Abstract:
An adaptive bias control circuit for use with a radio frequency (RF) power amplifier, the RF power amplifier having an input ( 112 ) for receiving an input signal having a varying amplitude, an output ( 116 ), a first transistor ( 110 ), and a plurality of operating performance characteristics responsive to a quiescent operating point established by a bias current in the RF power amplifier, the bias control circuit having: a first circuit ( 120 ) coupled to the RF power amplifier for receiving a portion of the input signal; and a second transistor ( 122 ) for generating a rectified signal from the portion of the input signal, the rectified signal for causing the bias current to be controlled as a function of the amplitude of the input signal, the second transistor having a first and second terminal connected together and coupled to the first circuit and a third terminal coupled to a ground potential.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to radio frequency (RF) power amplifiers, and more specifically to a circuit for causing a bias current in the RF power amplifier to be controlled as a function of an input signal into the RF power amplifier having varying amplitude.  
         BACKGROUND OF THE INVENTION  
         [0002]    Radio frequency (RF) power amplifiers are used in a wide variety of communications and other electronic applications. These amplifiers are 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 stage gain. 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 characteristic. This non-linearity adversely affects various amplifier operating characteristics such as gain performance, intermodulation performance and efficiency.  
           [0003]    Non-linear amplifier transfer characteristics give rise to a phenomenon, hereinafter referred to as gain expansion. Gain expansion is caused by the change in the amplifier&#39;s base-emitter voltage due to rectification of input signal power in the base-emitter junction. In effect, the input signal power to an RF amplifier changes the amplifier&#39;s quiescent operating point. As a result, an RF amplifier&#39;s gain will increase as a function of the input signal power, thereby giving rise to the gain expansion phenomenon. Gain expansion is typically an undesirable characteristic exhibited by RF power amplifiers. This is especially true when the amplifier must operate across a  
           [0004]    wide dynamic range of input signals, like the multi-tone linear power amplifiers disclosed in U.S. Pat. No. 5,307,022, entitled HIGH DYNAMIC RANGE MODULATION INDEPENDENT FEED FORWARD AMPLIFIER NETWORK and assigned to the assignee of the present application. In such multi-tone applications, constant amplifier gain over a wide dynamic range of input signals is required.  
           [0005]    This same non-linearity causes distortion of the amplifier&#39;s output signal so that it is no longer a perfect replica of the input signal. This distortion produces spurious  
           [0006]    signal components known as intermodulation products. Intermodulation products are typically undesirable because they cause interference, cross talk, and other deleterious effects on the performance of a system employing the amplifier. Of note, the quantity of intermodulation products generated by the amplifier is directly proportional to the magnitude of the signal applied to the amplifier&#39;s input.  
           [0007]    Yet another RF power amplifier operating characteristic hampered by nonlinear transfers is the amplifier&#39;s efficiency. By definition, an amplifier&#39;s efficiency is determined by POUT/PIN. The more efficient an amplifier is, the less input power required to achieve a desirable output level. Since gain expansion tends to distort the amplifier&#39;s output power level, it has the undesirable effect of decreasing an amplifier&#39;s efficiency at low output powers.  
           [0008]    Accordingly, the prior art reflects various methods and devices designed to improve one or more of the amplifier&#39;s operating characteristics, typically at the expense of others. As will be appreciated, optimizing for any one parameter adversely effects the others, since they are all closely interrelated. Thus, while biasing the amplifier&#39;s quiescent operating point low tends to improve the amplifier&#39;s efficiency, intermodulation performance and saturation point, it nonetheless compromises the maximum gain available and the amplifier&#39;s gain flatness (constant gain over wide dynamic range). Conversely, while biasing the amplifier&#39;s quiescent operating point higher tends to improve the maximum gain available and the amplifier&#39;s gain flatness, it nonetheless compromises the amplifier&#39;s efficiency, intermodulation performance and saturation point. In addition, one or more of the RF amplifier&#39;s operating performance characteristics may be affected by temperature. However, the above prior art requires additional circuitry for temperature compensation.  
