Abstract:
A system for sensing RF amplifier output power includes an amplifier transistor and a sampling transistor that is physically smaller than the amplifier transistor. The sampling transistor is configured to sample the same RF input signal that is amplified by the amplifier transistor. A bias circuit associated with the transistors includes a selection of components based upon operating parameters as well as actual physical sizes of the transistors. The selection of component values in association with transistor sizes is used to enable generation of a current sensing signal that is proportional to the power level of the RF output signal generated by the amplifier transistor.

Description:
BACKGROUND OF THE INVENTION 
     (1). Field of the Invention 
     The present invention generally relates to radio frequency (RF) power amplifier output power detection techniques and, more particularly, to a system and method for sensing power consumed by a RF power amplifier. 
     (2). Description of the Prior Art 
     Portable wireless transmitter systems generally require that a specified radio frequency (RF) output power be delivered to the radiating antenna. Further, many such systems are required to back-off or reduce the transmitted power to achieve a specific level depending upon signal strength. To meet the aforesaid requirements, the system architecture generally incorporates a closed-loop power control scheme. Typically, this scheme requires a “sampling” of the RF power amplifier output power that is subsequently fed back to predetermined control circuitry which generates a control signal that adjusts the output power until it is within the specified power level. Such sampling of the output power is disadvantageous in that it increases the insertion loss between the output of the power amplifier and the radiating antenna. Therefore, sampling of the output power increases the required output power from the power amplifier and reduces the overall talk time that is available. Talk time is a measure of the time a portable transceiver can be in the “talk” mode before the battery is fully depleted. Because the power amplifier delivers the largest signal within a portable transceiver (transmitter), it consumes the majority of the current and therefore dominates in the calculation of talk time. 
     A common technique for sampling the output power includes use of a directional coupler on the output of the power amplifier. The power coupled from the main signal path is diode detected to generate a video signal proportional to the output voltage delivered to the antenna. Use of directional couplers, however, adds loss to the system, forcing the power amplifier to deliver more power thereby reducing the talk time of the associated radio unit. In typical applications, the aforesaid loss is often 5-10% of the power amplifier output power and relates to a direct loss in available talk time. 
     Another common technique for detecting the output power includes measurement of the current consumed by the power amplifier. This current is directly related to the output power generated by the power amplifier and is also fed back to predetermined power control leveling circuitry. This technique is also disadvantageous due to the loss associated with the current measurement. This current measurement generally requires that a series “dropping” element be added between the associated battery and the power amplifier bias input. The voltage across this element will determine the current entering the power amplifier (for a known resistance across the element). In typical applications, the voltage across the dropping element will be about 3% of the total battery voltage. Because this is a loss in the dc input power to the power amplifier, the loss of talk time will be even higher than 3% due to the less than 100% dc-rf conversion efficiency of the power amplifier. For example, if the power amplifier efficiency is 60%, then the talk time loss will be 3/0.6 or 5%. 
     Thus, there remains a need for a new and improved technique for current sensing associated with RF amplifier power detection. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a system and method relating to current sensing to detect RF amplifier output power. A small current sensing transistor is added in parallel with a large transistor that delivers the high power to the antenna. The current in this sensing transistor is proportional to the current in the large transistor. The current in the large transistor is directly related to the output power generated by the power amplifier. By using a small current sensing transistor, talk time loss is reduced by less than 1%. In one embodiment, a small transistor is added in parallel with the much larger output transistor. This small transistor is dc biased and RF driven proportionally to the large output transistor. The small transistor (sampling transistor) is about {fraction (1/250)} th  the size of the larger output transistor which results in a scaling factor such that the additional current required for sensing is about 0.4% of the total current consumed by the power amplifier. 
     Accordingly, one feature of the present invention includes a technique for measurement of RF amplifier output power that reduces talk time by less than 1%. 
     Another feature of the present invention includes a technique for measurement of RF amplifier output power that is more efficient than known measurement techniques. 
     Still another feature of the present invention includes a technique for measurement of RF amplifier output power which consumes about an order of magnitude less power than that consumed by using known techniques. 
     These and other features of the present invention will become apparent to those skilled in the art after a reading of the following description of the preferred embodiment when considered with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a simplified schematic diagram illustrating a traditional method of RF power sensing known in the art. 
     FIG. 2 is a simplified schematic diagram illustrating a current sensing system constructed according to one embodiment of the present invention. 
     FIG. 3 is a more detailed schematic diagram illustrating a current sensing system according to another embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following description, like reference characters designate like or corresponding parts throughout the several views. Referring now to the drawings in general and FIG. 1 in particular, it will be understood that the illustrations are for the purpose of describing a preferred embodiment of the invention and are not intended to limit the invention thereto. In FIG. 1, a simplified schematic diagram illustrates a traditional RF power sensing system known in the art, generally designated  100 . As stated herein, many RF systems need to know how much power is being supplied. The classical solution is to obtain samples of the RF energy being supplied via a coupling scheme, rectify the samples and use the resulting voltage as an indication of the power being supplied. A pair of coupled transmission lines  102  are used as the directional coupler. The power coupled from the main signal path  104  is detected via a diode  106 , i.e. Schottky diode to generate a video signal, e.g. PWR_Sense  108 , proportional to the RF output voltage delivered to the antenna (not shown). As stated above, such directional couplers  102  add loss to the system  100 , forcing the power amplifier  110  to deliver more power, thus reducing the valuable talk time of the associated radio (not shown). 
