Abstract:
High impedance, high frequency nanoscale device electronics configured to interface with low impedance loads include an impedance transforming stage constructed of multiple nanoscale devices, such as carbon nanotube field-effect transistors. In an embodiment of the present invention, an impedance transforming output stage of a multistage amplifier is configured to drive a 50 ohm transmission line with unity voltage gain using multiple carbon nanotube field-effect transistors in parallel. In a further embodiment, a receiver provided for an electronically steered receive array is a monolithic, lumped-element system formed from nanoscale devices and configured to interface with the external electrical systems via a single transmission line.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to high frequency electronics. More specifically, the present invention relates to the integration and impedance matching of systems formed from high impedance nanoscale devices. 
     2. Description of Related Art 
     Carbon nanotubes were discovered in the early 1990s as a product of arc-evaporation synthesis of fullerenes. Scientists have since determined that carbon nanotubes have extraordinary physical characteristics, and their potential use in many different applications has attracted much attention. For example, field effect transistors formed using carbon nanotubes provide a linear response at extremely low power. Their linearity, and small size, makes the use of carbon nanotube field effect transistors ideal for low-power, highly linear systems such as radar and communications receivers or any battery powered device. 
     Due to their small size and structure, carbon nanotube field effect transistors are able to source only a small amount of RF current. Therefore, it is difficult to engineer systems from carbon nanotube field effect transistors that efficiently interface with the high current levels seen in the everyday world. Specifically, the input impedance of a carbon nanotube field effect transistor is capacitive, and this small capacitance gives carbon nanotube field effect transistors an input impedance on the order of 10 kΩ or more. Similarly, these devices have a very large output impedance as well. Because traditional transmission lines and loads are generally in the range of 5Ω-500Ω, there exists an inherent impedance mismatch between nanoscale devices and traditional high frequency devices and systems. Traditional matching techniques employing transmission line, capacitors, inductors, and resistors require the use of networks that can be large, extremely lossy, and/or narrow band and are therefore unsuitable for many high frequency applications. 
     Thus, there remains a need for systems and methods to effectively use high impedance carbon nanotube devices in high frequency systems. 
     SUMMARY OF THE INVENTION 
     According to an embodiment of the present invention, an output stage is provided that has a high input impedance and sufficient transconductance to drive a low impedance load, and is formed from active devices. This output stage may be formed using multiple carbon nanotube field effect transistors in parallel. After determining the individual field effect transistor properties necessary for the desired application, such as gate oxide thickness, contact material, and channel properties, the number of parallel field effect transistors is selected to provide the required transconductance for the given load. In this way, the output stage functions as an active impedance matching stage, functional over a wideband with substantial power benefits. 
     According to an embodiment of the present invention, a multistage amplifier is provided which includes at least one high impedance gain stage and an impedance transforming output stage formed from active devices. Using the appropriate number of carbon nanotube field effect transistors to achieve the desired transconductance for the given design constraints, the input resistance of the output stage is of the same order of magnitude as the output resistance of a high impedance gain stage. This allows the output stage to be placed in series with high impedance gain stages without need for matching. The initial stages provide voltage gain at high impedance levels while the output stage is used as an impedance transformer, stepping down the impedance with unity voltage gain. This impedance transformation essentially provides the power gain. Thus, the use of an active output stage overcomes the difficulties of impedance matching using conventional techniques at high frequencies. 
     According to an embodiment of the present invention, a narrow band receiver is provided for an electronically steered receive array using lumped element nanoscale components at high impedances. The receiver is comprised of a high-gain input amplifier formed from nanoscale devices and coupled to an antenna, a double-balanced image reject mixer, a phase shifter, a power combiner, and an impedance transforming output stage formed from active nanoscale devices. This output stage transforms the high-impedance devices of the receiver to 50Ω for interfacing with external electronic systems. In this configuration, the output stage consumes over half of the total system power. 
     Some of the advantages of the present invention over the prior art include substantial power benefits, matching benefits, and compact structure. Because the majority of system power is consumed at the output stage, the addition of front end features such as image rejection have little cost to the overall power budget. Additionally, the use of the output stage as a matching network generates an effective match between the high-impedance electronics and a low impedance load, without the difficulties of traditional matching techniques or narrow-band results. Further, systems according to the present invention may be fabricated from monolithic, lumped element devices such that the small scale of nanostructures may be fully utilized. Through the novel matching technique of the present invention, using an active output stage to transform impedance, the advantages achieved with nanoscale devices may be utilized in real world systems. 