           [0009]    It would be extremely advantageous therefore to provide a compensation circuit capable of equalizing an RF power amplifier&#39;s operating characteristics over a wide dynamic range of inputs and over temperature, so that the RF power amplifier achieves optimum linearity while avoiding the shortcomings of the prior art. U.S. Pat. No. 5,311,143, entitled RF AMPLIFIER BIAS CONTROL METHOD AND APPARATUS discloses a bias control circuit that changes the appropriate bias point relative to the average power out of the RF power amplifier. However, this circuit requires multiple circuit components that can track the envelope of the input signal in order to change the bias point as a function of the envelope. Moreover, the circuit disclosed in U.S. Pat. No. 5,311,143 also requires additional circuitry for temperature compensation.  
           [0010]    Thus, there exists a need for a simple space effective, power effective and cost efficient circuit that controls the bias point of an RF power amplifier as a function of the input signal and that also has the benefits of temperature compensation. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0011]    A preferred embodiment of the invention is now described, by way of example only, with reference to the accompanying figures in which:  
         [0012]    [0012]FIG. 1 illustrates a diagram of an RF power amplifier network in accordance with the present invention; and  
         [0013]    [0013]FIG. 2 illustrates the transfer function of the bias control circuit illustrated in  
         [0014]    [0014]FIG. 1. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]    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.  
         [0016]    Referring to FIG. 1, there is illustrated a diagram of a radio frequency (RF) power amplifier network  100  according to the present invention. Typically, but not necessarily, network  100  is a single stage in a power amplifier system used, for instance, in a communications device, wherein the power amplifier system comprises a plurality of cascaded power amplifier networks like the one illustrated in FIG. 1. Network  100  signaling preferably anticipates both narrow bandwidth modulated signals and wide bandwidth modulated signals, such as, for example, a Frequency Division Multiple Access (FDMA) format and/or a Code Division Multiple Access (CDMA) format. In addition to comprising multiple modulation formats, the anticipated signaling environment of RF power amplifier network  100  is further characterized by input signals that exhibit a wide and dynamic range of input power levels (or amplitudes).  
         [0017]    Referring back to FIG. 1, RF power amplifier network  100  includes an RF power amplifier having a plurality of operating performance characteristics responsive to a quiescent operating point. The RF power amplifier comprises a transistor  110 . Preferably, transistor  110  is a lateral double-diffused metal-oxide semiconductor (LDMOS) field effect transistor (FET) having its source coupled to a ground potential. The RF power amplifier further comprises input port  112  for receiving the input signal, and, preferably, an input match circuit  114  coupled between input  112  and the gate of transistor  110  for effectively delivering the input power from the source load (not illustrated) to transistor  110 . The RF power amplifier still further comprises an output port  116  and, preferably, an output match circuit  118  coupled between output  116  and the drain of transistor  110  for effectively delivering the output power from transistor  110  to an output load (not illustrated).  
         [0018]    RF power amplifier network  100  further includes an adaptive bias control circuit according to the present invention. The bias control circuit comprises circuit  120  that is preferably coupled to the junction of the input match  114  and the gate of transistor  110 . Preferably circuit  120  comprises a resistor  121 . Circuit  120  may also instead comprise an inductor or some other similar, preferably, passive network that provides for the needed functionality. The bias control circuit further comprises a transistor  122  that is also, preferably, an LDMOS FET that is, preferably, a small fraction of the size of transistor  110 , ideally {fraction (1/100)} the size to be most effective in power consumption. Transistor  122  is connected common gate, common drain, and its source is coupled to a ground potential. The junction of the gate and drain of transistor  122  is further coupled to circuit  120 . Finally the bias control circuit, preferably, further comprises a circuit  124 . Circuit  124  preferably includes a resistor  126  and a DC voltage source  128  coupled in series to the junction of the gate and drain of transistor  122 . However, those of ordinary skill in the art will realize that circuit  124  may comprise additional components for performing the same functionality.  
