Z-Copy Controlled-Gain Voltage Differencing Current Conveyor : Advanced Possibilities in Direct Electronic Control of First-Order Filter

1Abstract—A modified version of voltage differencing current conveyor (VDCC) and its performance in detail is presented in this paper. Modified VDCC, so-called z-copy controlled gain voltage differencing current conveyor (ZC-CGVDCC), offers interesting features from adjustability point of view. The active element allows independent electronic control of three adjustable parameters: intrinsic resistance of current input terminal, transconductance and current gain of the output stage which is not possible in case of conventional VDCC. The characteristics of proposed CMOS implementation designed using TSMC LO EPI 0.18 m technology process parameters are shown and discussed. Simple application in reconfigurable reconnection-less first-order voltage-mode multifunctional filter is shown and verified by SPICE simulations and experimentally. The filter tuning and change of the transfer function type is allowed by the controllable parameters of the ZC-CG-VDCC.


I. INTRODUCTION
Active elements [1] with more than one controllable parameter are very important for technical progress in this field, because they allow more effective electronic control, Manuscript received January 13, 2014; accepted April 9, 2014.Research described in the paper was supported by Czech Science Foundation project under No. 14-24186P, by internal grant No. FEKT-S-14-2281, and project Electronic-biomedical co-operation ELBIC M00176.The support of the project CZ. increasing variability in applications, simple circuit design and simple circuit structure.Frequently discussed active elements use principles based on electronic control of the intrinsic resistance (Rx) of the current input terminal x [2] (in the frame of current conveyor [3]) and control of the transconductance section (gm) [4].Active elements with possibility of current gain (B) control were presented by Surakampontorn et al. [5] and Fabre et al. [6].However, only the current gain control is realized.Some active elements that combine two externally controllable parameters (by bias voltage or current) were already discussed in applications of analog circuits and systems [7]- [9].Minaei et al. [7] proposed current conveyor with adjustable properties i.e. adjustable B between x and z terminals and adjustable intrinsic resistance of the current input terminal x.Marcellis et al. [8] introduced modified conveyor with controllable features of voltage gain between y a x and current gain between x and z terminals.Kumngern et al. [9] also combined Rx and B control in their version of translinear current conveyor.
Several combined active elements, based on transconductance section (OTA) [1], [4] and current conveyor of second generation (CCII) [2], [3], were already proposed.Typical examples of active elements with twoparameter control are some modifications of the very known current differencing transconductance amplifier (CDTA) [1], [10], where Rx and gm control is implemented by DC bias currents [11], [12].For example, the current conveyor transconductance amplifier (CCTA) [1], [13] also utilizes independent Rx and gm control in some of its modifications [14].One modification of CCTA [15] also employs current gain control, where current conveyor with gain-adjustable properties was used.Controllable current gain in frame of current conveyor [5], [6] seems to be an interesting and Z-Copy Controlled-Gain Voltage Differencing Current Conveyor: Advanced Possibilities in Direct Electronic Control of First-Order Filter valuable advantage.Active elements having this advantage attained many innovative modifications [16], [17].
The main aim of this paper is a design of an enhanced active element with useful and feasible controllable features.A voltage differencing current conveyor (VDCC) belongs to family of novel hybrid elements presented by Biolek et al. [1].In this contribution the VDCC, used as main core of the proposed new active device, is presented as z-copy variant with controllable parameters.We refer this modification as z-copy controlled gain voltage differencing current conveyor (ZC-CG-VDCC).Hitherto published works have already discussed so-called differential voltage current conveyor (DVCC) [1], [18]- [21].Nevertheless, both elements (VDCC and DVCC) have different behaviour and also different block structure that inheres from basic behaviour.The DVCC has two differential voltage input terminals y and realizes difference of two voltages [18]- [21].The rest of the conception is identical to common current conveyor [2], [3].One current input terminal x and current output terminal z (or multiple terminals z) are available.In comparison to discussed DVCC, the VDCC also consists of transconductance section (OTA) [1], [4] and offers additional auxiliary terminal for more universality of such element.Very interesting active elements, which belong to this family, were also proposed by Soliman [22] under designation pseudo-differential current conveyors (PDCCs).For example, the CCTA [13]- [15] utilizes same types of sub-blocks (CCII and OTA) as VDCC, however in reverse order of interconnection.This different order of interconnection also brings interesting features in applications.However, any of above discussed active elements and approaches do not allow control of three parameters within the frame of one active device as is presented in this paper.This approach allows construction of very simple applications with minimum number of passive elements.

II. Z-COPY CONTROLLED-GAIN VOLTAGE DIFFERENCING CURRENT CONVEYOR
Proposed active element, so-called z-copy controlled gain voltage differencing current conveyor (ZC-CG-VDCC) is shown in Fig. 1.This element was derived from basic theoretical concept of VDCC [1].The conventional VDCC consist of a transconductance amplifier [1], [4] connected to the y terminal of classical second generation current conveyor [3].Important fact is that only transconductance control is available in frame of conventional VDCC.Our modification allows simultaneous and mutually independent control of three important parameters, i.e. gm, Rx of current input terminal x in frame of the CCII section [2], and also B between x and z terminals of the CCII section [5]- [9].Behavioural model consists of the transconductor OTA-DISO (differential input and single output) and electronically controllable CCII (ECCII) [5]- [7], [9].The OTA section allows gm control and ECCII section realizes control of Rx and B.
Figure 1(b) explains behaviour of the proposed device in details and its possible block diagram is given in Fig. 1(c).Such active element seems to be complex, but internal CMOS topology is not challenging, see Fig. 2. Basic block structure form Fig. 1(c) was taken into account for design of CMOS realization in Fig. 2.There are three important parts (transconductor, current conveyor of second generation and adjustable current amplifier).The first part is the transconductance amplifier [1], [4], [23] with differential NMOS pair connected to a voltage input of CMOS current conveyor.Intrinsic resistance control was performed in frame of the current conveyor section [2], [23], which is separated from the last block forming adjustable current amplifier [24].The reason for this subdivision is simple.Bias control of intrinsic resistance in frame of current conveyor with current amplifier section influences at least dependence of current gain on control bias current significantly as was discussed in [25].Therefore, independent current conveyor and independent bias current serves for intrinsic resistance controlling purposes.

