Abstract:
A monolithic microwave integrated circuit (MMIC) includes a transistor, coupled line and multiple air bridges. The coupled line is configured to output a coupled signal from the transistor, the coupled line running parallel to a drain of the transistor. The air bridges connect the drain of the transistor with a bond pad for outputting a transistor output signal, the bridges being arranged parallel to one another and extending over the coupled line. The air bridges and the coupled line effectively provide coupling of the transistor output signal to a load.

Description:
BACKGROUND 
       [0001]    Microwave monolithic integrated circuit (MMIC) devices are widely used in microwave frequency applications, due to their relatively low cost and high degree of integration. For example, MMICs are incorporated into various wireless communication devices, such as cellular telephones, personal digital assistants and portable computers. MMICs may include power amplifiers, which typically consist of arrays of transistors coupled in parallel to form corresponding amplifier stages. The output of the final stage, which is typically a radio frequency (RF) signal, may be connected to a load through a coupler and an output impedance matching network. 
         [0002]    Typically, the coupler is an edge line coupler, which may be implemented by running a coupled line parallel to another matched impedance line.  FIG. 1  is a simplified block diagram application showing a conventional edge line coupler at the output of the amplifier. Referring to  FIG. 1 , MMIC  100  includes a power amplifier  110 , the final stage of which is indicted by transistor  115 . An (amplified) output signal of the power amplifier  110  is provided from drain  118  of the transistor  115 , and is received by printed circuit board (PCB) circuit  142  of module PCB  140 . 
         [0003]    In the depicted example, the PCB circuit  142  includes output impedance matching circuit  144  and edge line coupler  146 , the output of which is connected to load  150 . The edge line coupler  146  includes coupled line  147  running in parallel with matched impedance line  148  having predetermined resistance  149 , as mentioned above. The coupled line  147  may provide a coupled signal to an output detector (not shown), for example, which may determine a power level of the output of the power amplifier  110 . The edge line coupler  146  typically would have 50 ohm input and output impedances. Conventional edge line couplers, such as the edge line coupler  146 , provide good directivity, but they occupy a substantial amount of space relative to the size of the power amplifier  110  and/or the MMIC  100 . 
         [0004]    In cases where directivity is not critical, a conventional alternative to edge line couplers is a capacitive coupler located in the MMIC.  FIG. 2  is a simplified block diagram showing a conventional capacitive coupler, in which MMIC  200  includes a power amplifier  210 , the final stage of which is indicated by transistor  215 , and coupling capacitor  219 . An (amplified) output signal of the power amplifier  210  is provided from drain  218  of the transistor  215 , and the coupling capacitor  219  is connected in series between the drain  218  and an output detector (not shown), for example. That is, the coupling capacitor  219  is connected directly at the drain  218  of the transistor  215 , which is the last (or second to last) stage of the power amplifier  210 . 
         [0005]    The output signal of the power amplifier  210  is received by PCB circuit  242  of module PCB  240 . The PCB circuit  242  includes output impedance matching circuit  244  connected to load  250 , but no edge line coupler. Thus, the coupling capacitor  219  is able to approximate the coupling effect of an edge line coupler, using much less space than an edge line coupler, because the coupling capacitor  219  is implemented in the MMIC  200  itself. 
         [0006]    However, the coupling capacitor  219  has limited coupling effect. The coupling capacitor  219  is usually a few tenths of a picoFarad (pF), hence it is located at one edge of the output stage gain device. For example,  FIG. 3  is a top plan view showing a final stage of a conventional MMIC power amplifier and a corresponding coupling capacitor. Referring to  FIG. 3 , the final stage includes transistor  315 , which includes gate  316 , source  317  and drain  318 . The drain  318  is connected to a bond pad  330  through multiple air bridges  325 - 1  to  325 -n. Coupling capacitor  319  is connected to one edge of the bond pad  330 . The coupling capacitor  319  is therefore able to provide only limited coupling. 
