^{1}

^{*}

^{1}

^{1}

The recent advances in IC technology have led to the trend of designing hybrid systems to benefit both analog and the digital domain. Among analog circuits, multifunctional filter along with multiphase oscillator constitutes a building block of critical importance. In this paper, a digitally reconfigurable multi-input-multi-output voltage mode multifunctional biquadratic filter has been presented. The circuit comprises of two differential voltage current conveyors (DVCCs), two grounded capacitors and two floating resistors. The digital controllability is incorporated using a current-summing network (CSN). Tunability of quality factor is achieved by the use of a 3-bit digital control word while keeping the resonant frequency constant. PSPICE simulations using TSMC 0.25 μm CMOS technology have been performed to validate the theoretically predicted results.

Voltage-mode active filters exhibiting high input impedance are of great interest as a number of cells can be easily cascaded for the realization of the higher order filters. The significance of the digitally controlled blocks can be appreciated in situations where a filter needs to be designed for the given filter parameters, i.e. its cut-off frequency and quality factor. According to the given specifications and the transfer function of the circuit, the values of passive elements are selected to meet the requirements. If any parameter of the circuit is altered, the circuit needs to be designed again and again which is a tedious task. This problem can be alleviated by employing digitally controlled circuits which provide the flexibility to design the circuit for certain discrete values of the parameters instead of the single fixed value in conventional filters. The number of values will depend on the minimum resolution of the control word.

There have been countless applications that involve realization of active transfer functions employing current conveyors, which have received a considerable amount of research attention [

Among the several variations of current conveyor, the most successful type is second-generation current conveyor (CCII) introduced by Sedra and Smith [

The DVCC is a five-port, widely used analog building block defined by the following matrix equation [

The block level implementation of an ideal DVCC is shown in

The implemented voltage mode multifunctional filter [

If V_{i}_{1} = V_{i}_{3} = V_{in}(the input voltage signal) and V_{i}_{2} = 0, then

where the quality factor, Q and theresonant angular frequency, are given by:

It is to be noted that the low-pass, high-pass, band-pass and band reject functions can be simultaneously realized

without changing the circuit configuration. The multifunctional filter has been designed for a cut-off frequency of 1 MHz with C_{1} = C_{2} = 15.9 pF, R_{1} = R_{2} = 10 kΩ. The responses of the multifunctional filter for the above given design is shown in

For introducing the digital control in the multifunctional filter we have used Digitally Controllable DVCC (DCDVCC), shown in

As defined by the characteristics equations in Equation (8), the voltage on the X-terminal realizes the difference of potentials on terminals Y1 and Y2. Furthermore, the current injected at the X-terminal is replicated by a factor of k to the Z-terminals. For the Z+ and Z− terminals, the conveyed current flows in the same and opposite direction respectively, to the current flowing in the X terminal [

For digitally reconfiguring the DVCC and obtaining a digitally controlled DVCC (DC-DVCC), a Current Summing Network (CSN) has been employed at the Zand Z+ terminal for controlling the current transfer gain parameter k. The gain parameter k can be varied from 1 to (2^{n} – 1), where n represents the number of transistor arrays. The modified circuit of DVCC i.e. Digitally Controlled-DVCC employing a CSN is shown in

Furthermore, the current at the Z terminal, assumed to be flowing out of the DC-DVCC, can be expressed by:

Therefore, the proposed DC-DVCC provides a current transfer gain, k equal to:

where d_{i} represents the bits applied to the i-th branch in the CSN. Now depending upon its value, whether it’s a logic 1 or logic 0, the current is enabled or disabled to flow in that particular branch [13-15].

In this section, the voltage-mode multifunctional biquadratic filter of

tion of the digital control module, comprising of CSN leads to a significant change in the expression of the transfer function of multifunctional filter. The expression for the digitally programmable filter functions can now be expressed as:

The resonant angular frequency, ω_{o} and the quality factor, Q, are given by:

where

where d_{i} represents the bits applied to the i-th branch in the Current Summing Network (CSN).

It can be seen from Equations (16) and (17) that only the expression of quality factor is varying with control word, whereas the cut-off frequency is independent of it. Since quality factor indicates the selectivity characteristic of a filter and it’s significance is more with the band pass, as compared to the other filters, so for this digitally controlled circuit focus will be only on the response of band pass filter.Moreover, the circuit utilizes grounded capacitors for the filter’s realization, which makes it suitable for integrated circuit implementation.

The proposed digitally controllable multifunctional voltage mode filter has been simulated and all the realizations are designed and verified using PSPICE simulation tool with good results in support of the theory to ensure that the expected functionality is indeed obtained. The CMOS implementation of Digitally Programmable DVCC is shown in

the plots are in excellent mathematical conformity with the transfer functions given in equations. The slight decrease in the resonant frequency is because of the nonideal effects. The performance of the DVCC generally deviates from the ideal behavior since, the voltage and current conveying actions are not exact, therefore leading to slight degradation in performance of the circuit realized using these active elements. In order to account for these non ideal effects, the parameters of α and β are introduced, where α accounts for current transfer gains and β accounts for voltage transfer gains. These transfer gains differ from unity, by the voltage and current tracking errors of the DVCC.

The Digitally Programmed Multifunctional filter can be controlled by a 3-bit control word that can be varied from 1 to 8. However in order to achieve more variation in the Q-factor of the circuit, the digital control word could be increased by adding more transistor pairs in the current summing network of digitally controlled DVCC.

In this paper, a novel digitally controllable voltage-mode universal biquadratic filter based on DVCC was presented. Digital tuning has been achieved by the variation of 3-bit digital control word. PSPICE simulations were carried out to ascertain the working of the proposed filters and the results were found to match with the theoretical results.