Abstract:
An attenuator includes one or more series attenuation branches including one or more series field effect transistors (FETs) each having a gate; one or more shunt attenuation branches including one or more shunt FETs each having a gate; and a bias control FET. The bias control FET receives at its gate a first bias control signal and in response thereto produces at one of its drain and source terminals a second bias control signal. Either the first bias control signal is coupled to the gates of one or more series FETs, and the second bias control signal is coupled to the gates of the one or more shunt FETs; or the first bias control signal is coupled to the gates of the one or more shunt FETs, and the second bias control signal is coupled to the gates of the one or more series FETs.

Description:
BACKGROUND 
       [0001]    Radio Frequency (RF) signals and components are employed in a variety of devices, including mobile communication devices such as mobile telephones. One type of commonly employed RF component is an RF attenuator, which is sometimes employed to control an RF signal level in a device that employs RF signals. 
         [0002]      FIG. 1  illustrates a basic configuration of a series-shunt field effect transistor (FET) attenuator  100 . Attenuator  100  includes an input port  105  and an output port  115 . Attenuator  100  also includes a first series attenuation branch, or arm, including a first series field effect transistor  110 , connected in series with a second series attenuation branch, or arm, including a second series field effect transistor  120 , between input port  105  and output port  115  via an intermediate node  117 . Also, a shunt attenuation branch, or arm including a shunt field effect transistor  130  is connected between intermediate node  117  and ground. 
         [0003]    Attenuator  100  includes two attenuation control ports  125  and  135  which receive a series attenuation control signal Vg_series and a shunt attenuation control signal Vg_shunt, respectively. The series attenuation control signal Vg_series is applied to the gates of first and second series field effect transistors  110  and  120 , and the shunt attenuation control signal Vg_shunt is applied to the gate of shunt series field effect transistor  130 . 
         [0004]    Effectively, first and second series field effect transistors  110  and  120  and shunt field effect transistor  130  are operated as voltage controlled impedances to attenuate an input signal, particularly an RF input signal, received at input port  105  and to provide the attenuated signal at output port  115 . The voltages Vg_series and Vg_shunt are selected so that they operate in combination to provide a desired attenuation (e.g., X dB) while also maintaining desired input and output impedance values (e.g., 50Ω) within a desired tolerance. 
         [0005]    Unfortunately, maintaining the relationship between Vg_series and Vg_shunt to satisfy these requirements can be complicated. 
         [0006]      FIG. 2  shows a schematic diagram of an attenuator  200  that is designed to try to address this problem. Attenuator  200  includes an input port  205  and an output port  215 . Attenuator  200  also includes a first series attenuation arm, including a first series field effect transistor  210 , connected in series with a second series attenuation arm, including a second series field effect transistor  220 , between input port  205  and output port  215  via an intermediate node  217 . Also, a shunt attenuation arm including a shunt field effect transistor  230  is connected between intermediate node  217  and ground. Attenuator  200  includes one attenuation control port  225  which receive a series attenuation control signal Vg_series. Attenuator  200  also includes analog-to-digital converter (ADC)  240 , look-up table  250 , and digital-to-analog converter (DAC)  260 . 
         [0007]    In attenuator  200 , for each attenuation value, X, there exists a value of the attenuation control signal voltage Vg_series(X), and a corresponding value for Vg_shunt(X), which together yield the desired attenuation X, while also maintaining the desired input and output impedances. When attenuator  200  is designed and constructed, for each desired attenuation value X the corresponding values of Vg_series(X) and Vg_shunt(X) are determined that also maintain the desired input/output impedances. Vg_series(X) and Vg_shunt(X) are each “digitized”—i.e., converted to digital words. The digital word for Vg_shunt(X) is then stored in look-up table  250  at an “address” corresponding a digital word for Vg_series(X). 
         [0008]    In operation, when a particular attenuation value X is to be selected and applied by attenuator  200  to an input signal (e.g., an RF input signal), then the corresponding attenuation control signal voltage Vg_series(X) is applied to attenuation control port  225 . Vg_series(X) is converted by ADC  240  to a digital address for addressing look-up table  250 . Look-up table  250  then outputs a digital word representing the corresponding value for Vg_shunt(X). Finally, DAC  260  converts the digital word from look-up table  250  to produce the analog voltage Vg_shunt(X) which is then applied to shunt field effect transistor  230 . 
         [0009]    However, the attenuator  200  of  FIG. 2  is complicated, requiring a number of additional circuits beyond the simple attenuator  100  of  FIG. 1 . Furthermore, attenuator  200  lacks provisions for addressing variations in the attenuator response due to process variations and temperature changes. 
