Current-mode multiphase sinusoidal oscillator using CDTA-based allpass sections

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Abstract

In this work, a current tunable current-mode multiphase sinusoidal oscillator (MSO) employing current differencing transconductance amplifier (CDTA)-based first-order allpass sections is presented. The proposed MSO circuit, which uses only two CDTAs and one virtually grounded capacitor for each phase, can generate arbitrary 2n-phase current-output signals (n=2,3,4,) equally spaced in phase, all at high output impedance terminals. The oscillation condition and the oscillation frequency can be controlled electronically and independently by adjusting the bias current of the CDTA. The oscillator has low-sensitivity performance. Simulation results are also given to verify the functionality of the proposed oscillator.

Introduction

Multiphase sinusoidal oscillators (MSOs) are widely used in instrumentation control, power electronics and signal processing and measurement systems. As a result, a number of MSO circuits have been reported in the technical literature [1], [2], [3], [4], [5], [6], [7], [8], [9]. In [1], [2], [3], [4], several MSO circuits, using a second-generation current conveyor (CCII) as an active component, have been proposed. Most of the reported circuits suffer from the use of more passive components and the lack of the electronic controllability. In addition, they operate in voltage-mode. The current feedback operational amplifier (CFOA)-based MSO circuit proposed in [5] exploits the internal pole of the device to operate at relatively high frequencies, but this approach requires access to the device compensation terminal. Recently, the techniques to realize the voltage-mode MSO using operational amplifiers (op-amps) were developed [6], [7]. However, the drawback of these circuits is the well-known limitations of the op-amps. Moreover, they utilize too many external passive components and a number of them float. In recent years, the current-mode approach of signal processing has offered elegant solutions for analog circuit problems. The main advantages of this operation mode are wide signaling bandwidth, high slew rate and low-power consumption. Thus considering this fact, the current follower-based MSO structure operating in current-mode has then been proposed [8]. It employs two current followers, one floating resistor and one floating capacitor for each stage, and does not provide electronic tunability. More recently, the current-mode MSO circuit based on current-controlled conveyors (CCCIIs) has been reported [9]. This method utilizes the parasitic resistance (Rx) of the conveyor that makes electronic tunability possible through the bias current. However, it still requires an excessive number of external passive capacitors, i.e., two grounded capacitors for each stage. Moreover, its oscillation condition is adjusted by tuning the ratio of external passive capacitors, which is not well controlled. In viewpoint of integrated circuits (ICs), controlling circuit parameters electronically is much easier to realize than by changing the values of passive components.

Recently, a new current-mode active element with two current inputs and two kinds of current output, which is called a current differencing transconductance amplifier (CDTA), has been introduced [10]. This device is a synthesis of the well-known advantages of the current differencing buffered amplifier (CDBA) [11] and a multiple-output transconductance amplifier to facilitate the implementation of current-mode analog signal processing. As a result, many applications and advantages in the design of various current-mode circuits using CDTAs as active elements have received considerable attention [12], [13], [14], [15], [16], [17]. These applications have proven that the CDTA is a versatile active building block for current-mode signal processing applications. Until now, there is only current-mode quadrature oscillator circuit based on CDTAs, which is recently reported in [16]. However, it produces two sinusoidal output currents with only 90 phase difference.

In view of the above explanations, a novel current-controlled current-mode MSO topology realized by using CDTA-based first-order allpass sections is proposed. The proposed oscillator circuit can produce 2n-phase output current signals (n=2,3,) of identical frequency and equally spaced in phase. Benefits of the proposed circuit are the absence of passive resistors, and the capacity for orthogonal electronic tuning of the oscillation condition and oscillation frequency (ωo) through the bias current of the CDTA. It also exhibits low active and passive sensitivities, and possesses high output impedance that can be easily cascaded for current-mode systems. Simulation results verifying theoretical analyses are included.

