NEW TEMPERATURE-INSENSITIVE ELECTRONICALLY-TUNABLE GROUNDED CAPACITOR SIMULATOR

Over the years, the operational amplifier (OA) has been successfully used for realizing analog signal processing circuits. Inductors frequently required in such circuits are traditionally realized using the general impedance converter (GIC) [1]. The use of GIC to simulate resistively variable capacitors has been recently reported [2]. In the proposed circuits of [2], two operational amplifiers and three capacitors at most are required. Moreover, while the use of a grounded resistor paves the way for resistance control of the proposed circuits [2], the practical implementation of this resistance has not been discussed. The operational transconductance amplifier (OTA), configured as a grounded resistor [3], is a highly potential candidate for providing the electronic tunability of these realizations. However, since the transconductance gain g,n of the OTA is temperature-sensitive, then it is expected that the resulting electronically-tunable grounded capacitor realizations will be temperature sensitive. Thus, for a reliable operation particularly under varying temperature conditions, temperature-compensation schemes are required to provide thermal stabilization. Although such schemes are available [4], their practical implementation implies an increased circuit complexity. It is the major intention of this paper to present a new OA-OTA-based electronically-tunable temperature-insensitive grounded-capacitor realization. The proposed circuit is inherently thermally-stable and, thus, does not require special schemes for temperature stabilization.


INTRODUCTION
Over the years, the operational amplifier (OA) has been successfully used for realizing analog signal processing circuits.Inductors frequently required in such circuits are traditionally realized using the general impedance converter (GIC) [1].
The use of GIC to simulate resistively variable capacitors has been recently reported [2].In the proposed circuits of [2], two operational amplifiers and three capacitors at most are required.Moreover, while the use of a grounded resistor paves the way for resistance control of the proposed circuits [2], the practical implementation of this resistance has not been discussed.The operational transcon- ductance amplifier (OTA), configured as a grounded resistor [3], is a highly potential candidate for providing the electronic tunability of these realizations.However, since the transconductance gain g,n of the OTA is temperature-sensitive, then it is expected that the resulting electronically-tunable grounded capacitor realizations will be temperature sensitive.Thus, for a reliable operation particularly under varying temperature conditions, temperature-compensation schemes are required to provide thermal stabilization.Although such schemes are available [4], their practical implementation implies an increased circuit complexity.
It is the major intention of this paper to present a new OA-OTA-based electronically-tunable temperature-insensitive grounded-capacitor realization.The proposed circuit is inherently thermally-stable and, thus, does not require special schemes for temperature stabilization.

PROPOSED SCHEME
The proposed scheme is shown in Fig. 1.Assuming ideal OA, routine analysis yields the input imedance Z,,p,t, which can be expressed by Zi.., Z2 (I) Thus, a negative grounded capacitor will be obtained using positive resistances for Z and Z2 and a capacitor for Z3.A positive grounded capacitor will be obtained using a negative resistance for Z and a positive resistance for Z2 and a capacitor for Z3.Finally, a negative (or positive) grounded inductor will be obtained using resistors for Z and Z3 and a capacitor for Z2.A practical realization for the proposed positive capacitor simulator using the circuit of Fig. 1 and OTA-based grounded and floating resistors [3] is. shown in Fig. 2. Since the transconductance gain, g,, of the OTA can be expressed by g,,k I/2Vr, where Ik is the amplifier bias-current of the kth OTA and Vr is the thermal voltage, and assuming ideal OTAs with io,tk gmk(V+k V-to), where V/k and V-k are the positive and negative inputs of the kth OTA and ioutk is its output current, then (1) From (2) one can see that, while the proposed capacitance is independent of temperature, its value can be controlled by adjusting the ratio between the amplifier-bias-currents IB1 and I2.

EXPERIMENTAL RESULTS
The proposed current-controlled positive grounded capacitor was implemented using the LM13600 OTAs and the 741 OA.The simulated capacitor was tested using the voltage-divider circuit shown in Fig. 3.The measured and calculated capacitance values, shown in Fig. 4, are in good agreement with the theoretical calculations.

CONCLUSION
In this paper, a new circuit for realizing electronically tunable grounded capacitors has been presented.The circuit uses one capacitor, three OTAs, and one OA.The use of the OTAs makes it feasible to obtain a temperature-insensitive grounded capacitor with electronic tunability over a wide range with bias current(voltage)control.The experimental results are in good agreement with the theory.

FIGURE 2
FIGURE 2 Practical realization of an electronically tunable temperature-insensitive positive capacitance.

FIGURE 3 FIGURE 4
FIGURE 3  Voltage divider circuit used to test the simulated capacitor of Fig.2.