     FIG. 2 illustrates a simplified schematic diagram showing a current sensing system  200  constructed according to one embodiment of the present invention. A radio frequency signal is received by the system  200  at the rf-in signal port  202  where it is coupled to a RF amplifier transistor  204  via coupling capacitor  206 . An output matching and bias network  208  provides the necessary dc power and antenna matching to provide efficient transmission of the amplified RF signal to the associated antenna  210 . The output matching and bias network  208  details are not important to the present invention and are not discussed herein to preserve brevity and enhance clarity associated with the present discussion of the invention. It is sufficient to state that a typical output matching network generally consists of a combination of inductors and capacitors configured to provide an impedance match between the RF amplifier transistor  204  output impedance and the antenna  210  input impedance. FIG. 2 also illustrates an implementation of RF output power sensing that is accomplished in part via a small RF signal sensing (sampling) transistor  212 . The small RF sampling transistor  212  is deployed in parallel with the RF amplifier transistor  204  that is physically much larger than the small sampling transistor  212 . Coupling a small amount of the drive energy from the output stage into a small parallel device provides an integrated approach for indicating the power supplied by an amplifier. The small sampling transistor  212  is dc biased via a dedicated bias network  214 . Details of the dc bias network  214  are not central to the present invention and so will not be discussed herein except to state that such bias schemes are well known to those skilled in the art and may consist of nothing more than a voltage source coupled to the small sampling transistor  212  via a resistor (not shown). In one embodiment, the size of the small sampling transistor  212  is about {fraction (1/250)} th  the size of the RF amplifier transistor  204 . The present invention is not so limited however, and it shall be understood that other ratios associated with sizing of the RF amplifier transistor  204  and the small sampling transistor  212  will also be effective to practice the present invention. For example, the aforesaid ratio could just as well be any number between 50 and 500. Larger ratios could also be used in association with dedicated applications and processing technologies. With a scaling factor of 250, the additional current required for sensing is only about 0.4% of the total current consumed by the current sensing system  200  RF amplifier transistor  204 . This is an order of magnitude less than the loss incurred by power measurement and sensing systems known in the art. For example, directional couplers reduce a typical transmitter&#39;s total efficiency by about 5% or more. 
     FIG. 3 illustrates a more detailed schematic diagram of a current sensing system  300  according to another embodiment of the present invention. With reference to the foregoing discussion above regarding ratio sizes, a typical ratio of device sizes for RF amplifier transistor  302  and sampling transistor  304  can be 224 to 1. In this case, the RF signal coupling capacitors  306 ,  308  will employ a similar ratio while the bias resistors  310 ,  312  will optimally employ a ratio of 1 to 224. The values for coupling capacitor  306  and bias resistor  310  will be set by the design value associated with the specific application parameters. In operation, the RF drive signal through capacitor  306  and capacitor  308  turns on the RF amplifier transistor  302  and the sampling transistor  304  proportional to the RF signal amplitude. The current flowing through the sampling transistor  304  then flows through bias resistor R 2  and reduces the voltage across capacitor C 2 . The output power can then be determined by sensing the bias current flowing through bias resistor R 2  at the PWR_Sense junction  314  using techniques familiar to those skilled in the art of voltage/current transformations. As illustrated in FIG. 2, it can be seen that the varying voltage across capacitor C 2  can also be used in a feedback loop to change the gain of the RF amplifier transistor  204 . A differential amplifier  218  compares a reference voltage  220  with the changing voltage across capacitor C 2  and changes the DC quiescent bias current to affect a change in the RF amplifier transistor  204  gain characteristics. The output power can thus be increased or decreased as desired. Most preferably, the RF signal coupling capacitors  306 ,  308 , bias resistors  310 ,  312  and transistors  302 ,  304  are integrated on the same die, leaving the connections to transistor collectors open to maximize flexibility in configuring the power sense option desired. 
     Certain modifications and improvements will occur to those skilled in the art upon a reading of the foregoing description. By way of example, the embodiments described herein above are based upon specific circuit architectures. The present invention is not so limited however. It shall be understood that those skilled in the art can use a wide variety of circuit architectures including electronic and electromechanical components, either integrated or discrete or combinations thereof to practice the present invention so long as the transistor ratio functionality described herein is retained. Also, it shall be understood that the present invention is not limited to use of a particular processing technology, e.g. HBT, Silicon BJT, CMOS, and the like. It should be understood that all such modifications and improvements have been deleted herein for the sake of conciseness and readability but are properly within the scope of the following claims.