         [0007]    While it uses little area, the coupling capacitor  319  has a number of drawbacks. For example, the coupling capacitor  319  only couples signals out from one end of the transistor  315 , as discussed above, leading to erroneous sampling of actual power output from the power amplifier  310 . Also, the coupling capacitor  319  is sensitive to variations in the output mismatch at the load (e.g., load  250  of  FIG. 2 ). Variations in load impedance lead to large variations in the coupled signal output through the coupling capacitor  319 . 
       SUMMARY 
       [0008]    In a representative embodiment, a monolithic microwave integrated circuit (MMIC) includes a transistor, a coupled line and multiple air bridges. The coupled line is configured to output a coupled signal from the transistor, the coupled line running parallel to a drain of the transistor. The air bridges electrically connect the drain of the transistor with a bond pad for outputting a transistor output signal. The air bridges are arranged parallel to one another and extend over the coupled line, where the air bridges and the coupled line provide coupling of the transistor output signal to a load. 
         [0009]    In another representative embodiment, an MMIC includes a multi-stage amplifier including a final stage and at least one previous stage, a first coupled line at the final stage of the multi-stage amplifier configured to provide a first coupled signal, and multiple first air bridges electrically connecting an output of the final stage to a bond pad for outputting a radio frequency (RF) signal from the of the multi-stage amplifier. The first air bridges extend over the first coupled line, where the first air bridges and the first coupled line couple the RF signal output from the multi-stage amplifier to a load. 
         [0010]    In another representative embodiment, a coupling device in an MMIC, for coupling a power amplifier with a load, includes a coupled line and multiple air bridges. The coupled line is configured to provide a coupled signal from a transistor included in the power amplifier. The air bridges connect one of a drain or a collector of the transistor with a bond pad for outputting an RF signal from the power amplifier. The air bridges extend over the coupled line, where the air bridges and the coupled line couple the power amplifier to a load. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The example 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. 
           [0012]      FIG. 1  is a simplified block diagram showing a conventional edge line coupler. 
           [0013]      FIG. 2  is a simplified block diagram showing a conventional capacitive coupler included in a microwave monolithic integrated circuit (MMIC). 
           [0014]      FIG. 3  is a top plan view of a conventional transistor and capacitive coupler included in an MMIC. 
           [0015]      FIGS. 4A-4E  are simplified block diagrams showing air bridge couplers included in MMICs, according to representative embodiments. 
           [0016]      FIG. 5  is a top plan view of a transistor and air bridge coupler included in an MMIC, according to a representative embodiment. 
           [0017]      FIG. 6  is a simplified block diagram of an air bridge coupler included in an MMIC, according to a representative embodiment. 
           [0018]      FIGS. 7A-7B  are plots comparing conventional capacitive couplers and air bridge couplers included in an MMIC, according to a representative embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    In the following detailed description, for purposes of explanation and not limitation, representative embodiments disclosing specific details are set forth in order to provide a thorough understanding of 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 apparatuses and methods may be omitted so as to not obscure the description of the representative embodiments. Such methods and apparatuses are clearly within the scope of the present teachings. 
         [0020]    Generally, it is understood that the drawings and the various elements depicted therein are not drawn to scale. Further, relative terms, such as “above,” “below,” “top,” “bottom,” “upper,” “lower,” “left,” “right,” “vertical” and “horizontal,” are used to describe the various elements&#39; relationships to one another, as illustrated in the accompanying drawings. It is understood that these relative terms are intended to encompass different orientations of the device and/or elements in addition to the orientation depicted in the drawings. For example, if the device were inverted with respect to the view in the drawings, an element described as “above” another element, for example, would now be “below” that element. Likewise, if the device were rotated  90  degrees with respect to the view in the drawings, an element described as “vertical,” for example, would now be “horizontal.” 
         [0021]      FIGS. 4A-4E  are simplified block diagrams showing air bridge couplers included in MMICs, according to representative embodiments. 
         [0022]    Referring to  FIG. 4A , power amplifier  410  is included in MMIC  400 , along with air bridge coupler  420 . The amplifier  410  is a multi-stage amplifier, including at least two stages. In the depicted embodiment, the final stage of the multi-stage amplifier  410  includes second (or final stage) transistor  415  and first (or previous stage) transistor  411 . However, it is understood that the amplifier  410  may include three or more stages (indicated by the dots preceding the input to the first transistor  411 ), or that the amplifier  410  may be a single stage amplifier, without departing from the scope of the present teachings. 
