DVCC-based Low-power RF Filter with 32 nm CNFETs

This work presents a wide-band active ﬁlter for RF receiver. The design uses Carbon Nanotube-FET (CNFET) based diﬀerential voltage current conveyor (DVCC) for the implementation of the proposed ﬁlter. The ﬁlter is designed to operate Ku-band frequencies (12–18 GHz), which is used in satellite communication. Additionally, CMOS based circuit and CNFET-based circuit for DVCC are compared for the performance evaluation. HSPICE simulations have been carried out to test the design aspects of the circuit. The CNFET-based circuit has better results in terms of 60 % reduction in the power consumption and about six times improvement in the bandwidth. The ﬁlter utilizes low supply voltage of 0.9 V and consumes 524 µ W only. The proposed ﬁlter outperforms the existing CMOS-based designs which suggests its usage for low-power high-frequency analog circuits.

audio to RF and across the complete spectrum of frequencies. These filters are used in communication systems such as satellite communication, transponders etc. Moreover, military radios for multi-band or frequency-hopped transceivers also employ these filters. Additionally, Ku-band is used for satellite communications, mainly for fixed and broadcast based services. It ranges from [12][13][14][15][16][17][18] GHz as per the IEEE Standard [14]. In this work, CMOS-based DVCC is also realized with 32 nm model parameters. The 3-dB bandwidth of CMOS-based DVCC is measured 4.55 GHz. It means that CMOS-based DVCC can not implement the Ku-band filter circuit. As, CNFET-based DVCC has higher 3-dB bandwidth (29.8 GHz) than CMOS, it can efficiently realize the Ku-band frequency range.
Various techniques of RF filtering have been described in the literature [3,12,7,18,23]. An active inductor-capacitor based band-pass filter has been presented [12]. Further, this technique has been applied to RF filter with center frequency of 1 GHz. Moreover, a CMOS based high-Q RF filter has been presented [7], which utilizes the Q-enhancing technique over the frequency range of 625 MHz to 1.68 GHz. A PMOS cascode structure has been used as the negative transconductance of a gyrator to reduce noise [18]. The cascode structure was utilized for tunable active RF band-pass filter for the frequency range 3.9 -12.3 GHz. Additionally, a third order g m -c filter has been discussed. The frequency tuning is done by the current [3]. Further, a 12th-order complex filter structure has been illustrated for Bluetooth and Zigbee range of applications. This filter was based on controllable transconductors with low power requirement. All these techniques were employed with CMOS technology.
Moreover, because of increasing demand for hand-held/portable devices, low power design of the electronic circuit is essential. Also, due to downscaling of transistor dimension beyond 45 nm, short channel effects and source (S)-drain (D) tunneling arise. Therefore, now the industry is looking for new substitute materials and devices to combine with current CMOS process technology. The carbon nanotube (CNT) is one of the alternative material as it has very high drive current, less scattering and near ballistic transport of charge carriers [21,6]. The CNT-based circuits have advantages of lower leakage currents and better switching characteristics than conventional CMOS. Conductive metal contacts have been made on either end of the single-walled carbon nanotubes (SWCNTs) serve as the source and drain [8,1]. It tends to focus on the emerging ultra-low power devices such as CNFET, Nanowire-FET, etc. These technologies depend on the materials which are compatible with complementary metal oxide semiconductor (CMOS) processing, such as Carbon nanotubes (CNTs), Silicon nano wire, etc. [16].
In this paper, a Ku-band filter is realized using CNFET-based differential voltage current conveyor (DVCC). The CNFET-based DVCC has 3-dB bandwidth of 29.8 GHz. Thus, the proposed filter circuit is tuned to cover the entire Ku-band (12-18 GHz) which is not possible to implement by CMOSbased DVCC. The filter circuit utilizes low voltage supply of 0.9 V and estimated power dissipation of 524 µW. Moreover, the performance of CMOSand CNFET-based DVCC is also compared. The result shows that CNFET- Fig. 1 Circuit Symbol of DVCC [10] based DVCC reduces 60 % power requirement and improves 3-dB bandwidth approximately six times, then CMOS counterpart. This paper has been prepared in the following order. The brief overview of CNFET-based DVCC is presented in Section-2. A Ku-band filter has been realized in Section-3. Section-4 depicts the simulation results of proposed filter. Finally, Section-5 concludes the paper.

Overview of CNFET-based DVCC
The current conveyor was introduced by Sedra & Smith and after that many versions of current conveyors were discussed [5,13] but the second generation current conveyor (CC-II) proved to a versatile active element for the realization of current-mode (CM) and voltage-mode (VM) circuits. The CC-II is not suitable for differential input signals, as it has the one high impedance node (Y). However, the differential voltage current conveyor (DVCC) has covered this gap, as DVCC has an extra Y terminal to manage differential inputs. The symbol of DVCC has been depicted in Fig. 1.The input-output ports relation is given by (1).
The transistor level implementation of DVCC based on CNFET has been illustrated in Fig. 2. As the mobility of p-CNFET and n-CNFET is same thus the width of both types of transistors is taken same, which is not possible in case of CMOS design. The parameters for transistors (M 1 -M 18 ) of Fig. 2 are given in Table 1. Here, HSPICE 32 nm model file (CNFET) has been used for the design which is developed by Stanford University [9].
In Fig. 2, the transistors M 5 and M 6 , work as a current mirror which is set to drive two differential amplifiers consisting of transistors M 1 & M 2 and M 3 & M 4 . Additionally, transistors M 7 and M 11 provide the feedback action to make the voltage V X independent of current drawn from the node X. The current in terminal X is conveyed to the Z+ terminal with the help of transistors    Thus, the equation for node voltages of two differential pairs can be written as (2). After solving, the relation between X and Y nodes presented as (3).
The performance of CMOS-and CNFET-based designs should be compared so that the actual benefits of the proposed circuit should be evaluated. Therefore, the simulation of CMOS based DVCC is also performed. Table 2 shows the performance of CMOS-and CNFET-based differential voltage current conveyor. The CNFET represents its superiority with improvement in bandwidth and reduction in the power consumption with compared to CMOSbased design.