         [0010]      FIG. 3  shows a schematic diagram of another attenuator  300  that is also designed to try to address the issue of maintaining a proper relationship between Vg_series and Vg_shunt to achieve desired attenuation values and maintain the input and output impedances within a desired range. Attenuator  300  includes an input port  305  and an output port  315 . Attenuator  300  also includes a first series attenuation branch, including a first series field effect transistor  310 , connected in series with a second series attenuation branch, including a second series field effect transistor  320 , between input port  305  and output port  315  via an intermediate node  317 . Also, a shunt attenuation branch including a shunt field effect transistor  330  is connected between intermediate node  317  and ground. Attenuator  300  includes one attenuation control port  325  which receive a series attenuation control signal Vg_series. 
         [0011]    Attenuator  300  also includes a “dummy attenuator” or “replica attenuator”  340 . Replica attenuator  340  includes a first replica series attenuation branch, including a first replica series field effect transistor  360 , connected in series with a second replica series attenuation branch, including a second replica series field effect transistor  370 , between replica attenuator input load  304  and replica attenuator output load  314  via an intermediate node  367 . Also, a replica shunt attenuation branch including a replica shunt field effect transistor  380  is connected between intermediate node  367  and ground. 
         [0012]    Attenuator  300  further includes an operational amplifier  390  having a non-inverting input connected to a supply voltage  308  through a resistor divider comprising resistors  324  and  334 . The inverting input of operational amplifier  390  is connected to the replica attenuator input load  304 . Operational amplifier  390  can be integrated into the same chip as attenuator field effect transistors  310 ,  320  and  330 , or it can be provided off-chip 
         [0013]    Operationally, series attenuation control signal Vg_series is provided to control the series field effect transistors  310  and  320 , and also to control the replica series field effect transistors  360  and  370 . Through feedback operation with replica attenuator  340 , operational amplifier  390  outputs a shunt attenuation control signal Vg_shunt to replica shunt field effect transistor  380  to maintain the input and output impedances of replica attenuator  340  to match the impedances of input and output loads  304  and  314 . The same shunt attenuation control signal Vg_shunt output by operational amplifier  390  is coupled to shunt field effect transistor  330 . By an appropriate selection of scaling for replica field effect transistors  360 ,  370  and  380  versus attenuator field effect transistors  310 ,  320  and  330 , and for input and output loads  304  and  314  versus the source and load impedances for input and output ports  305  and  315 , the operational amplifier  390  will output a value for shunt attenuation control signal Vg_shunt that will maintain the input and output impedances at attenuator  300  at the desired values. 
         [0014]    However, attenuator  300  has some drawbacks, including the added size and complexity of replica attenuator  340  and operational amplifier  390 . 
         [0015]    What is needed, therefore, is a relatively uncomplicated attenuator. What is further needed is an attenuator with a single attenuator control voltage input terminal which is relatively compact and which is relatively insensitive to process and temperature variations. 
       SUMMARY 
       [0016]    In an exemplary embodiment, an attenuator comprises: an input port, an output port, an attenuation control port, and first and second supply voltage. The attenuator also comprises: a first series attenuation branch, including a first field effect transistor, connected between the input port and an intermediate node; a second series attenuation branch, including a second field effect transistor, connected between the node and the output port; a shunt attenuation branch, including a third field effect transistor, connected between the intermediate node and the supply voltage connection, a gate of third field effect transistor receiving the attenuation control signal from the attenuation control port; and a bias control circuit. The bias control circuit comprises a fourth field effect transistor receiving at a gate thereof the attenuation control signal from the attenuation control port, and having a first terminal connected to the first supply voltage, and a resistor connected between a second terminal of the fourth field effect transistor and the second supply voltage. The voltage at the second terminal of the fourth field effect transistor is coupled to gates of the first and second field effect transistors to supply a bias voltage thereto in response to the attenuation control signal. 
         [0017]    In another exemplary embodiment, an attenuator comprises: one or more series attenuation branches comprising one or more series field effect transistors, each having a gate; one or more shunt attenuation branches comprising one or more shunt field effect transistors, each having a gate; and a bias control field effect transistor. The bias control field effect transistor receives at its gate a first bias control signal and in response thereto produces at one of its drain and source terminals a second bias control signal. Either the first bias control signal is coupled to the gates of the one or more series field effect transistors, and the second bias control signal is coupled to the gates of the one or more shunt field effect transistors; or the first bias control signal is coupled to the gates of the one or more shunt field effect transistors, and the second bias control signal is coupled to the gates of the one or more series field effect transistors. 