III. SPICE SIMULATION RESULTS OF THE ZC-CG-VDCC
Main parameters of the proposed ZC-CG-VDCC with power supply voltages 1 V are given in following figures.Gain control of current from x to z terminals was verified by adjusting of bias current Iset_B from 20 A to 100 A in range of B from 3.76 to 0.36 (Fig. 3).

IV. APPLICATION EXAMPLE: RECONFIGURABLE FIRST-ORDER MULTIFUNCTION FILTER
Controllable features of proposed active device can be used in so-called reconfigurable reconnection-less tunable multifunctional filter (no change of output or input terminal or change of structure is required for change of the transfer function [27] -electronic control allows contactless reconfiguration) very beneficially.These solutions were not studied in detail in the past and our initial experience shows that such active elements like ZC-CG-VDCC with variety of controllable possibilities are very useful for their synthesis.Proposed solution is shown in Fig. 6.Note that circuit in Fig. 6 requires z_TA terminal with opposite (negative) polarity.It is indicated by dashed line in CMOS structure in Fig. 2. Transfer function (TF) of the circuit in Fig. 6 has form   And if gmRx = 1, the low-pass (LP) filter response is obtained ( ) .
Special type of TF so-called inverting direct transfer (iDT) is available for gmRx = 2 and B = 0.In these conditions the circuit works as direct connection (constant magnitude and phase response) between input and output terminal.Pure direct transfer (non-inverting) is available for gmRx  0 together with any value of positive gain B.
We provided simulations of the circuit in Fig. 6 with proposed CMOS model (Fig. 2 I. Proposed active element was implemented in simple electronically reconfigurable reconnection-less first-order multifunctional filtering structure working in voltage-mode which features were verified in frequency band of hundreds of kHz by Spice simulations and experimentally with behavioural model of the proposed active device employing commercially available devices.Electronic control allows change of the transfer type (direct transfer, inverting direct transfer, all-pass and low-pass response) and tuning of the fp,z frequency (even independent control of fz).Tuning was tested for three values of B (0.5, 1, 2) that results in simulated fp,z (187, 350, 696 kHz).Measurements provide 166 kHz, 313 kHz and 610 kHz (Fig. 14).Behavioural and CMOS models of the ZC-CG-VDCC represent expected behaviour of filtering application and confirm our assumptions.ZC-CG-VDCC seems to be interesting choice for circuit synthesis and possible fabrication.

1 .
Proposed ZC-CG-VDCC with independent control of three parameters: a) symbol, b) behavioral model, c) possible block conception.

Figure 5 ( 5 .
Figure 5(b) shows dependence of Rx on Iset_Rx.Input resistance is controllable from 2.53 k to 451  by Iset_Rx adjusted from 10 to 150 A.Important parameters of the proposed ZC-CG-VDCC model are summarized in TableI.Notes in Fig.3-Fig.5meaninformation for connection of remaining terminals for presented set of analyses.

Fig. 12 .
Fig. 12. Reconfiguration between iDT, DT, LP and AP (phase responses).Transient response (oscilloscope Rigol DS1204B) of the AP filter is shown in Fig. 13 (finp = 313 kHz, AP has the same setting as we give in caption of Fig. 8).Tuning features for AP and LP responses are shown in Fig. 14 and Fig. 15.Ideal values of pole/zero frequencies (fp,z) are 169, 339 and 667 kHz (B = 0.5, 1, 2).Values of fp,z, obtained from simulations were 187 kHz, 350 kHz and 696 kHz and experiments yield values 166 kHz, 313 kHz and 610 kHz

Fig. 15 .
Fig. 15.Tuning of the LP filter (magnitude responses).V. CONCLUSIONS Our work is focused on design of modification of the voltage differencing current conveyor, proposal of its model and example of its useful features in simple application.The most important advantages of proposed active element are availability of three types of independent electronic control and useful z-copy technique.Range of the B control of the CMOS model was verified between 0.36 and 3.76 (Iset_B from 100 A to 20 A), control of the gm value between 255 S and 1 919 S (Iset_gm from 10 A to 150 A) and Rx value in range from 2.53 k to 0.451 k (Iset_Rx from 10 A to 150 A).Frequency features allow operation of CMOS 1.07/2.3.00/20.0007WICOMT, financed from the operational program Education for competitiveness, is gratefully acknowledged.The described research was performed in laboratories .05/2.1.00/03.0072, the operational program Research and Development for Innovation.Dr. Herencsar was supported by the project CZ.1.07/2.3.00/30.0039 of the Brno University of Technology.

TABLE I .
SUMMARIZATION OF IMPORTANT FEATURES OF THE ZC-CG-VDCC CMOS MODEL.