Current-Mode Third-Order Quadrature Oscillator Using CDTAs
This paper describes a current-mode third-order quadrature oscillator based on current differencing transconductance amplifiers (CDTAs). Outputs of two current-mode sinusoids with phase difference are available in the quadrature oscillator circuit. The oscillation condition and oscillation frequency are orthogonal controllable. The proposed circuit employs only grounded capacitors and is ideal for integration. Simulation results are included to confirm the theoretical analysis.
Various new current-mode active building blocks have received considerable attentions owing to their larger dynamic range and wider bandwidth with respect to operational amplifier-based circuits. As a result, current-mode active components have been increasingly used to realize active filters, sinusoidal oscillators, and immittances.
Quadrature oscillator is used because the circuit provides two sinusoids with 90° phase difference, as, for example, in telecommunications for quadrature mixers and single-sideband generators or for measurement purposes in vector generators or selective voltmeters. Therefore, quadrature oscillators constitute an important unit in many communication and instrumentation systems [1–17]. Two-integrator loop technique was developed to realize quadrature oscillators by using operational amplifiers or transconductance elements in [1, 2]. Holzel  proposed a method for realizing quadrature oscillator consists of two allpass filters and an inverter using operational amplifiers. Keskin et al. [15, 17] proposed two quadrature oscillators that were designed out by the method in  using current differencing buffered amplifiers (CDBAs) or current differencing transconductance amplifiers (CDTAs). Ahmed et al.  proposed two quadrature oscillator circuits that were realized base on the allpass filters and the noninverting integrators as building blocks using operational transconductance amplifiers (OTAs). This method was also used in  to obtain a quadrature oscillator using CDBAs. Soliman  describes several quadrature oscillator circuits based on the modification of two-integrator loop technique using current conveyors. Because the high-order network has high accuracy and high-quality factor, it gives good frequency response with low distortion [9–12]. Prommee and Dejhan  proposed two third-order quadrature oscillators using OTAs. Horng et al. [10, 11] proposed four voltage-mode third-order quadrature oscillator circuits; each circuit uses three second-generation current conveyors (CCIIs). Maheshwari and Khan  proposed a current-mode third-order quadrature oscillator using four CCIIs.
In 2003, a new current-mode active element that is called current differencing transconductance amplifier (CDTA) was introduced . Owing to the current conveying property, the CDTA is one of the modifications of the current conveyor (CC). Many applications in the design of active filter  and multiphase sinusoidal oscillator  using CDTAs as active elements have received considerable attention. A second-order current-mode quadrature oscillator consists of two CDTAs, four resistors, and two capacitors was presented in . However, the capacitors used in this circuit are connected to the input terminals of the CDTAs. Since the input terminals of CDTA have parasitic resistances , this quadrature oscillator is not ideal for high-frequency applications. In 2006, Biolek et al. proposed a second-order current-mode quadrature oscillator based on two-integrator loop technique . The main disadvantage of this oscillator is that there is no control on the condition of oscillation.
In this paper, a CDTAs-based current-mode third-order quadrature oscillator circuit is presented. The oscillation condition and oscillation frequency of the proposed quadrature oscillator are orthogonal controllable. The proposed quadrature oscillator uses only grounded capacitors. The use of only grounded capacitors is especially interest from the fabrication point of view .
2. Proposed Circuit
The circuit symbol and the equivalent circuit of the CDTA are shown in Figure 1. The terminal characteristic of the CDTA can be described by the following equations : where and are input terminals, and are output terminals, is the transconductance gain, and is external impedance connected at the terminal. According to the above equation and equivalent circuit of Figure 1(b), the current flowing out of the terminal is a difference between the currents through the terminals and . The voltage drop at the terminal is transferred to the currents at the terminal by the transconductance gain , which is electrically controllable by an external bias voltage. These currents that are copied to a general number of output current terminals are equal in magnitude but flow in opposite directions. A possible CMOS-based CDTA circuit realization is given in Figure 2 .
The oscillation condition and oscillation frequency can be obtained as
From (3) and (4), the oscillation frequency can be controlled by or . The oscillation condition can be independently controlled by . From Figure 3, the current transfer function from to is Under sinusoidal steady state, (5) becomes The phase difference, , between and is ensuring the voltages Io2 and Io1 to be in quadrature.
The proposed quadrature oscillator employs only grounded capacitors. The use of grounded capacitors is particularly attractive for integrated circuit implementation . From (6), the magnitude of and need not the same. For the applications need equal magnitude quadrature outputs, another amplifying circuits are needed.
3. Nonideal Effects
Taking the nonidealities of the CDTA into account, Figure 4 shows the simplified equivalent circuit that is used to represent the nonideal CDTA . In the figure, and is the current tracking error from the terminal to the terminal of the CDTA, and is the current tracking error from the terminal to the terminal of the CDTA, and and is the output transconductance tracking error from the terminal to terminal of the CDTA. Moreover, there are parasitic resistances ( and ) at terminals and and parasitic resistances and capacitances (, and , ) from terminals and to ground. Reanalysing of the proposed quadrature oscillator in Figure 3 using the nonideal CDTA model and assuming that the operation oscillation frequencies, , are very much smaller than or and the parasitic resistances at the terminals are very much greater than the parasitic resistances at or terminals of CDTAs, the characteristic equation of Figure 3 becomes where
The modified oscillation condition and oscillation frequency are where , , , , , , and .
Because the values of and are slightly less than unity , the parasitic conductances () at the terminals of CDTAs are not zero and the capacitances , , and are greater than , , and , respectively. From (9) and (10), the oscillation condition and oscillation frequency are deviated from the ideal cases. Therefore, to compensate this effect, we can slightly adjust the or values. The oscillation condition still can be independently controlled by . The active and passive sensitivities of the quadrature oscillator are all low and obtained as
4. Simulation Results
The quadrature oscillators were simulated using HSPICE. The CMOS CDTA implementation is shown in Figure 2 (using 0.18 m MOSFET from TSMC). The aspect ratios of the MOS transistors were chosen as in Table 1. The multiple current outputs can be easily implemented by adding output branches. Figure 5 represents the current-mode quadrature sinusoidal output waveforms of Figure 3 with , , , and where was designed to be larger than the theoretical value to ensure that the oscillations will start. The bias voltages are , and . The power dissipation is 3.9486 mW. The results of the and total harmonic distortion analysis are summarized in Tables 2 and 3, respectively. Figure 6 shows the simulation results of the oscillation frequencies of Figure 3 by varying the value of the transconductance with , , and was varied with by (3) to ensures that the oscillations will start.
In this paper, a new current-mode third-order quadrature oscillator using three CDTA and three grounded capacitors is proposed. Outputs of two sinusoids with 90° phase difference are available in the proposed quadrature oscillator. The oscillation condition and oscillation frequency of the proposed quadrature oscillator are orthogonal controllable. Simulation results verify the theoretical analysis.
The author would like to thank the reviewers for their suggestions. The National Science Council, China, supported this work under Grant no. NSC 98-2221-E-033-054.
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