Abstract
Two alternate topologies of lossless grounded inductor have been proposed using operational transresistance amplifier (OTRA). Three applications using the proposed inductors are also included. PSPice simulation and experimental results have been included to demonstrate the performance and verify the theoretical analysis.
1. Introduction
Recently the operational transresistance amplifier (OTRA) has emerged as alternate important analog building block as it inherits all the advantages offered by current mode techniques. The OTRA is a high-gain current input voltage output device. The input terminals of OTRA are internally grounded, thereby eliminating response limitations due to parasitic capacitances and resistances at the input. Although the OTRA is commercially available from several sources under the name of current differencing amplifier or Norton amplifier, it has not gained attention until recently. These commercial realizations do not provide internal ground at the input port and allow the input current to flow in one direction only. The former disadvantage limits the functionality of the OTRA where as the later forces to use external DC bias current leading to complex and unattractive designs [1]. Several high-performance CMOS OTRA topologies have been proposed in literature [1–4] leading to growing interest in OTRA-based analog signal processing circuits. In the recent past OTRA has been extensively used as an analog building block for realizing a number of signal processing circuits such as filters [5–8], oscillators [9, 10], multivibrators [11, 12], and immittance simulation circuits [9, 13–15], an application which has been dealt with in this paper. A number of grounded parallel immittance topologies using single OTRA are proposed in [13]. However none of these configurations can realize a lossless grounded inductor. The structure in [14] presents simulation of lossless negative grounded inductance. Lossless grounded inductor simulators using two OTRAs, five resistors, and one capacitor are presented in [9, 15].
In this paper two additional topologies of lossless grounded inductor using two OTRAs, five resistors, and one capacitor are reported. In these topologies five passive elements out of six are grounded as compared to four grounded elements in [9, 15]. Some applications of the proposed topologies are also presented.
2. Circuit Description
OTRA is a three-terminal device, shown symbolically in Figure 1 and its port relations can be characterized by the following matrix: For ideal operations the transresistance gain approaches infinity and forces the input currents to be equal. Thus OTRA must be used in a negative feedback configuration.
The proposed lossless grounded inductor topologies are shown in Figure 2. Routine analysis of the circuit of Figure 2(a) results in the following expression for input admittance: will be purely inductive if the following condition is met Similarly for inductor topology of Figure 2(b), input admittance is given by This input admittance will be purely inductive provided that The equivalent inductance values along with conditions are given in Table 1.
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(b)
It is clear from Table 1 that for both the topologies the inductance value can be controlled independent of condition of realization.
The proposed topologies are verified through simulations using the CMOS implementation of the OTRA [4] as given in Figure 3. The SPICE simulation is performed using 0.5 μm CMOS process parameters provided by MOSIS (AGILENT) and supply voltages taken are ±1.5 V. Aspect ratios used for different transistors are given in Table 2.
An inductor of value mH is designed using the inductor topology of Figure 2(a) with the component values of KΩ, KΩ, pF. The frequency response of the impedance as obtained using this inductor is shown in Figure 4(a). The inset depicts the enlarged view of impedance response in lower frequency range.
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Similarly, Figure 4(b) shows the frequency response of the impedance of the inductor of value μH as obtained with component values of KΩ, KΩ, and pF for inductor topology given in Figure 2(b). In the inset, the variation of impedance in lower frequency range is shown.
3. Applications
In this section some applications of the proposed topologies have been presented. Both the topologies may be used for constructing filter and oscillator circuits.
3.1. High-pass Filter
A high pass filter, as shown in Figure 5(a), can be constructed using proposed inductors. The transfer function for high pass response is obtained as where
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(b)
The functionality of the high pass filter is verified using the inductor topology of Figure 2(a) and designed for lower cutoff frequency of 503.3 KHz. The component values are obtained as Ω and nF for mH. The value of mH is obtained using inductor topology of Figure 2(a) with component values of KΩ, KΩ, and pF. The frequency response of the filter simulated using PSPICE is depicted in Figure 5(b). Simulated value of lower cutoff frequency is obtained as 505 KHz which is in close agreement to the theoretical value of 503.2 KHz.
3.2. Band Pass Filter
The proposed inductor topologies may also be used to obtain band-pass response using the circuit given in Figure 6(a). The transfer function for band-pass response is obtained as where The theoretical proposition is verified using the topology of Figure 2(b). A band-pass filter is designed having center frequency of 503.3 KHz. The component values are obtained as KΩ and nF for mH. The value of mH is obtained for inductor topology of Figure 2(b) with component values of KΩ, KΩ, pF. The frequency response of the filter simulated using PSPICE is depicted in Figure 6(b). The simulated results are in close agreement with the theoretical prediction.
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3.3. LC Oscillator
An LC Oscillator can be realized using the proposed inductor topologies. Figure 7(a) shows the schematic of an LC oscillator using topology of Figure 2(a) for which the condition of oscillation (CO) and frequency of oscillation (FO) are obtained as Figure 7(b) shows the output of the oscillator for mH and pF. Simulated frequency of oscillation is 860 KHz as against the calculated value of 876.2 KHz with % error of 1.85%.
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3.4. Experimental Verification
The proposed inductor topologies are also tested experimentally to verify the theory. The OTRA is implemented using two CFOAs (IC AD844AN) as shown in Figure 8 with a supply voltage of ±5 V.
The high pass filter of Figure 5(a) is prototyped with nF, Ω, and mH. The inductor of value mH is implemented using inductor topology of Figure 2(a) with component values of KΩ, KΩ, and nF. Theoretical, simulated and experimental frequency responses are shown in Figure 9. It is observed that the experimental values more or less follow the theoretical and simulated values. The oscillator circuit of Figure 7(a) is also tested experimentally. The output waveform observed on oscilloscope is shown in Figure 10. The observed frequency of oscillation is found to be 872.5 KHz, which is in close agreement to theoretically calculated value of 876.2 KHz.
4. Conclusion
Two new OTRA-based lossless grounded inductor topologies are presented. A high pass filter, band-pass filter, and an oscillator are realized using the proposed inductor topologies to illustrate their applications. PSpice simulation and experimental results are included to verify the theoretical propositions. It is found that the results obtained are in close agreement with the ideal values. Hence it is expected that the proposed inductors will provide an option to integrated circuit designer where lossless grounded inductor is required.