Abstract:
A transmitter system includes a transmitter with a power amplifier. An antenna communicates with the power amplifier. A protection circuit generates a sensed signal that varies with an impedance of the antenna and reduces an output of the power amplifier in response to the sensed signal exceeding a predetermined threshold.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 10/839,679 filed May 4, 2004, which is a continuation of U.S. patent application Ser. No. 10/246,870, filed Sep. 19, 2002 (now U.S. Pat. No. 6,856,200). The disclosures of the above applications are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to power amplifiers, and more particularly to a protection circuit and method for a radio frequency (RF) power amplifier implemented a wireless local area network (WLAN) transceiver. 
     BACKGROUND OF THE INVENTION 
     Referring now to  FIG. 1 , a wireless transceiver  10  is shown and includes a transmitter  12  and a receiver  14 . The wireless transceiver  10  may be used in a local area network (LAN) and may be attached to a Baseband Processor (BBP) and a Media Access Controller (MAC) in either a station or an Access Point (AP) configuration. A network interface card (NIC) is one of the various “STATION” configurations. The NIC can be connected to a networked device  16  such as a laptop computer, a personal digital assistant (PDA) or any other networked device. When the transceiver  10  is attached to an access point (AP) MAC, an AP is created. The AP provides network access for WLAN stations that are associated with the transceiver  10 . 
     The wireless transceiver  10  transmits and receives frames/packets and provides communication between two networked devices. In AdHoc mode, the two devices can be two laptop/personal computers. In infrastructure mode, the two devices can be a laptop/personal computer and an AP. 
     There are multiple different ways of implementing the transmitter  12  and the receiver  14 . For purposes of illustration, simplified block diagrams of super-heterodyne and direct conversion transmitter and receiver architectures will be discussed, although other architectures may be used. Referring now to  FIG. 2A , an exemplary super-heterodyne receiver  14 - 1  is shown. The receiver  14 - 1  includes an antenna  19  that is coupled to an optional RF filter  20  and a low noise amplifier  22 . An output of the amplifier  22  is coupled to a first input of a mixer  24 . A second input of the mixer  24  is connected to an oscillator  25 , which provides a reference frequency. The mixer  24  converts radio frequency (RF) signals to intermediate frequency (IF) signals. 
     An output of the mixer  24  is connected to an optional IF filter  26 , which has an output that is coupled to an automatic gain control amplifier (AGCA)  32 . An output of the AGCA  32  is coupled to first inputs of mixers  40  and  41 . A second input of the mixer  41  is coupled to an oscillator  42 , which provides a reference frequency. A second input of the mixer  40  is connected to the oscillator  42  through a −90° phase shifter  43 . The mixers  40  and  41  convert the IF signals to baseband (BB) signals. Outputs of the mixers  40  and  41  are coupled to BB circuits  44 - 1  and  44 - 2 , respectively. The BB circuits  44 - 1  and  44 - 2  may include low pass filters (LPF)  45 - 1  and  45 - 2  and gain blocks  46 - 1  and  46 - 2 , respectively, although other BB circuits may be used. Mixer  40  generates an in-phase (I) signal, which is output to a BB processor  47 . The mixer  41  generates a quadrature-phase (Q) signal, which is output to the BB processor  47 . 
     Referring now to  FIG. 2B , an exemplary direct receiver  14 - 2  is shown. The receiver  14 - 2  includes the antenna  19  that is coupled the optional RF filter  20  and to the low noise amplifier  22 . An output of the low noise amplifier  22  is coupled to first inputs of RF to BB mixers  48  and  50 . A second input of the mixer  50  is connected to oscillator  51 , which provides a reference frequency. A second input of the mixer  48  is connected to the oscillator  51  through a −90° phase shifter  52 . The mixer  48  outputs the I-signal to the BB circuit  44 - 1 , which may include the LPF  45 - 1  and the gain block  46 - 1 . An output of the BB circuit  44 - 1  is input to the BB processor  47 . Similarly, the mixer  50  outputs the Q signal to the BB circuit  44 - 2 , which may include the LPF  45 - 2  and the gain block  46 - 2 . An output of the BB circuit  44 - 2  is output to the BB processor  47 . 
     Referring now to  FIG. 3A , an exemplary super-heterodyne transmitter  12 - 1  is shown. The transmitter  12 - 1  receives an I signal from the BB processor  47 . The I signal is input to a LPF  60  that is coupled to a first input of a BB to IF mixer  64 . A Q signal of the BB processor  47  is input to a LPF  68  that is coupled to a first input of a BB to IF mixer  72 . The mixer  72  has a second input that is coupled to an oscillator  74 , which provides a reference frequency. The mixer  64  has a second input that is coupled to the oscillator through a −90° phase shifter  75 . 