Section snippets

Current differencing transconductance amplifier (CDTA)

The circuit representation and the equivalent circuit of the CDTA are shown in Fig. 1. The terminal relation of the CDTA can be characterized by the following set of equations [10]:vp=vn=0iz=ip-inandix=gmvz=gmZzizwhere p and n are input terminals, z and ±x are output terminals, gm is the transconductance gain, and Zz is an external impedance connected at the terminal z. According to above equations and an equivalent circuit of Fig. 1(b), the current flowing out of the terminal z(iz) is a

Proposed current-mode MSO circuit

The proposed current-mode MSO topology is given in Fig. 5. It consists of n cascaded CDTA-based allpass sections of Fig. 3. The output current ion of the nth stage is fed back to the input of the first stage through the current amplifier of Fig. 4. Note that the current amplifier performing a feedback path has the gain of -K. An attractive benefit offered by this configuration is the absence of the external passive resistor, which is suitable for integration point of view [18], [19]. Assuming

Effects of CDTA non-idealities

In this section, the effect of CDTA non-idealities on the performance of the proposed circuit is studied. Fig. 6 shows the simplified equivalent circuit that will be used to represent the behavior of the non-ideal CDTA. These mainly result from its finite parasitic elements and non-ideal current transfers. As can be seen, there are parasitic resistances (Rp and Rn) at terminals p and n, and parasitic resistances and capacitances (Rz,Cz and Rx,Cx) from terminals z and x to the ground. In the

Simulation results

In order to validate the operation of the proposed current-mode MSO topology in Fig. 5, a six-phase MSO (n=3) has been designed. The circuit was simulated using PSPICE program. The CDTA was performed by the schematic bipolar implementation given in Fig. 2 with the transistor model parameters of PR100N (PNP) and NP100N (NPN) of the bipolar arrays ALA400 from AT&T [20]. The power supply voltages were chosen to be +V=-V=3V and the values of the bias currents were equal to IA=100μA and IB=50μA.

The

Concluding remarks

In this paper, an electronically controllable current-mode MSO topology is presented. The proposed MSO is implemented through the proposed CDTA-based current-mode first-order allpass filters and current-controlled current amplifier as the building blocks. The circuit can produce 2n-phase of equally spaced in-phase output currents, and all of them have high output impedance, which can be directly cascaded in current-mode operations. Moreover, it provides the attractive feature of independent

Acknowledgments

The authors would like to thank Professor Wanlop Surakampontorn of Faculty of Engineering, KMITL, for his valuable discussions and effort in helping to improve this work. The authors are also thankful to the editor and reviewers for their valuable comments and helpful suggestions, which substantially improved the quality of the manuscript.

Worapong Tangsrirat received the B.Ind.Tech. (Honors) degree in Electronics, M.Eng. and D.Eng. degrees in Electrical Engineering all from Faculty of Engineering, King Mongkut's Institute of Technology Ladkrabang (KMITL), Bangkok, Thailand, in 1991, 1997, 2003, respectively. Since 1995, he has been a faculty member at KMITL, where he is currently an Associate Professor in the Department of Control Engineering and serves as the leader of Mixed Signal Processing Laboratory, Research Center for

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Worapong Tangsrirat received the B.Ind.Tech. (Honors) degree in Electronics, M.Eng. and D.Eng. degrees in Electrical Engineering all from Faculty of Engineering, King Mongkut's Institute of Technology Ladkrabang (KMITL), Bangkok, Thailand, in 1991, 1997, 2003, respectively. Since 1995, he has been a faculty member at KMITL, where he is currently an Associate Professor in the Department of Control Engineering and serves as the leader of Mixed Signal Processing Laboratory, Research Center for Communications and Information Technology (ReCCIT) at the same institute. He has several published papers in leading international journals and conferences, and has authored books on electronics and control. At present, his research interests are mainly in integrated circuit design, analog signal processing, current-mode circuits, electronic instrumentation and measurement systems, and active filter design.

Wason Tanjaroen received the B.Eng. degree in Instrumentation System Engineering from King Mongkut's Institute of Technology North Bangkok (KMITNB) in 2004, and M.Eng. degree in Control Engineering from King Mongkut's Institute of Technology Ladkrabang (KMITL), Bangkok, Thailand, in 2007. He is currently working towards the D.Eng. degree in Electrical Engineering at KMITL. His research areas are mainly in analog integrated circuits and current-mode active filter design.

Tattaya Pukkalanun received the B.Eng. (Honors) degree in Control Engineering from King Mongkut's Institute of Technology Ladkrabang (KMITL) in 1998, M.Sc. degree in Advanced Electronic Engineering (with Distinction) from the University of Warwick, UK, in 2001, and M.Eng. degree in Electrical Engineering from KMITL in 2003. She is currently a lecturer at the Department of Control Engineering, Faculty of Engineering, KMITL. At present, she is studying towards the D.Eng. degree in Electrical Engineering at KMITL. Her research areas include analog circuit design, signal processing and electronic control engineering.

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