NOVEL CURRENT-CONVEYOR-BASED UNIVERSAL CURRENT-MODE BIQUAD FILTER WITH THREE INPUTS AND ONE OUTPUT

A novel universal current-mode filter with three inputs and one high imedance output is presented. The proposed circuit uses four plus-type second-generation current-conveyors, grounded resistors and grounded capacitors. The proposed circuit enjoys low active and passive sensitivities and independent control of the parameters ω0/Q0 using grounded resistors.


INTRODUCTION
Recently, Chang, Chien and Wang, 1994, proposed a universal active current filter with three inputs and one ouput using current conveyors.The proposed circuit uses two plus-type first-generation current- conveyors, two minus-type second-generation current conveyors, two grounded capacitors and two grounded resistors and enjoys the following attractive features: 1. Low filter sensitivity to passive components. 2. The use of grounded capacitors which is attractive for integrated circuit implementation.
3. The versatility to synthesize virtually any type of active filter transfer function.
However, the circuit suffers from the following disadvantages: 1. Use of different types of current conveyors.
2. Interdependent control of the parameters Wo and wo/Qo.Thus, while the parameter Wo can be adjusted without disturbing the parameter wo/Qo, the parameter wo/Qo cannot be adjusted without disturbing the parameter Wo. 3. The sensitivity of the circuit to the voltage and current tracking errors of the current conveyors is not clear.
This paper presents a novel three-input universal current-mode biquad active filter.The proposed circuit enjoys the following attractive features: 1. Use of one type of second-generation current-conveyor. 2. Independent control of the parameters Wo and wo/Qo.Thus the parameter Wo can be adjusted without disturbing the parameter Qo, and the parameter wo/Qo, can be adjusted without disturbing the parameter Wo. 3. Enjoys low active and passive sensitivities.4. Use of grounded capacitors and grounded resistors.

PROPOSED CIRCUIT
The proposed circuit is shown in Figure 1.The circuit uses plus-type second-generation current-conveyors (CCII +) only.Using the stan- dard notation, the CCII + characteristics can be described by iz aix, Vx Vy, where a e and E denotes the current-tracking error, /3 e and e denotes the voltage-traCking error.The single output current Io can be expressed as a4/4G7 $2C1C302fl213 sC1G4a212 + G2G40l a203/l I1 I: 64 (1) s2C1 C6 + sC1 G6 + G2GsOl c2ce3/31/33 From (1) the parameters 0o and oo/Qo can be expressed as (2) From (1) it can be seen that: 1.The lowpass response can be realised with 12 13 0 2. The highpass response can be obtained with 11 12 0 3.The bandpass response can be obtained with 11 13 0 4. The notch response can be obtained with 12 0 and 11 13 5.The allpass response can be obtained with 11 12 13, G4 G5 and C3--C 6.
From (1) it can also be seen that the lowpass gain, the highpass gain and the bandpass gain are approximately given by G7 (4) GL, -----G5 G7 C3 GHP G4 C6 (5) and G7 (6) GBp G6 From (2) (6) it can be seen that the parameter o can be adjusted by controlling the resistance R2= l/G2 without disturbing the parameters wo/Qo, GL,, GHp and Gap.Also, the highpass gain can be adjusted by controlling the resistance R4 l/G4 without disturbing the parameters Wo, wo/Qo, GLp and GBp.Moreover, the parameter Wo/ Qo can be adjusted by controlling the resistance R6 l/G6 without disturbing the parameter Wo.However, controlling the resistance R6 will disturb the bandpass gain GBp.A possible strategy for adjusting the parameters Wo, wo/Qo, GLp, GHp and GBp is, therefore, as follows: First the resistance R6 1/G6 is controlled to adjust the parameter o/ Qo, then the resistor R7 1/G7 is controlled to adjust the bandpass gain GBp; the resistance R4 l/G4 is controlled to adjust the highpass gain GHp; the resistance R5 l/G5 is controlled to adjust the lowpass gain GLp, and finally the resistance RE 1/G2 is adjusted to control the parameter Wo.From (2) and (3) it is easy to show that the active and passive sensitivities of the parameters o and Qo are sQo __sQo __sQo Thus, all the active and passive sensitivities are no more than unity.
It is worth mentioning here that, another output current can be obtained when I1=0.By using an additional second-generation current-conveyor and a grounded resistor as shown in the dotted box of Figure 1; this addition is, however, optional, the new output current can be expressed as G8 G5 /out O304f13f14 771 Io (7) Thus, when Io is realising a bandpass response, the current/out will realise a lowpass response.Also, when Io is realising a highpass response, the current lout will realise a bandpass response.

EXPERIMENTAL RESULTS
To verify the theoretical analysis, the proposed circuit was used to realise LP, HP, BP and notch filters using the AD844 current-conveyor.