Journal of Applied Science and Engineering, Vol. 19, No. 2, pp. 187-196 (2016) DOI: 10.6180/jase.2016.19.2.09 Third-Order Sinusoidal Oscillator Using a Single CMOS Operational Transresistance Amplifier Hung-Chun Chien Department of Electronic Engineering, Jinwen University of Science and Technology, Xindian, Taiwan 231, R.O.C. Abstract This paper presents the design of a compact third-order sinusoidal oscillator based on an operational transresistance amplifier (OTRA). The proposed circuit consists of a single OTRA combined with three resistors and three capacitors. A review of relevant literature revealed that this is the first study to design a third-order sinusoidal oscillator, constructed with a single OTRA and the minimal number of passive components, with independent control of the oscillation condition and frequency. This study involved a review of previous designs as well as related formulations, nonideal analyses, and sensitivity discussions regarding the proposed circuit. Because the proposed circuit features a low-impedance output, it can be applied in cascading and used without additional buffer circuits. This study conducted simulations for the proposed circuit using HSPICE, and used commercially integrated circuits (ICs) and discrete components for circuit implementation and testing to verify its feasibility. Simulation and experimental results confirmed the validity of the proposed oscillator circuit. Key Words: Active-RC Circuit, Analog Circuit Design, Current-mode Circuit, Norton Amplifier, Operational Transresistance Amplifier (OTRA), Sinusoidal Oscillator 1. Introduction Early active-RC circuit designs have been widely employed in voltage-dominated operational amplifiers (OAs) as the active components in manufacturing various electronic circuits in applications [1]. Although OAs have long been the predominant active components for analog circuit designs, the conventional method they employ has been challenged by the requirements of high performance circuit applications, such as higher signal bandwidth, larger dynamic range and slew rate, lower power consumption, functional versatility, and simple implementation. Previous research has revealed that the current-mode circuit technique is more adequate for use in analog circuit designs than in voltage-mode circuits. Therefore, interest in designing analog circuits based on the current-mode circuit technique has increased [2]. Sinusoidal oscillators are vital circuit cells, which are used *Corresponding author. E-mail: hcchien@just.edu.tw in numerous electronic and electrical applications, for example, measurement meters, AC machine control systems, power converter control circuits, telecommunication modules, and medical equipment, etc. For decades, OAs have been used as the active component in constructing sinusoidal oscillators [3]; however, their use excessive amounts of passive components and have been unable to provide an adequate performance in high-frequency operation (< 100 kHz). To overcome this problem in designing sinusoidal oscillators, current active components based on the current-mode circuit technique have become a feasible option, providing superior circuit performances compared with OA-based solutions. Several typical studies have reported designs employing various active components [4-9]. A three-terminal active component called the operational transresistance amplifier (OTRA), introduced in [10,11], was an improved version of commercial Norton amplifiers. Because the OTRA exhibited a higher operating speed, wider signal bandwidth, and was free of the parasitic capacitance ef- 188 Hung-Chun Chien fect [12], there was an increased interest in sinusoidal oscillator designs based on this device. The first reported single-phase sinusoidal oscillators employing OTRAs as the active component appeared in [13]. However, the circuits depicted in [13] (topologies in Figures 3(b) and 3(c) of [13]) required numerous passive components to construct the second-order sinusoidal oscillators. Consequently, an improved topology designed with an OTRA, three resistors, and two capacitors was proposed to reduce the number of passive component and enable a resistance-controlled oscillation frequency [14]. However, that