         [0023]    For purposes of explanation, it may be assumed that each of the first and second transistors  411  and  415  is a field effect transistor (FET), such as a metal oxide semiconductor FET (MOSFET) or a metal semiconductor FET (MESFET). Accordingly, the first transistor  411  includes gate  412 , source  413  connected to ground and drain  414  connected to the second transistor  415 . The second transistor  415  includes gate  416 , source  417  connected to ground and drain  418 , which corresponds to the output of the amplifier  410 . 
         [0024]    However, it is understood that each of the first and second transistors  411  and  415  may be any of various alternative types of transistors, such as a high electron mobility transistor (HEMT), a pseudomorphic high electron mobility transistor (pHEMT), an enhancement-mode pseudomorphic high electron mobility transistor (E-pHEMT), a bipolar junction transistor (BJT), a heterojunction bipolar transistor (HBT), or the like, without departing from the scope of the present teaches. If the first and second transistors  411  and  415  were BJTs, for example, it is further understood that the first transistor  411  would include base  412 , emitter  413  connected to ground and collector  414  connected to the second transistor  415 , and the second transistor  415  would include base  416 , emitter  417  connected to ground and collector  418 . Likewise, it is understood that the sources (or emitters) and drains (or collectors) of the first and second transistors  411  and  415  may be reversed in alternative configurations. 
         [0025]    The output signal of the amplifier  410  may be an amplified RF signal, output from the drain  418  of the second transistor  415 . The output signal passes through the air bridge coupler  420 , which is also located in the MMIC  400 , along with the amplifier  410 . The output signal is provided to load  450  via a module PCB  440 , where the module PCB  440  includes an impedance matching circuit  444 . The air bridge coupler  420  enables coupling of the output signal to the load  450 . 
         [0026]    In the depicted embodiment, the air bridge coupler  420  includes multiple air bridges  425 - 1  to  425 -n and coupled line  427 , which passes through the space beneath each of the air bridges  425 - 1  to  425 -n. As discussed below, the air bridges  425 - 1  to  425 -n may electrically connect the drain  418  to a bond pad (e.g., bond pad  430  in  FIGS. 5A and 5B ) or other electrical connection for providing the output signal from the amplifier  410 . Generally, the air bridges  425 - 1  to  425 -n connect the drain  418  to bond pad  430 , for example. The air bridges  425 - 1  to  425 -n may be substantially parallel to one another, and the coupled line  427  may run substantially perpendicular to the air bridges  425 - 1  to  425 -n. The coupled line  427  is shown as having two ports, a first port being connected to output detector  460  and a second port being connected to ground through resistor  421 . The output detector  460  may determine a power level of the output of the power amplifier  410 , which may be used for system power level setting or amplifier linearization, for example. 
         [0027]    It is understood that the first port and/or the second port of the coupled line  427  may be connected to various other devices, such as auxiliary amplifiers or other circuits used for linearization of the amplifier  410 , and/or passive networks, such as resistance, capacitor and/or inductor circuits, without departing from the scope of the present teachings. For example, the second port of the coupled line  427  may be connected to an auxiliary amplifier while the first port is connected to the output detector  460 . Also, the coupled line  427  may be connected to an input of another amplifier for driving the other amplifier. Alternative configurations are discussed below with reference to  FIGS. 4B to 4E . It is further understood that, in various configurations, the air bridge coupler  420  may be connected to the output of one or more previous stages (e.g., the drain  414  of the first transistor  411 ) of the amplifier  410  instead of, or in addition to, being connected to the output of the final stage (e.g., the drain  418  of the second transistor  415 ) of the amplifier  410 . 
         [0028]      FIG. 5A  is a top plan view of a transistor and an air bridge coupler included in an MMIC, and  FIG. 5B  is a cross-sectional view of the air bridge coupler of  FIG. 5A  taken along line A-A′, according to a representative embodiment. More particularly,  FIGS. 5A and 5B  depict one example of the second transistor  415  and the air bridge coupler  420  of  FIG. 4A , in more detail, where the second transistor  415  is depicted as a large FET, e.g., relative to the wavelength of the coupled signal. 