     Other objects and advantages will be apparent to those of skill in the art upon review of detailed description of the preferred embodiments and the accompanying drawings, in which like components are designated with like reference numerals. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an electronic circuit with an impedance transforming output stage used to interface high impedance devices with an external load; 
         FIG. 2  is a diagram of a field-effect transistor having a carbon nantotube grown into the channel thereof; 
         FIG. 3  is a diagram of a multistage microwave amplifier; 
         FIG. 4  is a schematic of the individual stages of the amplifier shown in  FIG. 3 ; 
         FIG. 5  is a diagram of a narrow band receiver for an electronically steered phased array; 
         FIG. 6A  is a diagram of an amplifier array and  FIG. 6B  is a diagram of a narrow-band common-source amplifier based on nanotechnology used in the receiver of  FIG. 5 ; 
         FIG. 7A  is a diagram of a mixer array and  FIG. 7B  is a diagram of a double-balanced image reject mixer based on nanotechnology used in the receiver of  FIG. 5 ; 
         FIGS. 8A-C  are schematics of the individual elements of the mixer shown in  FIG. 7 ; 
         FIG. 9A  is a diagram of a phase shifter array and  FIG. 9B  is a schematic diagram of a phase shifter based on nanotechnology used in the receiver of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     While the present invention may be embodied in many different forms, a number of illustrative embodiments are described herein with the understanding that the present disclosure is to be considered as providing examples of the principles of the invention and such examples are not intended to limit the invention to preferred embodiments described herein and/or illustrated herein. 
       FIG. 1  shows a block diagram of a circuit according to an embodiment of the present invention. The circuit  100  includes one or more input or initial stages  102  and an output stage  104  electronically coupled to one of the initial stages and configured to receive an electronic signal there from. The initial stages,  110  and  112 , of the circuit are comprised of high frequency, high impedance devices which include nanoscale devices. As used herein, the term “high frequency” refers to devices and systems operational in the microwave, millimeter wave, and sub-millimeter wave frequency regions. Examples of suitable nanoscale devices include, but are not limited to, nanowires, quantum dots, molecular transistors and carbon nanotube field-effect transistors (CNT FETs). A high frequency signal is received at the initial stages from a source  106 . The initial stages may each perform one or more functions including, but not limited to, amplification, switching, or signal processing. Following the high impedance stages is an impedance transforming output stage  104  which delivers the signal to a low impedance load  108 . 
     The output stage  104  has a high input impedance and low output impedance, and is formed from active devices. Specifically, the output stage may be formed using multiple CNT FETs in parallel.  FIG. 2  is a diagram of a CNT FET  200  having a carbon nantotube  202  grown into the channel thereof. This configuration was reported by International Business Machines Corporation and is published at “Single- and multi-wall carbon nanotube field-effect transistors”, R. Martel, T. Schmidt, H. R. Shea, T. Hertel and Ph. Avouris, Applied Physics Letters, 73, 17, pp 2447-9, 1998, the contents of which are incorporated herein by reference. 
     After determining the individual FET properties necessary for the desired application such as gate oxide thickness, contact material, and channel properties, the number of parallel FETs used in the output is selected to provide a suitable transconductance to drive a low impedance load. In this way, the output stage functions as an active impedance matching stage, functional over a wideband with substantial power benefits. Thus, a circuit according to the present invention may be designed and fabricated using high impedance nanoscale devices, yet easily interface with external systems. 
       FIG. 3  shows a wideband amplifier  300  according to an embodiment of the present invention. The amplifier is divided into first  302 , second  304 , and third  306  high frequency gain stages and is designed to drive a low impedance load  308  with a signal received from source  310 . Each stage is a common-source gain stage including a plurality of CNT FETs arranged in parallel. The first  302  and second  304  stages are electronically coupled such that an electronic signal is amplified by the first stage  302  and then transmitted from the drain of the first stage  302  to the gate of the second stage  304 . Similarly, after being amplified in the second stage  304 , the electronic signal is transmitted to the gate of the third and final stage  306 . The drain of the final stage  306  is connected to a low impedance load  308 , e.g., a 50Ω transmission line. 
     The first  302  and second  304  stages provide voltage gain at a high impedance level while the final stage  306  acts as an impedance transformer, stepping down the impedance with unity voltage gain. Specifically, the input impedance of the first stage and the input impedance of the second stage are at least an order of magnitude greater than the impedance of the low impedance load. The power gain (or current gain) is attributed to the impedance transformation provided by the final stage  306 . The number of CNT FETs used in the final stage  306  is selected to provide sufficient transconductance to drive a low impedance load  308 , such as a 50Ω transmission line, thus providing an interface to an external electronic system  320 . External electronic systems may include, but are not limited to, transmitters and receivers, signal processing circuitry, imaging systems, or a testing system such as a network analyzer. 
     Referring to  FIG. 4 , the final stage  306  can be configured to drive a 50Ω load  308  with unity voltage gain. The transconductance may be defined as:
 