circuit still lacked an independent control for adjusting the oscillation condition. Thus, a specific design process regarding the oscillation condition must be first established for this circuit, to ensure the start of an oscillation procedure. In 2010, researchers presented an OTRA-based single-phase sinusoidal oscillator with an independent control of the oscillation condition and frequency [15]. However, that topology exhibited the complex circuitry and was considered a noncanonical design, because it employed three capacitors to realize a second-order sinusoidal oscillator. Recent studies have reported feasible topologies to enrich the list of the OTRA-based sinusoidal oscillator [16,17]. However, all of the suggested oscillators presented in [16,17] still did not enable an independent control of the oscillation condition and frequency. One study [18] presented another feasible scheme featuring independent control of the oscillation condition and frequency. However, the main problem of this design is that it requires an additional buffer circuit, thus indicating that the reported oscillator cannot be treated as a single OTRA-based design. Other types of sinusoidal oscillator using OTRAs as the active component include quadrature oscillators [13,19,20] and multiphase oscillators [21]. An available method for constructing quadrature oscillators is based on cascading the active-RC filters and integrators in a closed loop connection [19,20], but requires more active and passive components. For highly precise signal processing applications, third-order sinusoidal oscillators are preferable to second-order sinusoidal oscillators. Third-order sinusoidal oscillators first appeared in 2002 [22]; subsequently, third-order sinusoidal oscillators have since attracted increased attention [23-26]. Although secondorder OTRA-based single-phase sinusoidal oscillators have appeared in previous studies [13-18], third-order single-phase sinusoidal oscillators employing a single OTRA as the active component have not yet been discussed. Only two OTRA-based third-order quadrature oscillators were explored in [27]. However, both of the presented circuits employed three OTRAs, five resistors, and three capacitors, and were therefore considerably complex. This paper proposes the design of a third-order OTRA-based single-phase sinusoidal oscillator. The proposed circuit is composed of a single OTRA, three resistors, and three capacitors, which enables independent control of the oscillation condition and frequency. The proposed oscillator, which features a compact circuitry, contributes a novel approach to the study of third-order sinusoidal oscillator designs. Table 1 displays a comparison of various solutions to illustrate the novelty and advancement of the proposed circuit. Among the existing OTRA-based single-phase sinusoidal oscillators [1318], the proposed circuit exhibits the following advantages: 1) this is the first study to construct an OTRAbased third-order single-phase sinusoidal oscillator with a single OTRA and low number of passive components; 2) independently controllable oscillation condition and frequency (except for [15,18]); 3) low-impedance voltage output enabling cascading applications without supplementary buffer circuits; and 4) an operating frequency higher than that exhibited by operational-amplifier-based designs. The remainder of this paper is organized as follows. Section 2 introduces the OTRA followed by a circuit description of the proposed OTRA-based third-order sinusoidal oscillator and related governing equations. Section 3 discusses the nonideal effects and sensitivity analyses. Section 4 demonstrates the effectiveness of the proposed circuit by presenting computer simulation and experimental results. Finally, section 5 provides the conclusion to summarize this study. 