         [0029]    Referring to  FIG. 5A , the second transistor  415  includes gate  416 , source  417  and drain  418 , each of which is formed of conductive material, such as gold (Au), copper (Cu), aluminum (Al), and the like, or combinations thereof, and/or plated conductive material. The conductive material may be formed on a substrate, which may be various types of materials compatible with semiconductor processes, such as silicon (Si), GaAs, or the like, for example. The drain  418  is electrically connected to bond pad  430  via the multiple air bridges  425 - 1  to  425 -n. In other words, the metallization of the drain  418  is connected to the metallization of the bond pad  430  via the conductive air bridges  425 - 1  to  425 -n. The air bridges  425 - 1  to  425 -n are likewise formed of conductive material, and may be formed of the same material as one or both of the drain  418  and the bond pad  430 . As mentioned above, the air bridges  425 - 1  to  425 -n are arranged substantially parallel to one another. Therefore, the gaps defined beneath the air bridges  425 - 1  to  425 -n form channel  424 , separating the drain  418  and the bond pad  430 . The coupled line  427  extends beneath the air bridges  425 - 1  to  425 -n, within the channel  424 . Therefore, the coupled line  427  runs between the drain  418  and the bond pad  430 , and is arranged substantially perpendicular to the air bridges  425 - 1  to  425 -n. The coupled line  427  may be a conductive trace on the surface of the substrate, for example, formed of a conductive material, such as Au, Cu, Al, and the like, or combinations thereof. 
         [0030]    As mentioned above,  FIG. 5B  shows a cross-sectional view of the air bridge coupler  420  taken along line A-A′ of  FIG. 5A . Referring to  FIG. 5B , representative air bridge  425 - 2  is shown electrically connecting the drain  418  and the bond pad  430 . The air bridge  425 - 2  forms a gap and corresponding channel  424 , in which the coupled line  427  is arranged. As shown, the coupled line  427  does not come into physical contact with the air bridge  425 - 2 , the drain  418  or the bond pad  430 . 
         [0031]    Generally, the air bridge coupler  420  is used to provide coupling to first level metallization of the MMIC. In the depicted configuration, the air bridge coupler  420  effectively acts as a distributed capacitor, where the air bridges  425 - 1  to  425 -n collectively act as a top plate and the coupled line  427  acts as a bottom plate of the distributed capacitor. Significantly, the coupled line  427  extends along the entire length of the second transistor  415 , parallel to the drain  418 . Therefore, unlike a conventional capacitance coupler, the air bridge coupler  420  is able to couple out the output signal from the entire (large) transistor  415 , without taking up additional space within the MMIC. In other words, the output signal of the transistor  415  is sampled across the entire length of the connection of the drain  418 . In comparison, a conventional capacitance coupler, as shown in  FIG. 3 , for example, pulls the coupled output signal from one corner of the large transistor, which leads to erroneous sampling. The air bridge coupler  420  is also less sensitive to variations in output mismatch at the load, and the coupling coefficient is easily and accurately controllable, as discussed below with reference to  FIG. 6 . 
         [0032]      FIG. 6  is a simplified block diagram of an air bridge coupler included in an MMIC, according to a representative embodiment. More particularly,  FIG. 6  depicts an illustrative configuration of the air bridge coupler  420 , discussed above. 
         [0033]    Referring to  FIG. 6 , the air bridge coupler  420  includes representative air bridges  425 - 1 ,  425 - 2 ,  425 - 3 ,  425 - 4  . . .  425 -n, which electrically connect the drain  418  of the second transistor  415  and the bond pad  430 . The air bridges  425 - 1 ,  425 - 2 ,  425 - 3 ,  425 - 4  . . .  425 -n are arranged substantially parallel to one another, and form channel  424  between the drain  418  and the bond pad  430 . Although only five air bridges are depicted, it is understood that the total number may vary to provide unique benefits for particular situations or to meet application specific design requirements of the implementations, as would be apparent to one of ordinary skill in the art. The coupled line  427  runs substantially perpendicular to the air bridges  425 - 1 ,  425 - 2 ,  425 - 3 ,  425 - 4  . . .  425 -n within the channel  424 . The coupled line  427  has width W 1 , and is separated from the drain  418  by distance D 1  and from the bond pad  430  by distance D 2 . 