     Outputs of the mixers  64  and  72  are input to a summer  76 . The summer  76  combines the signals into a complex signal that is input to a variable gain amplifier (VGA)  84 . The VGA  84  is coupled to an optional IF filter  85 . The optional IF filter  85  is connected to a first input of an IF to RF mixer  86 . A second input of the mixer  86  is connected to an oscillator  87 , which provides a reference frequency. An output of the mixer  86  is coupled to an optional RF filter  88 . The optional RF filter  88  is connected to a power amplifier  89 , which may include a driver. The power amplifier  89  drives an antenna  90  through an optional RF filter  91 . 
     Referring now to  FIG. 3B , an exemplary direct transmitter  12 - 2  is shown. The transmitter  12 - 2  receives an I signal from the BB processor  47 . The I signal is input to the LPF  60 , which has an output that is coupled to a first input of a BB to RF mixer  92 . A Q signal of the BB processor  47  is input to the LPF  68 , which is coupled to a first input of a BB to RF mixer  93 . The mixer  93  has a second input that is coupled to an oscillator  94 , which provides a reference frequency. The mixer  92  has a second input that is connected to the oscillator  94  through a −90° phase shifter  95 . Outputs of the mixers  92  and  93  are input to the summer  76 . The summer  76  combines the signals into a complex signal that is input the power amplifier  89 . The power amplifier  89  drives the antenna  90  through the optional RF filter  91 . The RF and IF filters in  FIGS. 2A ,  2 B,  3 A and  3 B may be implemented on-chip or externally. 
     The power amplifier drives current to create magnetic fields that propagate from the antenna. Typically, the impedance of the antenna has a specific value for the operating frequency of the power amplifier. For example, a WLAN transmitter operating at 2.4 GHz typically has a 50Ω antenna. When the impedance of the antenna changes, the voltage across one or more transistors in the power amplifier may substantially increase. If the voltage exceeds a breakdown voltage of the output transistor, then the transistor may be damaged. 
     For example, the impedance of the antenna changes when users inadvertently handle the antenna or when the antenna contacts other objects. The impedance of the antenna also varies with the operating frequency of the transmitter. Some conventional transceivers are implemented using transistor technologies (such as GaAs) that have high breakdown voltages. However, the selection of the transistor technology may be based on other design considerations such as cost and the selected transistor technology may not have a high breakdown voltage. 
     SUMMARY OF THE INVENTION 
     A protection circuit for a power amplifier of a radio frequency transmitter according to the invention includes a sensing circuit that generates a sensed signal based on an output of the power amplifier. A reference signal generator generates a reference signal. A comparator communicates with the sensing circuit and the reference signal generator and outputs a first state when the reference signal exceeds the sensed signal and a second state when the sensed signal exceeds the reference signal. 
     In other features, the comparator communicates with the power amplifier and turns off the power amplifier when the comparator is in the second state. The sensing circuit includes first and second transistors having a gate that is biased at a first voltage, sources that communicate with positive and negative outputs of the power amplifier, and drains that communicate with one input of the comparator. A current source communicates with the drains of the first and second transistors. 
     In yet other features, a turn-off circuit turns the power amplifier off when the comparator is in the second state. The turn-off circuit includes third and fourth transistors that communicate with inputs of the power amplifier. Gates of the third and fourth transistors communicate with the comparator. Alternately, the turn-off circuit includes a fifth transistor that shorts inputs of the power amplifier when the comparator is in the second state. 
     A radio frequency (RF) wireless local area network (WLAN) transceiver according to the present invention includes a power amplifier that includes a transistor. A protection circuit senses an output voltage of the power amplifier and turns off the power amplifier when the output voltage exceeds a predetermined level to prevent damage to the transistor. 
     In other features, the protection circuit includes a sensing circuit that generates a sensed signal based on an output voltage of the power amplifier. The protection circuit includes a reference signal generator that generates a reference signal. The protection circuit includes a comparator that communicates with the sensing circuit and the reference signal generator. The comparator outputs a first state when the reference signal exceeds the sensed signal and a second state when the sensed signal exceeds the reference signal. The comparator communicates with the power amplifier and turns off the power amplifier when the comparator is in the second state. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a functional block diagram of a wireless local area network (WLAN) transceiver according to the prior art; 
         FIG. 2A  is a functional block diagram of an exemplary super-heterodyne receiver architecture according to the prior art; 
         FIG. 2B  is a functional block diagram of an exemplary direct receiver architecture according to the prior art; 
         FIG. 3A  is a functional block diagram of an exemplary super-heterodyne transmitter architecture according to the prior art; 
         FIG. 3B  is a functional block diagram of an exemplary direct transmitter architecture according to the prior art; 
         FIG. 4  is a functional block diagram of a radio frequency transmitter and a power amplifier protection circuit according to the present invention; 
         FIG. 5  is a functional block diagram of the power amplifier protection circuit of  FIG. 4  in further detail; 
         FIG. 6  is an electrical schematic of a first exemplary implementation of the power amplifier protection circuit; 
         FIG. 7  is an electrical schematic showing the antenna coupled to the output stage of the power amplifier; and 
         FIG. 8  is an electrical schematic of a second exemplary implementation of the power amplifier protection circuit. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements. 