Proposed Ku-Band Filter based on CNFETs
Carbon nanotubes (CNTs) have emerged as a key material for modern day nano-scale systems design. Apart from their widespread use in biotechnology, material science, nano-electro-mechanical-systems (NEMS), etc., CNTs have encroached upon conventional MOSFETs for the design of high performance and low-power analog circuits [11]. In this section, a DVCC based voltagemode second order Ku-band filter has been presented. The applications using DVCCs as active elements have received considerable attention [10,19,17]. The design of active filter generally needs one (or more) active devices and passive components like capacitors and resistors. The inductors are not used due to their incompatibility with the standard CMOS fabrication process. Here, the active building block (DVCC) has been realized by CNFETs, which is further used for the realization of the proposed filter as shown in Fig.3. The filter circuit has capacitors (C 1 &C 2 ) which are connected to Z+ port of DVCC first & second. The proposed circuit uses two grounded resistors (R 1 &R 2 ) which is attractive for integrated circuit implementation. The circuit exhibits a good frequency performance. The given circuit can realize the standard band pass filter functions. The filter has following features: (i) use of CNFET-based DVCC as active elements with low power requirement; (ii) the employment of all grounded resistors; (iii) operated with low supply voltage of 0.9 V; (iv) suitable alternative for CMOS-based design. The proposed circuit is verified using HSPICE, a powerful tool for verifying new circuits based on active elements, which are either not commercially available or their implementation using available ICs is not very economical.
The analysis of circuit of Fig.3 illustrates the transfer function of proposed Ku-band filter and is given by (4). The equations for pole frequency and quality factor of proposed filter are represented as (5) and (6). The cut-off frequency is calculated as (7).
Here, resistors (R 1 &R 2 ) and capacitors (C 1 &C 2 ) are used as passive elements for the proposed filter design.

Simulation Results
The circuit analysis of the proposed filter is performed in the previous section which satisfies the band-pass filter function response. HSPICE simulations have been used to test the performance of the proposed circuit. In order to simplify (5), we choose R 1 = R 2 = R and C 1 = C 2 = C. The quality factor of the filter will be not affected by this selection though the cut-off frequency can be tuned (6). The cut-off frequency of the filter is further solved in term of R and C as (8). Now, as the design requires the cut-off frequency is in GHz range (12)(13)(14)(15)(16)(17)(18). Therefore, the value of resistor is consider in KHz range and calculate the capacitor value which is found to be in fF range. Further, to calculate the exact value of resistor, value of capacitor is considered in fF range initially. For example, capacitor value is taken five fF then value of the resistor can be calculated by (8). To validate the performance of the circuit, four different center frequencies have been opted such as 12 GHz, 14 GHz, 16 GHz & 18 GHz and the values of the resistor are found to be 2.6 K, 2.3 K, 2.1 K, and 1.7 K as illustrated in Table 3. The tuning is performed through the resistor as the resistor has fewer process steps than a capacitor in a semiconductor IC.
The tuning of the center frequency of the filter has been presented in Fig.4. Moreover, it has been observed that proposed filter utilizes carbon nanotube field effect transistor and places the valuable alternative to CMOS. The filter is also suitable for low power applications. The proposed CNFET-based design has the power requirement of 524 µW only. The performance of the proposed filter is compared with the literature as illustrated in Table 4. It shows that the proposed work has least power requirement.

Conclusion
CNFET has emerged as one of the most promising candidate for ultra-low power analog integrated circuits. The advantages of the carbon nanotube-FET are the smaller size, ballistic transport of carrier and higher mobility. This paper has explored the possibilities of the analog integrated circuit design using carbon nanotube-FETs. A low power RF band-pass filter is proposed using CNFET-based DVCC. Moreover, the power dissipation of filter is 524 µW only. HSPICE simulations confirm the capability of the circuit to work in the Ku-band of frequencies (12)(13)(14)(15)(16)(17)(18). The proposed filter could be a suitable alternative for CMOS-based circuit design in the future. CNFET based analog device has exhibited better results than its CMOS counterpart. DVCC based on CNFET has higher bandwidth and less power consumption. Thus, CNFET and their future integration on silicon platform could offer novel opportunities for the circuit design innovations.

Acknowledgement
The authors are thankful for Nanoelectronics group (Jie Deng, Albert Lin, and Gordon Wan), Stanford University, USA, for providing open access SPICEcompatible CNFET Model. It was not possible to realize the CNFET-based circuits without CNFET model.

Data Availability Statement
The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.

Declaration
We declare that all information on what should be included under each heading/subheading is related to our research work only. We have included proper citation and reference to literature work.