         [0034]    As mentioned above, the coupling coefficient of the coupling between the amplifier  410  and the detector  460  and/or feedback signals in the coupled line  427  is adjustable in response to changes in the configuration of the air bridge coupler  420 . For example, the coupling coefficient is directly proportional to the number of air bridges  425 - 1  to  425 -n. That is, the coupling coefficient increases as the number of air bridges  425 - 1  to  425 -n increases and decreases as the number of air bridges  425 - 1  to  425 -n decreases. The coupling coefficient is also directly proportional to the width W 1  of the coupled line  427 . That is, the coupling coefficient increases as the width W 1  of the coupled line  427  increases, and decreases as the width W 1  of the coupled line  427  decreases. 
         [0035]    The coupling coefficient is indirectly proportional to the proximity of the coupled line  427  with each of the drain  418 , the bond pad  430  and the air bridges  425 - 1  to  425 -n. That is, the coupling coefficient increases as the distance D 1  between the coupled line  427  and the drain  418  and/or the distance D 2  between the coupled line  427  and the bond pad  430  decreases, and the coupling coefficient decreases as the distance D 1  between the coupled line  427  and the drain  418  and/or the distance D 2  between the coupled line  427  and the bond pad  430  increases. Similarly, the coupling coefficient increases as the distance (not shown in  FIG. 6 ) between the coupled line  427  and the air bridges  425 - 1  to  425 -n (above the coupled line  427 ) decreases, and the coupling coefficient decreases as the distance between the coupled line  427  and the air bridges  425 - 1  to  425 -n increases. Of course, the coupling coefficient would be affected by other factors, as well, such as frequency of the output signal and the type of material filling the gap between the air bridges  425 - 1  to  425 -n and the coupled line  427 . For example, the coupling coefficient is minimum when the gap is a vacuum or is filled with air, and is higher when the gap is filled with silicon nitride, or other dielectric material. 
         [0036]    As mentioned above, the air bridge coupler  420  may be incorporated into any number of various configurations, an example of which is depicted in  FIG. 4A , discussed above.  FIGS. 4B-4E  are simplified block diagrams showing additional illustrative configurations of air bridge couplers included in MMICs, according to representative embodiments. 
         [0037]    Referring to  FIG. 4B , power amplifier  410  is included in MMIC  400 , along with air bridge coupler  420 . In the depicted embodiment, the amplifier  410  is a multi-stage amplifier, including first and second transistors  411  and  415 . The output signal (e.g., amplified RF signal) of the amplifier  410  is output at the drain  418  of the second transistor  415 . As discussed above with respect to  FIG. 4A , the output signal passes through the air bridge coupler  420 , located in the MMIC  400 , which provides coupling between the amplifier  410  and the load  450  via the module PCB  440 . 
         [0038]    The air bridge coupler  420  includes multiple air bridges  425 - 1  to  425 -n and coupled line  427 , which passes through the space beneath each of the air bridges  425 - 1  to  425 -n. In the embodiment depicted in  FIG. 4B , the coupled line  427  is connected to feedback network  429  leading to a signal processing circuit, as opposed to the output detector  460 , such that the coupled signal in the coupled line  427  provides feedback. For example, the signal processing circuit may include the input of a different stage of the amplifier  410  or some digital processing circuit (not shown). That is, the feedback network  429  may provide feedback to the input of the first transistor  411 , for example, in which case a first port of the coupled line  427  may be connected to the gate  412  of the first transistor  411 , and a second port of the coupled line  427  may be connected to ground through resistor  421 . Other examples include the coupled line  427  being connected to the final stage of the amplifier  410  (e.g., the gate  416  of the second transistor  415 ) via the feedback network  429 , or when the amplifier  410  includes more than two stages, the coupled line  427  being connected to another previous stage of the amplifier  410 , without departing from the scope of the present teachings. 
