DESIGNING AN IMPROVED RESISTIVE SENSOR INTERFACE WITH CNFET.

Sensor interface has been widely used in applications areas such as pharmacy, food-processing, biotechnology, industries, and laboratories etc. The differential voltage current conveyor (DVCC) is a very versatile active building block and attained specialinterest for current-mode circuits. This paper attempts to explore an application of CNFET-based current conveyor (DVCC) for the design of a resistive sensor interface. The proposed interface circuit reduces 16.6% power than existing work. The design also minimizes the number of active and passive components in the circuit. Further, a DVCC based Instrumentation amplifier is presented which is also utilized for the simpler design of sensor interface. HSPICE simulations with 32 nm CNFET model are performed to test the design.

In this work, a comparison between CMOS and CNFET based DVCC is presented. Moreover, an improved CNFET based resistive sensor interface is proposed which reduces the active & passive components and power requirement than the existing work [9]. Further, the work also attempts to investigate the use of CNFET based Instrumentation amplifier (IA) for a simpler interface design. This rest of the paper is prepared in the following manner. The implementation of the differential voltage current conveyor (DVCC) using carbon nanotube-FETs is discussed in Section 2. Section 3 presents the improved circuit of the resistive sensor interface. Section 4 deals with simulation results and discussion. Section 5 presents the conclusion.

Brief Discussion on DVCC
The current conveyor was introduced by Sedra& Smith and after that many versions of current conveyors were discussed [1,3] 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) filled 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 is illustrated and given in Fig. 2. Here, HSPICE 32 nm CNFET parameters are used for the design which is developed by Stanford University [7]. The design parameters of CNFET are presented in Table 1. Considering the circuit of Fig. 2, basic operation can be explained as follows. The transistorsM5 andM6, work as a current mirror which is set to drive two differential amplifiers consisting of transistors M 1 & M 2 and M3 & M 4 . Additionally, the sum of drain currents of M 1 and M 4 is equal to drain currents of M 2 and M 3 . Further, since transistors M 9 and M 10 are biased with equal gate voltages (and since their source voltages are also equal), they would have equal drain currents. The relation between X and Y nodes presented as (2). Additionally, transistors M 7 andM 11 provide the necessary feedback action to make the voltage VX independent of current drawn from the terminal X. The current in terminal X is conveyed to the Z+ terminal with the help of transistors M 7 , M 8 , M 11 and M 12 .

Overview of Nanotube Transistor
The  Fig. 3. Here, carbon nanotubes work as channel material for the MOSFET-like device.
The variation in sensor's resistance (R S ) is rejected in the output voltage of first DVCC (3). Thus, voltage (V X2 ) of second DVCC is also change (4), which finally alter the output voltage (V out ) according to change in the resistance of sensor (8). Therefore, variation in sensor's resistance is measured by the change in the output voltage of proposed sensor interface. In addition to this, it is important to compare the parameters of existing and proposed circuits of the resistive sensor interface. Table 2 presents the fair comparison between these two circuit designs.

Resistive Sensor Interface using Instrumentation Amplifier
In the last section, an improved circuit of sensor interface is discussed. But, there is a simpler design proposed for resistive sensor interface. The feature of DVCC can also be utilized as an Instrumentation Amplifier (IA) [12]. That concept can be further proposed as a circuit for sensor interface as shown in Fig. 6. It utilizes a DVCC and a resistor with resistive sensor (R S ).
The relation (9) and (10) can be understood by properties of DVCC [18]. Moreover, change in (RS) is incorporated by the output voltage (11).

Simulation Results:-
The performance of CNFET-based DVCC with 32 nm feature size of CNFET is presented. The differential voltage current conveyor is simulated and tested through HSPICE, satisfying the characteristics as depicted in (1). Further, the current and voltage transfer characteristics of proposed building block are also tested with the fundamental properties. Fig. 7 and Fig. 8 illustrate the current and voltage transfer curves of the CNFET-based current conveyor. The input and output voltages have the linear relation i.e. V X = (V Y 1 -V Y 2 ). Additionally, to test the validity of DVCC circuit, transient-mode simulations are also performed. The response of input-output voltages is given in Fig. 9, while Fig. 10 reflects the currents of X and Z terminals of DVCC, as given in relations described by (1).
The comparison of CMOS and CNFET based DVCC is also performed. Table 3 illustrates the superiority of CNFET based DVCC over its CMOS counterpart. The CNFET based DVCC has 3.26 times improvement in bandwidth and 5.16 times reduction in power consumption than CMOS.   Fig. 6 is simulated for the resistance range of 1-100 K. The corresponding output voltage is depicted in Fig. 11. Thus, CNFET based design can also be used in the circuits where low power electronic interface is required.

Conclusion:-
This work has explored the design of low-voltage low-power resistive sensor interface circuit. The proposed circuit used CNFET-based active building block for the interfacing of resistive sensor. The power requirement of the circuit is about 175 µW with the supply voltage of 0.75 V. Further; the results showed that the proposed circuit has the advantage over existing circuit as it utilizes only two active and three passive components. The sensor interface circuit could be used in various research and industrial applications. The work also presented the comparison between CMOS and CNFET based DVCC. The CNFET based current conveyor has 5.16-and 3.26-times improvement in power requirement and 3-dB bandwidth respectively than its CMOS counterpart. It shows that CNFET gives superior performance than CMOS. The circuit performed HSPICE simulations with 32 nm model to test the functionality.