2. Proposed OTRA-Based Third-Order Sinusoidal Oscillator An OTRA is a transimpedance-type active building block with two current input terminals (I+ and I-) and a voltage output terminal (Vo) [10,11]. The circuit symbol of an OTRA is depicted in Figure 1. The terminal relations of an OTRA are characterized in (1), using a matrix form, which indicates that the output voltage (Vo) is the difference between two input currents (I+ and I-) multiplied by the transresistance gain (Rm). Ideally, the transresistance gain (Rm) approaches infinity, forcing the two Third-Order Sinusoidal Oscillator Using a Single CMOS Operational Transresistance Amplifier 189 Table 1. Comparisons among various OTRA-based single-phase sinusoidal oscillators Circuit Component numbers and connected type Independent control Circuit of OC and OF/Type order/Canonical of OC and OF design control Implement technology/ Supply voltage Measured highest frequency/Total harmonic distortion Topology in OTRA ´ 1 Figure 3(b) of Resistor ´ 4 [13] (Only one grounded) Capacitor ´ 2 (Floating) Second-order/ Yes No (only of the oscillation condition)/ OC: by R OF: NA CMOS realization (NA)/±2.5 V Hundreds of kHz (790 kHz)/NA Topology in Figure 3(c) of [13] OTRA ´ 1 Resistor ´ 3 (Floating) Capacitor ´ 3 (Only one grounded) Second-order/ No No (only of the oscillation frequency)/ OC: NA OF: by C CMOS realization (NA)/±2.5 V Hundreds of kHz (790 kHz)/NA Topology in Figure 1 of [14] OTRA ´ 1 Resistor ´ 3 (Only one grounded) Capacitor ´ 2 (Floating) Second-order/ Yes No (only of the oscillation frequency)/ OC: NA OF: by R CMOS realization (MIETEC 1.2-mm CMOS process technology)/±5 V Several MHz (1.59 MHz)/NA Topology in Figure 3 of [15] OTRA ´ 2 Resistor ´ 3 (Floating) Capacitor ´ 3 (Floating) Second-order/ No Yes/ OC: by R OF: by R Commercial ICs (AD844AN)/ NA Hundreds of kHz (159 kHz)/NA Topology in Figure 4(a) of [16] OTRA ´ 1 Resistor ´ 2 (Floating) Capacitor ´ 2 (Floating) Second-order/ Yes NO/ OC: NA OF: NA CMOS realization (TSMC 0.35-mm CMOS process technology)/ ±2.5 V Hundreds of kHz (108 kHz)/2.6% Topology in OTRA ´ 1 Figure 4(b) of Resistor ´ 3 [16] (Only one grounded) Capacitor ´ 2 (Floating) Second-order/ Yes No (only of the oscillation condition)/ OC: by R OF: NA CMOS realization (TSMC 0.35-mm CMOS process technology)/±2.5 V Hundreds of kHz (150 kHz)/2.7% OTRA ´ 1 Resistor ´ 2 (Floating) Capacitor ´ 3 (Only one grounded) Second-order/ No No (only of the oscillation frequency)/ OC: NA OF: by C CMOS realization (TSMC 0.35-mm CMOS process technology)/ ±2.5 V Hundreds of kHz (260 kHz)/NA Topology in OTRA ´ 1 Figure 4(a) of Resistor ´ 3 [17] (Only one grounded) Capacitor ´ 2 (Floating) Second-order/ Yes NO/ OC: NA OF: NA CMOS realization (NA)/±2 V Tens of kHz (21.8 kHz)/NA Topology in Figure 4(c) of [16] 190 Hung-Chun Chien Table 1. Continued Component numbers and connected type Circuit Topology in Figure 6 of [18] Proposed (Figure 3) Independent control Circuit of OC and OF/Type order/Canonical of OC and OF design control Implement technology/ Supply voltage Measured highest frequency/Total harmonic distortion OTRA ´ 1 Voltage buffer ´ 1 Resistor ´ 3 (Only one grounded) Capacitor ´ 2 (Floating) Second-order/ Yes Yes/ OC: by R OF: by R Commercial ICs (AD844AN)/±5 V Hundreds of kHz (424 kHz)/NA OTRA ´ 1 Resistor ´ 3 (Floating) Capacitor ´ 3 (Only one floating) Third-order/ Yes Yes/ OC: by C OF: by R CMOS realization (TSMC 0.35-mm CMOS process technology)/ ±2.5 V Several MHz (1.08 MHz)/1.9% OC: oscillation condition. OF: oscillation frequency. NA: not available or not tested. Figure 1. Circuit symbol of an OTRA. input currents (I+ and I-) to be equal. Several CMOS OTRA realizations have been explored from the perspective of analog integrated-circuit implementation [10,11, 28]. using the commercially ICs (AD844ANs) [29] presented in [16]. The AD844AN IC is a current-feedback amplifier, which differs from a conventional OA in that the voltage at the noninverting input terminal is transferred to the inverting input terminal. Thus, an inherent virtual short exists between these two terminals. Also, the current flowing into the inverting input terminal is duplicated to the terminal Tz, and the output voltage is the same as the voltage appearing at Tz. The implemented circuit (Figure 3) was composed of two AD844ANs to perform the functions of the virtual ground of the two input terminals (V+ and V-), and the output voltage (Vo) proportional to the difference between the noninverting (1) Figure 2 depicts a feasible CMOS OTRA internal circuit construction consisting of two main functional blocks: a differential-current-controlled