         [0039]      FIGS. 4C and 4D  are similar to  FIGS. 4A and 4B , except that the air bridge coupler  420  includes multiple coupled lines, indicated by representative first and second coupled lines  427  and  428 . The first and second coupled lines  427  and  428  run substantially parallel to one another, and are arranged within the channel  424  beneath the air bridges  425 - 1  to  425 -n of the air bridge coupler  420 . Therefore, both of the first and second coupled lines  427  and  428  are substantially perpendicular to the air bridges  425 - 1  to  425 -n, as discussed above. The first and second coupled lines  427  and  428  carry corresponding coupled signals. The configuration and effects of the second coupled line  428  (and corresponding coupled signal) are essentially the same as those discussed above with reference to the first coupled line  427 . The second coupled line  428  may be positioned beside the first coupled line  427  within the channel  424  (on the substrate), or it may be positioned above or below the first coupled line  427  within the channel  424 , without departing for the scope of the present teachings. Notably, as in the case of the first coupled line  427 , the coupling coefficient varies directly proportionally to the width of the second coupled line  428  and indirectly proportionally to the proximity of the second coupled line  428  with each of the drain  418 , the bond pad  430 , and the air bridges  425 - 1  to  425 -n. The overall coupling coefficient would depend on the aggregate effect of both of the first and second coupled lines  427  and  428  (as well as the number of air bridges  425 - 1  to  425 -n, the material filling the gap, etc.), for example. 
         [0040]    Referring to  FIG. 4C , the first coupled line  427  has a first port connected to the output detector  460  and a second port connected to ground through resistor  421 . The second coupled line  428  has a third port connected to feedback network  429  to provide feedback to any of a variety of possible signal processing circuits, as discussed above, and a fourth port connected to ground through resistor  421 . Referring to  FIG. 4D , the first coupled line  427  has a first port connected to first feedback network  429 - 1  to provide feedback to a first signal processing circuit and a second port connected to ground through resistor  421 . The second coupled line  428  has a third port connected to second feedback network  429 - 2  to provide feedback to a second signal processing circuit (which may be the same as or different from the first signal processing circuit) and a fourth port connected to ground through resistor  421 . 
         [0041]    Also, as discussed above with reference to  FIG. 4A , it is understood that the first through fourth ports may be connected to various other devices, such as auxiliary amplifiers or other circuits used for linearization of the amplifier  410 , and/or passive networks, such as resistance, capacitor and/or inductor circuits, without departing from the scope of the present teachings. 
         [0042]      FIG. 4E  is similar to  FIGS. 4A-4D , except that the MMIC  400  includes a second air bridge coupler  470 , in addition to air bridge coupler  420 , and corresponding coupled lines, indicated by representative first and second coupled lines  427  and  428 . Referring to  FIG. 4E , power amplifier  410  is included in MMIC  400 , along with air bridge couplers  420  and  470 . In the depicted embodiment, the amplifier  410  is a multi-stage amplifier, including first and second transistors  411  and  415 . The output signal (e.g., amplified RF signal) of the amplifier  410  is output at the drain  418  of the second transistor  415 . As discussed above with respect to  FIG. 4A , the output signal passes through the air bridge coupler  420 , located in the MMIC  400 , which provides coupling between the amplifier  410  and the load  450  via the module PCB  440 . The air bridge coupler  420  includes multiple air bridges  425 - 1  to  425 -n and coupled line  427 , which passes through the space beneath each of the air bridges  425 - 1  to  425 -n. In the embodiment depicted in  FIG. 4E , the coupled line  427  is connected to output detector  460 , as discussed above. 
         [0043]    In addition, an output signal from the first transistor  411 , output from the drain  414 , passes through the second air bridge coupler  470 , also located in the MMIC  400 . The air bridge coupler  470  includes multiple air bridges  475 - 1  to  475 -n and coupled line  477 , which passes through the space beneath each of the air bridges  475 - 1  to  475 -n. The configuration and functionality of the air bridge coupler  470  is substantially the same as that discussed above with regard to the air bridge coupler  420 . In embodiment depicted in  FIG. 4E , the coupled line  477  is connected to feedback network  479  leading to a signal processing circuit, such that the coupled signal in the coupled line  477  provides feedback. For example, the signal processing circuit may include the input of a different stage of the amplifier  410  or some digital processing circuit (not shown). That is, the feedback network  479  may provide feedback to the input of an earlier stage transistor (not shown), for example, in which case a first port of the coupled line  477  may be connected to the gate of the earlier stage transistor, and a second port of the coupled line  477  may be connected to ground through resistor  471 . Notably, in various configurations, the air bridge coupler  470  may include features discussed above with regard to air bridge coupler  420 , such as the inclusion of one or more additional coupled lines (along with the coupled line  477 ) and/or connection to an output detector (such as output detector  460 ). 