 g   m   =A   V   /Z   load  
 
Therefore, a transconductance of 20 mS permits the system to drive a 50Ω load with unity voltage gain. A CNT FET has a transconductance of only about 155 μS. In an embodiment, a transconductance of 20 mS can be produced using 130 CNT FETs arranged in parallel  400 .
 
     A CNT FET device with a 0.5 micron channel has a capacitive input impedance of 10 −4  pF. Thus, the final stage has a total input impedance of 0.013 pF. At X-band, this is an input impedance of approximately 1 kΩ and the final stage has a total dissipated power of 130 μW. 
     As shown in  FIG. 4 , the output of the second stage  304  can be connected directly to the gate  402  of the final stage  306 . Therefore, the second stage  304  can be designed to drive a load of approximately 1 kΩ, the input impedance of the final stage  306 . Having 10 CNT FETs in parallel  404 , the voltage gain of the second stage  304  is
 
 A   V   =gm*Z   load =1.55 mS*1 kΩ=4 dB.
 
At X-band, the second stage  304  has an input impedance of approximately 13 kΩ and dissipates about 10 μW of power.
 
     As shown in  FIG. 4 , the output of the first stage can be connected directly to the gate  406  of the second stage  304 . In this configuration, the first stage  302  is designed to drive a load of approximately 13 kΩ, the input impedance of the second stage  304 . Because it is driving a much higher impedance load, the first stage  302  provides a substantial gain even with a relatively low total transconductance. Having only 3 CNT FETs in parallel  408 , the voltage gain of the first stage is
 
 A   V   =g   m   *Z   load =465 μS*13 kΩ=16 dB.
 
The input impedance of the first stage  302  is approximately 43 kΩ and the power dissipated in this stage is about 1 μW. The total power dissipated in the amplifier  300  is thus 143 μW. Were the amplifier to be designed with a single gain stage of CNT FETs operable to drive a low impedance load, the amplifier would dissipate as much as 1.3 mW.
 