     Referring now to  FIG. 4 , a transmitter  120  includes transmitter circuits  122  (such as those depicted in  FIGS. 3A and 3B ), a power amplifier  124 , and an antenna  126 . A power amplifier protection circuit  130  monitors a voltage output of the power amplifier  124 . The power amplifier protection circuit  130  shuts down the power amplifier  124  when the output voltage exceeds a predetermined voltage value to prevent damage to the power amplifier  124 . For example, the power amplifier protection circuit  130  prevents damage that may occur when a user or an object touches the antenna  126 , alters the impedance of the antenna, and causes a voltage increase. 
     Referring now to  FIG. 5 , the power amplifier protection circuit  130  is illustrated in further detail. The power amplifier protection circuit  130  includes a sensing circuit  140  that senses an output of the power amplifier  124 . An output of the sensing circuit  140  is input to a comparator  144 . A reference signal generator  146  generates a reference signal. An output of the reference signal generator  146  is also input to the comparator  144 . When the output of the sensing circuit  140  exceeds an output of the reference signal generator  146 , the comparator  144  changes state and turns off the power amplifier  124 . 
     Referring now to  FIG. 6 , a first exemplary implementation of the power amplifier protection circuit  130 - 1  is illustrated. The power amplifier protection circuit  130 - 1  communicates with an output stage  150  of a power amplifier. The output stage  150  is typically coupled by capacitors  154  and  156  to other transmitter circuits. A negative input in n  is coupled to a gate of a first transistor  158 . A positive input in p  is coupled to a gate of a second transistor  160 . A first inductor  164  is connected between a voltage source V DD  and a drain of the first transistor  158 . A second inductor  166  is connected between the voltage source V DD  and a drain of the second transistor  160 . The sources of the transistors  158  and  160  are coupled to a common potential such as ground. Positive and negative outputs V outp  and V outn , which drive the antenna, are taken between the inductors  164  and  166  and the drains of the transistors  158  and  160 . In  FIG. 7 , the output stage of the differential power amplifier is typically coupled through an output transformer, which performs differential to single-ended conversion. 
     The power amplifier protection circuit  130 - 1  includes first and second transistors  180  and  182  having drains connected to the gate of the transistors  158  and  160 , respectively. Gates of the transistors  180  and  182  are connected to an output of a comparator  186 . A first input of the comparator  186  is connected to a reference signal V th . 
     The outputs V outp  and V outn  of the output stage  150  are connected to sources of transistors  190  and  192 . Gates of the transistors  190  and  192  are connected to V bias . Drains of the transistors  190  and  192  are connected together, to a current source  194 , and to a second input of the comparator  186 . 
     In use, the voltage V bias  is set above the normal operating voltage of the transistors  158  and  160 . Transistors  190  and  192  are off under normal operating conditions with proper signal voltage at the drains of transistors  158  and  160 . Since neither transistors  190  and  192  are conducting, current source  194  will pull the second input of the comparator  186  towards ground potential. When the operating voltage exceeds V bias , the transistors  190  and  192  begin conducting. The non-inverting input exceeds the threshold voltage of the comparator  186  and the comparator  186  changes state. The comparator  186  biases the gates of the transistors  180  and  182 , which begin conducting. The inputs to the output stage  150  of the power amplifier are shorted to ground and the power amplifier is turned off. When the operating voltage falls below the V bias , the transistors  190  and  192  stop conducting and the comparator  186  changes state. The comparator  186  turns off the transistors  180  and  182  and normal operation of the power amplifier can be resumed if the effective impedance of the antenna returns to nominal range. 
     Referring now to  FIG. 8 , a second exemplary implementation of the power amplifier protection circuit is illustrated at  130 - 2 . The transistors  180  and  182  are replaced by a transistor  200 . The transistor  200  shorts the gates of the transistors  158  and  160  when the comparator  186  changes state when the second signal exceeds the reference signal. This will suppress the AC signals applied to the PA and reduce signal swing at PA outputs, which prevents transistor breakdown or overstress. 
     In the exemplary implementations in  FIGS. 6 and 8 , CMOS technology is employed. Transistors  158 ,  160 ,  180 ,  182  and  200  are implemented using n-channel CMOS transistors. Transistors  190  and  192  have been implemented using p-channel CMOS transistors. Skilled artisans will appreciate that the present invention has application to other transistor technologies having low breakdown voltages and that these other transistor technologies may be employed without departing from the scope of the present invention. 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.