected voltage is higher than the threshold voltage, decreasing the gain of the power amplifier.

12. The method of claim 11, wherein the first and second detectors are peak detectors.

13. The method of claim 12, wherein the first and second peak detectors are matched.

14. The method of claim 11, wherein the gain of the power amplifier is increased if the detected output power is less than a desired power level, and if the detected voltage of the first node is less than the threshold voltage.

15. The method of claim 12, wherein the gain of the power amplifier is decreased if the detected output power is greater than a desired power level, or if the detected voltage of the first node is higher than the threshold voltage.

16. A method of controlling an RF power amplifier comprising the steps of:
detecting the output power of the RF power amplifier;
detecting the voltage at a first node of the RF power amplifier by providing a first detector coupled to the first node and a second detector coupled to a reference tone;
increasing the gain of the power amplifier if the detected output power is less than a desired output power level and if the detected voltage does not exceed a threshold voltage; and
decreasing the gain of the power amplifier if the detected output power is greater than the desired output power level or if the detected voltage exceeds a threshold voltage.

17. The method of claim 16, wherein the first and second detectors are peak detectors.

18. The method of claim 17, wherein the first peak detector and the second peak detector are matched.

19. The method of claim 17, wherein the outputs of the first and second peak detectors are combined by subtracting the output of the second peak detector from the output of the first peak detector.