current source (M1 - M12) followed by a voltage buffer circuit (M13 – M20) [28]. In this circuit, the substrate terminals of all PMOS and NMOS transistors are connected to positive and negative supply voltages, respectively. Although previous studies have presented several implementations of CMOS OTRA [10,11,28], a practical implementation of OTRA is shown in Figure 3, which was constructed by Figure 2. CMOS internal circuit construction of an OTRA [28]. Third-Order Sinusoidal Oscillator Using a Single CMOS Operational Transresistance Amplifier and inverting input currents (I+ and I-) multiplied by the transresistance gain (Rm) of the OTRA [16]. If the Tz node of the second AD844AN is open-circuited (Rm ® ¥), the circuit (Figure 3) is nearly an ideal OTRA. Therefore, the realization shown in Figure 3 can provide a viable method to practically implement an OTRA, which is currently commercially unavailable. Figure 4 depicts the circuit diagram of the proposed third-order OTRA-based single-phase sinusoidal oscillator. The proposed circuit features compact circuitry that employs a single OTRA, three resistors, and three capacitors to meet the minimum requirements of a third-order oscillator. It is crucial to note that the output terminal of the OTRA features a low-impedance voltage output. Thus, the proposed oscillator does not require supplementary voltage buffers for cascading applications. Assuming an ideal OTRA, characterized in (1), a routine circuit analysis using (1) yields the characteristic equation of the circuit expressed in (2). (2) Based on (2), (3) and (4) express the formulas of the oscillation condition and frequency for the oscillator, respectively. Equations (3) and (4) indicate that the oscillation condition is independently controllable by using the capacitor Cf without affecting the oscillation frequency, whereas the oscillation frequency can be adjusted by varying the resistor R. Thus, both the oscillation condition and oscillation frequency have non-interactive adjustment manners. Because the oscillation frequency is independently controllable by using the resistor, a resistor-controlled sinusoidal oscillator is feasible. C = 12Cf (3) Figure 3. OTRA constructed using commercially integrated circuits (AD844ANs) [16]. 191 (4) Compared with the existing OTRA-based single-phase sinusoidal oscillators reported in [13-18], this paper is the first study to realize a noninteractive control oscillation condition and frequency third-order single-phase sinusoidal oscillator employing a single OTRA with fewer passive components. No previously suggested single OTRA-based single-phase sinusoidal oscillators have enabled independent tuning of the oscillation condition and frequency. Because OTRA-based signal-processing and signal-generation circuits have been widely discussed in analog circuit designs, the new proposed circuit presents a practical design for OTRA application and for teaching purposes in current analog circuit systems. 3. Non-Idealities Analysis This section presents nonideal problems to determine the influences of the nonideal effects on the proposed oscillator circuit (Figure 4). Non-idealities can originate from finite transresistance gain and nonideal parasitic elements of the OTRA. However, reports have shown that because the input terminals of an OTRA are internally grounded, the influences of the parasitic elements on the OTRA can be largely ignored [12]. Thus, the main nonideality problem is caused by the finite transresistance gain of the OTRA [19]. As discussed in section 2, ideally the transresistance gain of an OTRA approaches infinity. In practice, the transresistance gain was a frequency-dependent finite value. This might have caused the performance of the oscillator circuit (Figure 4) to deviate from the ideal scenario. A practical transresistance gain Rm (s) can be expressed as in (5), which is determined from a single-pole model [19]. Figure 4. Circuit diagram of the proposed OTRA-based thirdorder sinusoidal oscillator. 