All-pass networks using current conveyors

All-pass networks using current conveyors K. Pal

Department of Earthquake Engineering, University of Roorkee, Roorkee (U.P.), India Two circuits, each realizing a second order all-pass transfer function, are described. One circuit uses a grounded capacitor, and is thus suitable from the point of view of IC implementation. The other circuit employs a minimum of active and passive components required for a second order allpass realization.

1. Introduction All-pass networks have a voltage transfer function with constant magnitude and a phase response which is a function of frequency. There are basically two types of all-pass networks, first order and second order giving phase shifts from 0 to - 180 ° and 0 to - 3 6 0 °, respectively. Comer [1] has discussed the broad use of these networks, e.g. in designing highly selective tuned circuits, oscillators, phase and frequency modulators, current controllers, etc., besides its main application as a phase correction circuit. There exist a n u m b e r of circuits realizing all-pass filters using operational amplifiers [2-7]. However, the use of a second generation current conveyor (cc II) [8] has resulted in having some notable advantages over operational amplifierbased all-pass circuits [9-11]. The aim of this paper is to report on two active allpass circuits which have certain advantages over earlier cc II realizations.

2. Circuit descriptions The proposed circuits are shown in Fig. 1 and Fig. 2, respectively. The voltage transfer function of the circuit shown in Fig. 1 (assuming an ideal cc II for which iy = 0, Vx = Vy, iz = ix) can be expressed as

Vo

- - +

1 + s2CIC2RIR2 + s ( C I R l + C2R2 - a f i R 2 )

K

v, where a

1 + s2CIC2RIR2 + s(CIR 1 +

=

R4fR 3. If Cl = C2 = C, R l

=

(1)

C2R2)

R 2 =

R and a = 4

MICROELECTRONICS JOURNAL Vol. 22 No. 4 © 1991 Elsevier Science Publishers Ltd., England

53

K. Pal

54

Vi

:

Y X

R°

x~'

C~ (IcCaz YC CCi)lIz"IR4 y]CCC)I

: Vo

K-Ro

Fig. 1.

ii cl | R1'")

Vi •

CC II

®

I

z

,I

eVo

Fig. 2.

Vo ~ +

1 + szCZR 2 - s 2 R C

V~

1 + s2CR 2 - s2RC

(2)

Thus this circuit, like the earlier one [10], has the advantage of using grounded capacitors that are suitable from an integration point of view [12]. However, it does have the advantage of tuning through a ganged potentiometer for R, and R2. This circuit is suitable for phase shift from 0 to -360 °. Another circuit is shown in Fig. 2. Its voltage transfer function is given by

v0

(1 + s2CIC2R~R~) + s(C~R~ + C2R2 - C2RO

v,

(1 + s2C1Cz~RIR2) -t- s(C1R1 + CzR2)

--+

(3)

All-pass networks using current conveyors

55

In the above analysis, again an ideal cc II has been considered. Let CiRl + C:R2 = 1

(4)

and

(5)

C2R1 = 2

Dividing, one gets Ci

1

R2

C2

2

Rj

(6)

and eqn. (3) now becomes Vo --+

1 + s2CtC2RiR2 - s

Vi

1 + s2CIC2RIR2 + s

(7)

Thus the circuit exhibits a second order all-pass transfer function. Compared to available second order current conveyor all-pass circuits, the present circuit may be considered to be novel as it uses a minimum of passive and active components for the desired realization. The effect of a non-ideal cc II (% = ix" h), when taken into consideration, modifies eqn. (6) as follows: Ci

h

R_~

2

Ri

-

C2

(8)

Thus a non-ideal cc II only modifies the design: it does not change the filter response.

3. Conclusions

Two all-pass filters have been described. The first circuit uses grounded capaciters, and phase tuning is done through a ganged potentiometer. This circuit is superior to an earlier realization [10], as that circuit is not easily tunable. The other circuit uses a minimum of active and passive components, and the same has not been achieved in the literature using one active and four passive components. References

[1]

Comer, D.J., ~'The utility of the all-pass filter", I E E E Trans. Instrumentation a n d Measurement, vol. IM-28, p. 164, 1979.

56

[2]

K, Pal

Genin, R., "'Realisation of an all-pass transfer function using operational amplifiers", Proc. IEEE, vol. 56, p. 1746, 1968. [3] Dutta Roy, S.C., "RC active all-pass networks using a differential input operational amplifier", Proc. IEEE, vol. 57, p. 2055, 1969. [4] Deliyannis, T., "RC active all-pass sections", Electron. Lett., vol. 5, p. 59, 1969. [5] Bhattacharya, B.B., "Realisation of an all-pass transfer function",Proc. IEEE, vol. 57, p. 2092, 1969. [6] Aronhime, P. and Budak, A., ~'An operational amplifier all-pass network", Proc. IEEE, vol. 57, p. 1677, 1969. [7] Aronhime, P., "Transfer function synthesis using a current conveyor", IEEE Trans. Circuits and Systems, vol. CAS-21, p. 312, 1974. [8] Sedra, A. and Smith, K.C., "'All second generation current conveyor and its applications", IEEE Trans. Circuit Theory, vol. 17, p. 132, 1970. [9] Pal, K,, "Realisation of current conveyor all-pass networks", Int. J. Electronics, vol. 50, p. 165, 1982. [10] Pal, K. and Singh, R., "Inductorless current conveyor all-pass filter using grounded capacitors", Electron. Lett., vol. 18, p. 47, 1982. [11] Higashimura, M. and Fukui, Y., "Realisation of all-pass networks using a current conveyor", vol. 65, p. 249, 1988. [12] Bhusan, M. and Newcomb, R.W., '~Grounding of capacitors in integrated circuits", Electron. Lett., vol. 3, p. 148, 1967.