     In this embodiment, the majority of the total power dissipated in the amplifier  300  is spent in the impedance transforming final stage  306 . Therefore, one may add additional high impedance voltage gain stages without significantly changing the power budget of the amplifier. This is important in both low-power systems as well as extremely small systems, where thermal management due to power dissipation is of critical concern. 
       FIG. 5  shows a narrow band receiver  500  for an electronically steered array according to an embodiment of the present invention. The receiver includes an array of low noise amplifiers (LNAs)  502 , an array of image-reject mixers  504 , an array of phase shifters  506 , and a power combiner  508  built from high impedance nanoscale components as well as an impedance transforming output stage  510  designed to drive a low impedance load  514 . Because it is not possible to construct effective transmission lines at the impedance levels of nanoscale devices, it may be necessary to construct phase shifters and power combiners from active components. 
     Referring to  FIG. 6A , the LNA array  502  used in the receiver of the present invention is comprised of individual LNAs  600 . As shown in  FIG. 6B , each LNA is common source amplifier based upon a CNT FET  602  that is configured to provide a suitable voltage gain for the application, e.g., a voltage gain of 20 dB. Each LNA receives a high frequency signal from a source  512 , e.g., an antenna electronically coupled to the input  604  of the LNA. The LNA may include a single CNT FET  602  and achieve a high gain through the use of on-resonance impedance of an RF filter  606 . In the present embodiment, the RF filter  606  is an LC parallel resonator which provides a high impedance in the pass-band of the receiver and low impedance out of band. When selecting the value of capacitor  608  used in the filter  606 , the input capacitance of the next stage may be included. Additionally, more sophisticated filters may be used to address size or frequency constraints. After being amplified, the signal is then routed to a double-balanced image-reject mixer. 
       FIG. 7A  shows an image reject mixer array  504  comprised of individual mixers  700 . As shown in  FIG. 7B , a double-balanced image-reject mixer  700 , comprising an element in the image reject mixer array  504 , includes two double-balance mixers  702  and  704 , two 90 degree phase shifters  706  and  708 , and a differential power combiner  710 . The electronic signal is received at node  716  from an amplifier  600  at a radio frequency (RF) port  718 , and is then routed to a first double balanced mixer  704  where it is mixed with a signal from the local oscillator (LO) received at LO port  722 , which has been shifted 90 degrees by phase shifter  708 . Separately and simultaneously, the RF signal  716  from the amplifier  600  received at RF port  720  and then shifted 90 degrees by phase shifter  706 , is transmitted to second double balance mixer  702  where it is combined with a signal from the LO received at port  714 . The intermediate frequency (IF) signals outputted from the mixers are combined in a CNT FET based differential power combiner  710  generating an image rejected IF signal outputted at port  712 . Adding a DC bias current to an RF port of the mixer can substantially increase device linearity without adding significantly to the power budget. 
       FIG. 8A  shows an exemplary CNT FET based double balance mixer  802  which may be used in the image-reject mixer  700  of the present embodiment. The double balance mixer  802  includes four CNT FETs in a ring configuration  804 , adapted to receive an RF signal at node  806 , an LO signal at node  808 , and produce an IF signal at node  810 . Similarly, both the 90 degree phase shifter  812  shown in  FIG. 8B  and the differential power combiner  814  shown in  FIG. 8C  are CNT FET based as well. 
     After the signals have been mixed, they are then phase-shifted and combined.  FIG. 9A  illustrates a phase shifting element  900  in the phase shifter array  506 .  FIG. 9B  is a device level schematic of the phase shifting element  900 . As shown in  FIG. 9B , a variable phase shifter  900  may be formed from two nanoscale devices  902  and  908 . Specifically, the quantum capacitance (small density of states) of the nanoscale device  902  makes it an ideal ultra-low capacitance varactor, which is used in the present system to tune an LC resonator  904 . By adjusting the voltage applied to the nanoscale device  902 , the phase of the signal at output node  906  may be variably shifted up to +/−45 degrees. If a larger phase shift is required, multiple phase shifters may be used in series. The output signals of each of the phase shifting elements in the array  506  are fed to the input of a combining element  508  where the signals are combined. 
     With reference to  FIG. 5 , after the input signals have been mixed, phase-shifted, and added together, they are fed to an amplifier output stage  510  comprising multiple CNT FETs in parallel as discussed in prior embodiments. The number of CNT FETs used in the output stage is selected to provide sufficient transconductance to drive the transmission line  514 , thus providing an interface to external electronic systems  516 . When the linearity to dissipated power ratio is maximized, the output stage  510  consumes over half of the total system power. 
     From the above, it will be appreciated that the novel matching technique of the present invention, using an active output stage to transform impedance, makes it possible to use nanoscale devices in an advantageous manner in real world systems. 
     Thus, a number of preferred embodiments have been fully described above with reference to the figures. Although the invention has been described based upon these preferred embodiments, it would be apparent to those of skill in the art that certain modifications, variations, and alternative constructions would be apparent, while remaining within the spirit and scope of the invention.