Practical Realization of Current Mode Active Elements Using AD844 and some Applications

In this paper various Current Mode active elements like first generation current conveyor (CCI)), third genera- tion current conveyor (CCIII), Differential voltage Current Conveyor (DVCC) and Dual-X-CCII (DXCCII) have been realized using commercially available chip AD844. The DXCCII is used as a building block to show its application as Amplifier, Integrator and All-pass filter. The proposed circuits are composed of only grounded passive components, which is suitable for designing in the Integrated circuits. PSPICE simulation has been done to verify the results.


Introduction
The operational amplifiers (opamps) suffer from the finite gain-bandwidth product which limits accuracy and reduces the frequency range of operation.[1-6] Therefore, many circuits such as oscillators, current-mode and voltage mode filters, simulated inductors, rectifiers etc. are constructed employing different types of current conveyors (CC) such as second-generation positive-type current conveyor(CCII+), second-generation negative-type current conveyor(CCII-), dual-output second-generation current conveyor (DOC-CII), first-generation current conveyor (CCI), third-generation current conveyor(CCIII), differential voltage current conveyor(DVCC), dual X current conveyor (DXCC), positivetype current gain variable CCII(CGVCCII+), positive-type voltage gain variable CCII(VGVCCII+). Current conveyors (CCI, CCII and CCIII) have received considerable attention since their introduction mainly due to their wider bandwidth and their capability to give current as well as voltage output. But one of the disadvantages of this current mode circuits is that there is only one commercially available chip in the market that is AD844 [7]. In fact, the AD844 is a current feedback Operational amplifier (CFOA) which can be used also as a positive type second generation current conveyor (CCII+). In this paper the CCII+ is not realized because the AD844 verifies its port characteristics. For practical purposes, it is very important to realize different types of CCs and other active devices using AD844 or CCII+s.
2. Current Mode Active elements 2.1Positive-type first-generation Current Conveyor (CCI+) The port characteristics of CCI+ are shown as matrix equation (1) (1) It can be realized with three CCII+s and three resistors as shown in fig.1. The current gains of the Z+ and Y terminals are equal to α=R 1 / R 2 and γ= R 1 / R 3 Fig.1 Positive-type first-generation current conveyor 2.2 Positive-type third-generation Current conveyor (CCIII+) The Port relation of CCIII+ can be described as: ( 2) It can be realized with three CCII+s and two resistors as shown in fig.2. The current gains of the Z+ terminal is equal to α=R 1 / R 2 It can be realized with three CCII+s and two resistors as shown in fig.3. The voltage gain of the DVCC is equal to β

Dual X Current conveyor (DXCCII)
A DXCCII can be implemented with two CCII+s, one CCII-(CCII-is formed by two CCII+) and two resistors as shown in fig.4 and symbol of DXCCII is shown in fig.5 The DXCCII is a five port active element with One high impedance, voltage input terminals:Y Two low impedence, current input terminal: Xp, Xn Two high impedence, current output terminals: Zp, Zn The input -output characteristic of DXCCII is defined as: (4) 3. Simulation Results PSPICE simulation using AD844 Model has been done. This simulation has been done at supply voltage of ± 10 volt and R 1 = R 2 =1K.   The simulated results which are shown in fig.6(a), 6(b) and 6(c) show the port characteristics verification of the CCI+. Fig. 7(a), 7(b) and 7(c) show the practical realization of CCIII+, fig.8 (a),8(b) and 8(c) is of DVCC and fig.9(a), 9(b) and 9(c) is of DXCCII. All the above result verified its port relations.

Applications of DXCCII 4.1Amplifier
Zn Xp  The transfer function of the proposed current mode lossy Integrator (Low pass filter) is given by:

All pass filter
The transfer function of the proposed current mode All pass filter is given by: ( )

Simulation Results
PSPICE simulation using AD844 Model has been done. This simulation has been done at supply voltage of ± 10 volt. Taking R 1 = 0.5k, R 2 =1k, R 3 =2k, and Iin=2mAp-p, the relation given by eqn. (5) shows non-inverting Amplifier with no error in Fig.13 of the CM integrated circuit.

Conclusion
The Current mode active elements have been realized using commercially available chip AD844. CCI+, CCIII+, DVCC and DXCCII port relations are verified. The DXCCII is used as a building block to show its applications as Amplifiers, Integrators and All-pass filters in the current mode topology. All the proposed circuits are composed of only grounded passive components, which are suitable for designing them in the integrated circuits.