         [0044]      FIGS. 7A-7B  are plots comparing conventional capacitive couplers and an air bridge coupler included in an MMIC, according to a representative embodiment. 
         [0045]      FIG. 7A  shows actual measured output when each of the conventional capacitive couplers and the air bridge coupler is connected to an output detector. For purposes of the measurement, the output signal had a frequency of about 2.0 GHz, the size of the gain device (e.g., the length of the drain) was about 28 mm, and the ambient temperature was about 25° C. The output of the output detector is plotted as output voltage (y-axis) versus output power (x-axis), where curves  701 - 703  correspond to conventional capacitive couplers having capacitances of 0.5 pF, 1.0 pF and 1.5 pF, respectively, and curve  710  corresponds to an air bridge coupler, according to a representative embodiment. As shown  FIG. 7A , the air bridge coupler coupling coefficient is lower than that of the capacitive coupler in this particular embodiment. However, as mentioned above, the coupling coefficient can be easily increased if necessary. 
         [0046]      FIG. 7B  shows effects of the output detector connected to conventional capacitive couplers and the air bridge coupler when the voltage standing wave ratio (VSWR) of the load is varied across 360° of phase angles, and the VSWRs are set to 2:1 and 4:1. For purposes of the measurement, the output signal had a frequency of about 2.0 GHz, the size of the gain device (e.g., the length of the drain) was about 28 mm, and the fixed output power Pout was about 28 dBm. The output of the output detector is plotted as output voltage (y-axis) versus phase (x-axis). Curves  721 - 722  correspond to conventional capacitive couplers having a capacitance of about 0.5 pF, and where the VSWRs are 1:2 and 1:4, respectively. Curves  731 - 732  correspond to air bridge couplers, according to a representative embodiment, where the VSWRs are 1:2 and 1:4, respectively. As shown in  FIG. 7B , the air bridge coupler has about half the variation across various load phase angles as the conventional capacitor couplers when the VSWR is high. 
         [0047]    According to various embodiments, an air bridge coupler included in an MMIC enables improved coupling between the MMIC and a load, without increasing space needed to accommodate the coupling circuit. This enables the MMIC to remain relatively small, while improving operational characteristics and reliability. For example, the air bridge coupler may be easily incorporated into various MMIC layouts to provide a means to sense the actual output power of a gain device, e.g., for multi-stage amplifier designs. Also, both ends of the coupled signal (e.g., the first and second ports of the coupled line  427 ) may be used to equalize the output power distribution of a physically large transistor (e.g., FET). For example, phase variations fed into a large FET gate may cause loss of output power. However, the air bridge coupler described herein is able to feed the coupled signal back to provide a power sensing means, such as output detector  460 , to improve power delivery from across the entire transistor. Further, the air bridge coupler may be used as means to inject power into the output of a power amplifier, such as amplifier  410 , via an auxiliary amplifier for distortion cancellation purposes. Accordingly, the power injection would be uniformly coupled into the transistor output (e.g., along the entire length of drain  418 ), such that combining at the output matching network (e.g., output impedance matching circuit  444 ) is consistent. 
         [0048]    The various components, materials, structures and parameters are included by way of illustration and example only and not in any limiting sense. For example, although various representative embodiments were discussed assuming that the MMIC included an amplifier, this configuration is described only for purposes of illustration. It is therefore understood that air bridge couplers as described herein may be included in other types of MMICs, without departing from the scope of the present teachings. In view of this disclosure, those skilled in the art can implement the present teachings in determining their own applications and needed components, materials, structures and equipment to implement these applications, while remaining within the scope of the appended claims.