192 Hung-Chun Chien (5) where Rmo represents the DC transresistance gain, and wo is the pole angular frequency of the OTRA. Rmo is typically in the order of several hundreds of dBW and wo is in the range of several hundreds of rad/sec. The standard values of the parameters (Rmo and wo) for the realized OTRA (Figure 2) are Rmo = 130 dBW (3.16 MW) and wo = 703.72 rad/sec (112 kHz) [19]. For highfrequency applications, the Rm (s) can be expressed as in (6). (6) After the high-frequency single-pole model of the OTRA was applied to the circuit (Figure 4) and a routine circuit analysis was repeated, tedious derivations yielded the following modified characteristic equation: (7) Based on (7), the modified oscillation condition and frequency were determined using (8) and (4), respectively. Equation (8) indicates that the finite transresistance gain of the OTRA caused the oscillation condition to deviate from the expected values of the ideal scenario. This problem was overcome by slightly readjusting the value of the capacitor Cf, to ensure the oscillation starting procedure in the circuit (Figure 4). C = 12(Cp + Cf) (8) The non-idealities analysis result shows that the oscillation frequency could still be independently controlled by the resistor R without affecting the oscillation condition. By calculating (4), the sensitivities of the oscillator circuit were derived in (9). (9) Because the proposed third-order oscillator circuit used a single OTRA and few passive components, the realization constituted a compact configuration. The following section presents computer simulation and experimental results to verify the feasibility of the proposed circuit. 4. Simulation and Experimental Results To verify the theoretical analyses, the proposed oscillator circuit (Figure 4) was simulated using HSPICE circuit simulation program with a CMOS implementation of the OTRA, as shown in Figure 2 with the supply voltages: VDD = -VSS = 2.5 V, the bias voltages Vg1 = -Vg2 = 1 V, and the bias currents IB1 = IB2 = 30 mA. The NMOS and PMOS transistor models were adopted from the Taiwan Semiconductor Manufacturing Company (TSMC) 0.35 mm CMOS process parameters with the designed aspect ratios of the MOS transistors: (W/L) M1-M18 = 2 mm /1.5 mm and (W/L) M19-M20 = 8 mm /0.8 mm. As an example, the oscillator circuit (Figure 4) was designed for an oscillation frequency of fo = 100 kHz with the component values R = 227 W and Cf = 1 nF. In practice, the value of a capacitor C = 12.1 nF was designed to be slightly larger than the theoretical value (C = 12 nF) to start the oscillations. This slight deviation was caused by the finite-transresistance-gain effect, which was similar to that anticipated based on (8). Figure 5 provides the simulation results of the time waveform in a steady state, using these component values, and its corresponding frequency spectrum for the output. Simulation results exhibited an oscillation frequency of fo = 99.1 kHz, which was 0.9 % in disagreement with the designed value. The percentage of the total harmonic distortion (THD) and total power consumption were determined as 1.9 % and 6.8 mW, respectively. However, the output voltage swing (Figure 5) seems to be a low voltage level. To overcome this problem, an additional voltage amplifier is required to use the voltage across the output for circuit applications. Figure 6 presents the simulation result of the transient response of the circuit (Figure 4) when building the oscillations for the component values Cf = 1 nF, C = 12 nF, and R = 227 W. To investigate the lowest-and highest-applicable operating frequencies of the oscillator circuit (Figure 4), the specific simulation tests were conducted as follows. Figure 7(a) displays the simulation result for the output of the oscillator circuit in the steady state with fo = 1.08 MHz, when adopting the component values Cf = 0.5 nF, C = 6.06 nF, and R = 41.4 W . Under this condition, the deviation of the oscillation frequency between the theoretical value and the simulation result was 1.82%. The highest operating frequency of the proposed oscillator was in the range of several MHz. To at- Third-Order Sinusoidal Oscillator Using a Single CMOS Operational Transresistance Amplifier 193 Figure 5. Simulation results of the output Vo for the circuit (Figure 4) in the steady state, and its corresponding frequency spectrum. Figure 6. Simulation result of the output Vo for the circuit (Figure 4) during the start-up