         [0019]    RF power amplifier network  100  preferably functions as follows. Prior to the input signal being received into input port  112 , resistor  126  and voltage source  128  are used to set a predetermined drain current through transistor  122 . This drain current through transistor  122 , in turn, causes a DC bias voltage, Vbias, to be coupled through circuit  120  to the gate of transistor  110  for setting the quiescent operating point of the RF power amplifier, which in the case of an LDMOS FET is established by a bias current IDQ into the drain of transistor  110 . The values of resistor  126  and of voltage source  128  are selected to generate an IDQ that causes the RF power amplifier to be characterized in a particular class of operation. For instance, the RF power amplifier can be characterized as Class A. In that case, the values of resistor  126  and of voltage source  128  are, preferably, selected to cause the RF power amplifier to operate with optimal linearity.  
         [0020]    As stated above, once the input signal begins to be received into input port  112 , it may cause transistor  110  to exhibit some non-linearity, for instance, due to the input signal having varying amplitude. The bias control circuit compensates for such non-linearity by adjusting Vbias, the bias voltage on the gate of transistor  110 , for controlling the bias current IDQ as a function of the amplitude of the input signal, for maintaining optimal linearity as the amplitude of the input signal changes. Specifically, circuit  120  causes a portion of the input signal (determined by the size of resistor  121 ) to be coupled to transistor  122 . Transistor  122  rectifies this portion of the input signal, such that as the amplitude of the input signal increases, this causes transistor  122  to draw current and, thus, causes the bias voltage at the gate of transistor  110  to decrease, which decreases the bias current IDQ. Conversely, as the amplitude of the input signal decreases, this causes transistor  122  to draw less current and, thus, causes the bias voltage at the gate of transistor  110  to increase, which increases the bias current IDQ. In an LDMOS RF power amplifier, this is desirable because it is known that these type of amplifiers require a harder bias point at lower power out than at peak power out for optimal linearity operation. By adaptively controlling the bias point over the envelop of the input signal, an LDMOS power amplifier can be configured to have an optimal linearity condition over the envelope with overall efficiency improvement.  
         [0021]    [0021]FIG. 2 illustrates the transfer function of the adaptive bias control circuit illustrated in FIG. 1. FIG. 2 shows the decreasing bias voltage, Vbias, at the gate of transistor  110  as the power level at input port  112  increases, thereby adjusting the bias current IDQ as a function of the input power level in order to control the quiescent operating point of transistor  110  to optimize overall linearity and efficiency of transistor  110 .  
         [0022]    Preferably, many of the components of the RF power amplifier network  100  illustrated in FIG. 1 are housed in an integrated circuit (IC) chip. The dashed box  130  illustrates those components of RF power network  100  that are preferably on the same IC ship. Particularly transistor  110  is on the IC chip, and match circuits  114  and  118  may be either on- or off-chip. Preferably, circuit  120  and transistor  122  in the bias control circuit are also manufactured on the same IC chip with transistor  110 . Having both transistor  110  and transistor  122  on the same IC chip enables the bias control circuit of FIG. 1 to also maintain the desired bias point fixed over temperature since transistor  122  will thermally track transistor  110  and appropriately change the bias voltage to transistor  110 . The key to thermal tracking is that both transistors are on the same IC chip such that their thermal and electrical characteristics are essentially the same independent of temperature and process variations.  
         [0023]    An advantage of the present invention is that it uses a FET in the adaptive bias control circuit, which is fast enough to track the envelope of the input signal, thereby eliminating the need for the more complex and expensive additional circuitry used in the prior art.  
         [0024]    Another advantage of the present invention is that bias control in an RF power amplifier may be achieved using an adaptive bias control circuit having a single active element and a minimum of one additional passive element, which greatly decreases the cost of manufacturing the circuit compared to the prior art.  
         [0025]    Yet another advantage of the present invention is that since the adaptive bias control circuit is, preferably, located on-chip with the RF power amplifier such that both the amplifier transistor and the bias control circuit transistor have the same electrical and thermal characteristics, the bias control circuit further provides for temperature compensation.  
         [0026]    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. For instance, those of ordinary skill in the art will realize that the present invention may be modified, wherein different types of transistors are used, including but not limited to Bipolar and Gallium Arsanide transistors, which also have similar linearity versus bias behavior as LDMOS FETs, or wherein the RF power amplifier transistor and the bias control circuit transistor are each a different type of transistor (if temperature considerations are not considered). 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.