         [0018]    In yet another exemplary embodiment, a method is provided for attenuating a signal. The method comprises: providing one or more series attenuation branches comprising one or more series field effect transistors each having a gate; providing one or more shunt attenuation branches comprising one or more shunt field effect transistors each having a gate; receiving a first bias control signal and providing the bias control signal to a bias control field effect transistor; at the bias control field effect transistor, producing from the first bias control signal a second bias control signal having a voltage which changes in an opposite direction with respect to a change in voltage of the first bias control signal; and either: (1) coupling the first bias control signal to the gates of each of the one or more series field effect transistors, and the second bias control signal is applied to the gates of the one or more shunt field effect transistors; or (2) coupling the first bias control signal is to the gates of the one or more shunt field effect transistors, and the second bias control signal is applied to the gates of the one or more series field effect transistors. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The exemplary embodiments are best understood from the following detailed description when read with the accompanying drawing figures. It is emphasized that the various features are not necessarily drawn to scale. In fact, the dimensions may be arbitrarily increased or decreased for clarity of discussion. Wherever applicable and practical, like reference numerals refer to like elements. 
           [0020]      FIG. 1  shows a schematic diagram of an attenuator. 
           [0021]      FIG. 2  shows a schematic diagram of another attenuator. 
           [0022]      FIG. 3  shows a schematic diagram of yet another attenuator. 
           [0023]      FIG. 4  shows a schematic diagram of one embodiment of an attenuator which has a single attenuation control port. 
           [0024]      FIG. 5  shows a schematic diagram of another embodiment of an attenuator which has a single attenuation control port 
           [0025]      FIG. 6  illustrates an input impedance characteristic of the attenuator of  FIG. 5  as a function of attenuator control voltage. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    In the following detailed description, for purposes of explanation and not limitation, example embodiments disclosing specific details are set forth in order to provide a thorough understanding of an embodiment according to the present teachings. However, it will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure that other embodiments according to the present teachings that depart from the specific details disclosed herein remain within the scope of the appended claims. Moreover, descriptions of well-known apparati and methods may be omitted so as to not obscure the description of the example embodiments. Such methods and apparati are clearly within the scope of the present teachings. 
         [0027]    As used herein, the term “radio frequency” or “RF” pertains to VHF, UHF, SHF and even millimeter wave frequencies to the extent that technology permits the devices and circuits disclosed herein to be fabricated and operated at such frequencies. Also, unless otherwise noted, when a first device is said to be connected to a second device, this encompasses cases where one or more intermediate devices may be employed to connect the two devices to each other. However, when a first device is said to be directly connected to a second device, this encompasses only cases where the two devices are connected to each other without any intermediate or intervening devices. Similarly, when a signal is said to be coupled to a device, this encompasses cases where one or more intermediate devices may be employed to couple the signal to the device. However, when a signal is said to be directly coupled to a device, this encompasses only cases where the signal is directly coupled to the device without any intermediate or intervening devices. 
         [0028]      FIG. 4  shows a schematic diagram of one embodiment of an attenuator  400  which has a single attenuation control port. Attenuator  400  includes an input port  405  and an output port  415 . Attenuator  400  also includes a first series attenuation branch, or arm, including a first series field effect transistor  410 , connected in series with a second series attenuation branch, or arm, including a second series field effect transistor  420 , between input port  405  and output port  415  via an intermediate node  417 . Also, a shunt attenuation branch, or arm, including a shunt field effect transistor  430  is connected between intermediate node  417  and ground. 
         [0029]    Attenuator  400  includes a single attenuation control port  425  which receives a bias control signal Vg_shunt. 
         [0030]    Attenuator  400  also includes a bias control circuit  440 . Bias control circuit  440  includes a bias control field effect transistor  450  and a resistor  404 , which are connected in series to a supply voltage  408 . 
         [0031]    As can be seen in  FIG. 4 , bias control circuit  440  operates to receive a first bias control signal Vg_shunt which is also coupled to shunt field effect transistor  430 , and to produce therefrom a second bias control signal Vg_series to be coupled to the gates of first and second series field effect transistors  410  and  420 . In particular, the same voltage Vg_shunt which is coupled to the gate of shunt field effect transistor  430  is also coupled to the gate of bias control field effect transistor  450 . One terminal (e.g., the drain) of bias control field effect transistor  450  outputs the second bias control signal Vg_series which exhibits a voltage which changes in an opposite direction with respect to a change in voltage of the first bias control signal Vg_shunt which is coupled to the gate of bias control field effect transistor  450 . 
         [0032]    The exact relationship between the first bias control signal Vg_shunt and second bias control signal Vg_series is governed by proper selection of supply voltage  408 , resistor  404 , and the size of bias control field effect transistor  450 . In particular, the supply voltage  408 , resistor  404 , and the size of bias control field effect transistor  450  are selected in concert to yield the minimum variation in attenuator port impedance from the desired value, as a function of attenuation value. The selection of supply voltage  408 , resistor  404 , and the size of bias control field effect transistor  450  to produce the desired characteristics can be easily accomplished by one skilled in the art in a very short time using conventional design tools. 
