PROGRAMMABLE CF-OTA-BASED LOWPASS AND BANDPASS FILTERS

A new circuit is proposed for realizing lowpass and bandpass filter responses. The circuit uses one current-follower and one operational transconductance amplifier. The circuit enjoys low temperature sensitivities, high input impedance and its important parameters are electronically programmable. The use of grounded capacitors is an additional attractive feature for integration. Simulation results confirming the theory presented are included.


INTRODUCTION
At present there is a growing interest in designing current-follower (CF)-based biquad filters [1,2]. This is attributed to their wide bandwidth and accurate performance. Moreover, the CF has only the current tracking error. This makes CF-based realizations more attractive than second-generation-current-conveyor (CCII)-based realizations which, in addition to the current-tracking error, suffer also from the voltage tracking error. However, the CF-based biquad filter realizations available in the literature [1,2] can not be cascaded to produce higher-order filters. Moreover, the important parameters, such as the pole frequency (tOo) and the pole-Q (Qo) are not programmable. On the other hand, the operationaltransconductance-amplifier (OTA) can be utilized for the realization of active filters with electronic tunability over a wide range of control current (or voltage) [3,4]. This is attributed to the dependence of the OTA-transconductance (gm) on the auxiliary bias current. However, the resulting active-filter structures, usually using a pair of OTAs, suffer from relatively high temperature-sensitivities [3,4].
In this paper, an OTA-CF-based lowpass and bandpass biquad filter circuit is proposed. The circuit uses a single OTA and enjoys relatively low temperature sensitivity. The circuit has a high input-impedance and, therefore, is cascadable to produce higher-order filters. Moreover, the important parameters tOo and Qo are programmable. Furthermore, the proposed circuit uses two grounded capacitors. These are attractive features for integration.

PROPOSED CIRCUIT
Consider the circuit shown in Fig. 1 Ianc is the auxiliary bias current of the OTA, VT is the terminal voltage, and v/ and 267 M.T. ABUELMA'ATrI AND A. QUDDUS v_ are the positive and negative input voltages of the OTA, routine analysis of the circuit of Fig. 1 shows that Vol Ggm/C1C2 V s 2 q-sG/C 4-Ggm/C1C 2 (1) and

Vo2
Sgm/C1 V s 2 q-sG/C q-Ggm/C1C 2 (2) Equation (1) corresponds to the transfer function of a lowpass filter and equation (2) corresponds to the transfer function of a bandpass filter. From (1) and (2)  Similarly, the temperature-sensitivity of too and Qo can be expressed by 1 2 Thus the to o and Qo sensitivities to passive-elements and temperature are low.

Effect of CF Non-idealities
The effect of CF-non-idealities on the performance of the biquad filter circuit of Fig. 1 can be studied by assuming that the characteristic of a CF with current tracking error can be described by iy kix, k -,11 < Reanalysis of the circuit of Fig. 1 shows that Vol kGgm/CiC2 V s 2 + sG/C + kGgm/C 1C2

SIMULATION RESULTS
The biquad filters proposed in this paper have been checked using PSPICE Evaluation Version 5.4. Although there are several ways to simulate the CF required, the simulation was performed using a second-generation currentconveyor (CCII). The CF can be obtained using a CCII with its high impedance terminal grounded. The CCII was simulated using the circuit proposed by Senani [5] because of its simplicity. The kernel of the work presented here is independent of the particular simulation selected. The CCII was simulated using one OTA, one resistor and one operational amplifier (OA) as shown in Fig. 2. The OA was simulated using the uA741 model contained in the file called EVAL.LIB available in the PSPICE Evaluation Version. The OTA was simulated assuming a finite input resistance of 2 MfL Figs. 3 and 4 show the simulated results obtained from a lowpass and a bandpass filter realization. The simulation results appear to be in good agreement with the theory presented.

CONCLUSION
In this paper, a new CF-OTA-based lowpass and bandpass filter realization has been presented. The proposed circuit uses one CE one OTA, one floating resistor, and two grounded capacitors. The circuit has a high input impedance and is, therefore, cascadable to produce higher order filter realizations. The important parameters of the filters, too and Qo are programmable by adjusting the transconductance of the OTA. Thus, current(voltage) digital programming of these  parameters is feasible. The use of grounded capacitors is another attractive feature for integration. The CF current-tracking error has a slight effect on the parameters to o and Qo and does not affect the bandwidth (too/Qo).
It is worth mentioning here that the proposed circuit can realize lowpass and bandpass biquad filter responses. Other responses, such as highpass, allpass, and notch, are also realizable by using an additional summing circuit [6].