oscillation. tain the lowest-applicable operating frequency of the oscillator circuit, a simulation example with component values Cf = 0.1 mF, C = 1.21 mF, and R = 227 W was conducted to explore this characteristic. Figure 7(b) presents the simulation result for the lowest frequency of the oscillator with an oscillation frequency fo = 1.004 Hz which deviated from the theoretical value by 0.59%. The simulation results demonstrated that the proposed sinusoidal oscillator (Figure 4) operated in a wide frequency range from several hertz to several megahertz. Although the output waveform seems immature in Figure 7(a), an advanced CMOS process technology or a highspeed OTRA component can be used to extend the frequency higher. For comparison, the limited frequency range of the conventional OA-based oscillator is also ve- rified by experiments. Experimental results demonstrate that the conventional OA-based oscillator performs well in a frequency range from several hertz to several tens of kilohertz. Furthermore, the power consumption of the OA-based oscillator is also determined as several tens of mW. Thus, the proposed circuit (Figure 4) operates at a higher operating frequency and a lower power consumption than that used in the OA-based design. To demonstrate the property used to independently control the oscillation frequency by using the resistor R, the capacitance values Cf = 1 nF, C = 12.1 nF were assigned to enable starting the oscillation procedure; R was varied from R = 0.1 kW to 1 kW in 0.1 kW steps to vary the oscillation frequency. Figure 8 presents the theoretical and simulation results for the variation of the oscillation frequency. Sim- 194 Hung-Chun Chien ulation results confirmed that the proposed circuit (Figure 4) enabled independent control of tuning the oscillation frequency through resistor R without affecting the oscillation condition. If concerned with the non-linearity problem of the proposed oscillator, the total harmonic distortion analysis is required. The simulation result was recorded in Figure 9. This study summarized that the percentage of total harmonic distortion fluctuates from 0.91% to 1.93% when the oscillation frequency of the circuit (Figure 4) varies from 10 kHz to 100 kHz. Due to very simple structure, the proposed oscillator enjoys an extremely low output harmonic distortion. Re- garding the experimental tests for the proposed oscillator, Figure 3 presents a viable implementation of OTRA for experimental testing. A prototype circuit of the proposed oscillator (Figure 4) was constructed using AD844AN ICs and discrete passive components. The AD844AN ICs were biased with a supply voltage of ±5 V. For example, the circuit (Figure 4) was designed for the oscillation frequency of fo = 100 kHz with the component values of R = 215 W , Cf = 1 nF, and C = 12.8 nF. In practice, the value of C was designed slightly larger than the theoretical value (C = 12 nF) to start the oscillations. Figure 10 shows the experimental results of the Figure 7. Simulation results of the circuit (Figure 4) for (a) the highest-applicable oscillations and (b) the lowest-applicable oscillations. Figure 8. Variation of the oscillation frequency against R for the circuit (Figure 4). Figure 9. Simulation results of the total harmonic distortion versus oscillation frequency for the circuit (Figure 4). Third-Order Sinusoidal Oscillator Using a Single CMOS Operational Transresistance Amplifier time waveform of the output voltage, and its output frequency spectrum. The experimental result in Figure 10(a) shows an oscillator frequency fo = 99.28 kHz, which is close to the theoretical prediction (fo = 100 kHz). The percentage error between theoretical and experimental result is 0.72%. The percentage total harmonic distortion was measured as 2.55%. The experimental result shows the slight distortion on this circuit because Figure 4 circuit suggests a compact circuit topology design. Simulation and experimental results supported the theoretical analysis, confirming the feasibility of the proposed sinusoidal oscillator. 5. Conclusions This study presented the design of a third-order OTRA-based single-phase sinusoidal oscillator. 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