A NEW CMOS DESIGN AND ANALYSIS OF CURRENT CONVEYOR SECOND GENERATION (CCII)

: This paper describes the current conveyors used as a basic building block in a variety of electronic circuit in instrumentation and communication systems. Today these systems are replacing the conventional Op-amp in so many applications such as active filters, analog signal processing. Current conveyors are unity gain active building block having high linearity, wide dynamic range and provide higher gain bandwidth The proposed current conveyors are simulated using TSMC 0.18μm CMOS technology on Advanced Design System and the results are also tabulated for comparison. The main features of these current conveyors are low voltage, less power, high slew rate and wide bandwidth for voltage transfer ( V y to V x ) and current transfer ( I x to I z ) which make them suitable for high frequency and low power applications.


INTRODUCTION
One of the most basic building blocks in the area of current-mode analogue signal processing is the current conveyor (CC).The principle of the current conveyor of the first Generation was published in 1968 by K. C. Smith and A. S. Sedra [1].Current Conveyor First Generation CCI was then replaced by a more versatile second-generation device in 1970 [2], the CCII Current conveyor designs have mainly been with BJTs due to their high transconductance values compared to their CMOS counterparts.They are used as currentfeedback operational amplifiers like the MAX477 high-speed amplifier and the MAX4112 The current mode circuits such as Current conveyors (CCs) have received considerable attention and emerged as an alternate building block to the Op-Amp (voltage mode circuit) in the field of analog signal processing [3] due to its potential performance feature.In CCs, the use of current rather than voltage as the active parameter can result in a higher usable gain, accuracy and bandwidth due to reduced voltage excursion at sensitive nodes [4].The current conveyors are not only useful for current processing, but also offer certain important advantages in voltage processing circuits.The nonlinear circuits and dynamics [5] can easily be developed using CCs.
With the reduction in the supply voltage and device threshold voltage of CMOS technology, the performance of CMOS voltage mode circuits has greatly affected which results in a reduced dynamic range, an increased propagation delay and reduced noise margins.The CCs have simple structure, wide bandwidth and capability to operate at low voltage.It also offer unity current gain, unity voltage gain, higher linearity, wider dynamic range and better high frequency performance.

THE CURRENT CONVEYOR CC
The current conveyor is functionally flexible and versatile in nature as it has precise unity voltage gain between X and Y; unity current gain between Z and X as shown in Fig. 1, rather than the high ill-defined open loop gain of Op-Amps.Because of this fact, CCII is generally used without feedback in amplifier applications [6,7].[ The current conveyor is a grounded three-port network represented by the black box (Fig with the three ports denoted by X, Y, and Z.Its terminal characteristics can be represented best by a hybrid matrix giving the outputs of the three ports in terms of their corresponding inputs [8].

CURRENT CONVEYOR SECOND GENERATION CCII
The second-generation current conveyor (CCII) is used as a basic building block in many current-mode analog circuits.It offers high input impedance at voltage input port Y, which is preferable in order to avoid loading effect.Therefore, second generation current conveyor is developed to overcome the problem loading effect of CCI.The CCII is considered as a basic building block in analog circuit design because all the analog applications can be developed by making suitable connections of one or more CCIIs with passive and active components.
The second-generation current conveyor is a grounded three-terminal (X, Y and Z) device as shown in Fig. 2 (a), and the equivalent circuit of the ideal CCII is shown in Fig. 2 (b).The characteristics of ideal CCII are represented by the following hybrid matrix An ideal CCII has the following characteristics:

 Amplifiers using CCII
The CCII can easily be used to form the current output amplifiers and voltage-output amplifier as shown in Fig. 3.The voltage-and current-gains are as follows:   = −( 1 +  2 ) (7)  Differentiators using CCII Current-and voltage-mode versions are shown in Fig. 6.The output signals are as follows:

5.PROPOSED CMOS CURRENT CONVEYOR SECOND GENERATION
A new connection of Bulk-driven OTA is used to realize the CCII.In the OTA-based approach, presented in Fig. 7, Bulk-driven OTA is used to implement the unity gain buffer between the Y and X inputs [9].The X input current Ix is sensed by duplicating buffers, output transistors M6 and M7 using transistors M8 and M9, and extracting the X current from them as Iz.Since transistors M8 and M9 have the same size and gate-source voltage as the output stage transistors M6 and M7, the current Iz should be a copy of the current flowing through M6 and M7 which is Ix.Transistors M10-M15are used to generate Iz.Since no additional transistors need to be inserted between the OTA and rails, the approach will not increase the minimum operating voltage over that of the operational core.In addition the voltage follower is based on an OTA, thus it will maintain all the benefits and also the disadvantages of such a circuit i.e. a good voltage follower at the cost of lower bandwidth [10].The aspect ratios of each of the transistors used the CCII in Fig. 7 are listed in Table1.

SIMULATION RESULTS
The simulated frequency responses of current gains Iz+/Ix, Iz-/Ix are given in Fig. 8.The cut off frequencies for the gains are 20 MHz and 52 MHz, respectively.
In Fig. 9, the input voltage buffer behaviour is shown.A DC sweep simulation has been performed, to check the range in which the voltage on X node is equal to the voltage applied to Y node.

CONCLUSION
In this paper Bulk-driven CCII based on operational Transconductance Amplifier OTA is simulated using TSMC 0.18um CMOS technology with 0.6V power supply.Differential pair partially improves for power dissipation and terminal impedances but bandwidth reduces when scaled down from 0.35um to 0.18um.CCII can be used as a voltage buffer and current buffer.
amplifier, which both feature current feedback rather than the conventional voltage feedback used by standard operational amplifiers.Current conveyors are used in highfrequency applications where the conventional operational amplifiers can not be used, because the conventional designs are limited by their gain-bandwidth product.

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Infinite input impedance at terminal Y (RY = ∞ and IY = 0)  Zero input impedance at terminal X (RX = 0)  Accurate voltage copy from terminal Y to X (VX = VY)  Accurate current copy from terminal X to Z with infinite output impedance at Z (IZ = IX and RZ = ∞)