MULTI-FUNCTION ACTIVE-ONLY HIGH-ORDER CURRENT-DRIVEN FILTER

A new multi-function high-order current-driven filter is proposed. The filter uses only operational amplifiers, and operational transconductance amplifiers (OTAs). Without using any external passive elements, a variety of high-order input-current/output-current and/or input-current/output-voltage responses can be realised without changing the circuit topology and without any matching or cancellation conditions. The parameters of the high-order filter responses can be electronically tuned by adjusting the bias currents of the OTAs.


INTRODUCTION
At present, there is a growing interest in designing capacitorless- resistorless current-mode active-only filters using only active-elements, such as operational amplifiers (OAs) and operational transconduct- ance amplifiers (OTAs).This is attributed to their integratability, programmability and wide frequency range of operation [1,2].The circuits reported in [1,2] can realise second-order lowpass, highpass, bandpass and notch transfer functions, without any matching or cancellation conditions.Realisation of allpass transfer functions is also feasible.The work to be presented in this paper is an attempt to extend the applications of capacitorless-resistorless active-only realisations to the domain of high-order filters.Obviously, high-order filters can be obtained by cascading second-order realisations.However, this approach usually requires an extensive number of active components.For example, the realisation of an Nth-order response, N=even integer, using the structure proposed in [2] requires N OAs and 5N/2 OTAs.Similarly, the realisation of an Nth-order response using the structure proposed in [1] requires 3N/20TAs and N OAs.It will be shown, in this paper, that an Nth-order response, with N=even integer, can be realised using N OAs and N+ 10TAs only.Thus, the proposed realisation saves (N/2-1) OTAs.Moreover, while the realisations of [1] and [2] can support only one output response at a time, the proposed realisation can simultaneously support a variety of output responses.

PROPOSED CIRCUIT
The proposed circuit is shown in Figure 1.Using standard notations, the OTA can be characterised by gmk(V+-V_), where gmk (IABCk/2Vr) is the transconductance of the kth OTA, 1.4BCk is the auxiliary bias-current, Vr is the thermal voltage and v+ and v_ are Von FIGURE Proposed active-only nth-order filter.
From (1) and (2) the parameters ak, k 0-n and Ck, k 1-n can be expressed as k ak gm'--Z H nm  From (5) it can be seen that the parameters b, l= 1-n can be independently adjusted by controlling the transconductances gmt, l= 1-n.Thus, the proposed circuit enjoys electronic tunability of its parameters.

CONCLUSION
A new Nth-order current-driven filter has been presented.The pro- posed filter uses only operational amplifiers and operational trans- conductance amplifiers.No external passive elements are used.The proposed circuit enjoys the following advantages: (a) Independent current control of the filter parameters.
(b) Simultaneous support of n+ different high-impedance output- current responses and n different low-impedance output-voltage responses.
(c) Insensitivity to the temperature variations of the transconduct- ances of the OTAs.Moreover, using operational amplifiers with temperature compensated gain-bandwidth products [5], high- order responses with temperature-insensitive parameters can be realised.