         [0033]    In operation, input port  405  receives an input signal that is to be attenuated. Typically, the input signal is an RF signal. Also, single attenuation control port  425  receives first bias control signal Vg_shunt having a voltage selected to provide a desired attenuation to the input signal. First bias control signal Vg_shunt is coupled to the gate of shunt field effect transistor  430 , and also to the gate of bias control field effect transistor  450 . The drain of bias control field effect transistor  450  becomes the second bias control signal Vg_series and is coupled to the gates of first and second series field effect transistors  410  and  420 . Field effect transistors  410 ,  420  and  430  operate, in response to corresponding bias control voltages, as voltage controlled impedances. The voltage of the first bias control signal Vg_shunt biases shunt field effect transistor  430  to present a particular shunt impedance to ground for the RF input signal, and the voltage of the second bias control signal Vg_series biases series field effect transistors  410  and  420  each to present a particular series impedance to the RF input signal. As a result of the selected series and shunt impedances of field effect transistors  410 ,  420  and  430 , the RF input signal is attenuated and output at output terminal  415 . Furthermore, due to the proper selection of supply voltage  408 , resistor  404 , and the size of bias control field effect transistor  450 , Vg_series is generated such that the input and output impedances of attenuator  400  are set to a desired value (e.g., 50Ω) within a desired tolerance (e.g., 45-63Ω) over the range of attenuation values. 
         [0034]    In one particular embodiment: first and second series field effect transistors  410  and  420 , and shunt field effect transistor, are each of a size of 200 μm; supply voltage  408  has a voltage of 5V; resistor  404  has a value of 15 kΩ; and bias control field effect transistor  450  has a size of 20 μm. 
         [0035]    In a beneficial arrangement, all of the field effect transistors  410 ,  420 ,  430  and  450  and resistor  404  are fabricated in a vicinity to each other in an integrated circuit. In this case, process and temperature variations in the attenuator field effect transistors  410 ,  420  and  430  will be mirrored in bias control field effect transistor  450 . 
         [0036]      FIG. 5  shows a schematic diagram of another embodiment of an attenuator  500  which has a single attenuation control port  425 . Attenuator  500  is similar to attenuator  400 , and like-numbered elements are the same. For brevity, only the differences between attenuator  500  and attenuator  400  will now be described. 
         [0037]    In attenuator  500 , the first bias control signal Vg_shunt is coupled to the gate of shunt field effect transistor  430  via a corresponding gate resistor  536 , and the second bias control signal Vg_series is coupled to the gates of first and second series field effect transistors  410  and  420  via corresponding gate resistors  516  and  526 . Also, the first bias control signal Vg_shunt is coupled to the gate of bias control field effect transistor  450  via a corresponding gate resistor  556 . Attenuator  500  also includes first and second shunt resistors  513  and  523  each connected in parallel across a source and drain of a corresponding one of the first and second field effect transistors  410  and  420 . First and second shunt resistors  513  and  523  allow first and second field effect transistors  410  and  420  to be operated in a pinch-off condition without presenting an undesirably high impedance to the external circuitry. In a beneficial arrangement, resistors  516 ,  526 ,  536  and  556  all have relatively high resistance values (e.g., 10 kΩ), and first and second shunt resistors  513  and  523  each have a same value as the desired port impedance (e.g., 50Ω). 
         [0038]      FIG. 6  shows an input port impedance characteristic of the attenuator of  FIG. 5  as a function of attenuator control voltage (e.g., Vg_shunt). It can be seen from  FIG. 6  that the input impedance only varies from about 45-63Ω across a wide range of attenuation control voltage. This implies a VSWR of less than about 1.3:1, which represents a good match. 
         [0039]    Although the embodiments illustrated in  FIGS. 4 and 5  are in a so-called “T” configuration with a single shunt attenuation branch disposed between two series attenuation branches, the invention is not so limited. The attenuator could include additional series and shunt branches while still operating within the principles disclosed above. Also, while the particular embodiment derives a Vg_series bias control voltage for series attenuation transistors from a Vg_shunt bias control voltage for a shunt attenuation transistor, in an alternative arrangement the Vg_series bias control voltage could be applied to a bias control transistor to develop therefrom the Vg_shunt bias control voltage. 
         [0040]    While exemplary embodiments are disclosed herein, one of ordinary skill in the art appreciates that many variations that are in accordance with the present teachings are possible and remain within the scope of the appended claims. The embodiments therefore are not to be restricted except within the scope of the appended claims.