Versatile Tunable Current-Mode Universal Biquadratic Filter Using MO-DVCCs and MOSFET-Based Electronic Resistors

This paper presents a versatile tunable current-mode universal biquadratic filter with four-input and three-output employing only two multioutput differential voltage current conveyors (MO-DVCCs), two grounded capacitors, and a well-known method for replacement of three grounded resistors by MOSFET-based electronic resistors. The proposed configuration exhibits high-output impedance which is important for easy cascading in the current-mode operations. The proposed circuit can be used as either a two-input three-output circuit or a three-input single-output circuit. In the operation of two-input three-output circuit, the bandpass, highpass, and bandreject filtering responses can be realized simultaneously while the allpass filtering response can be easily obtained by connecting appropriated output current directly without using additional stages. In the operation of three-input single-output circuit, all five generic filtering functions can be easily realized by selecting different three-input current signals. The filter permits orthogonal controllability of the quality factor and resonance angular frequency, and no inverting-type input current signals are imposed. All the passive and active sensitivities are low. Postlayout simulations were carried out to verify the functionality of the design.


Introduction
Since a low-voltage operating circuit becomes necessary, the current-mode technique is ideally suited for this purpose. Current-mode analog circuits have emerged in the implementation of analog functions. Current-mode active elements have been receiving considerable attention due to their potential advantages such as better linearity, wider bandwidth, larger dynamic range, and lower power consumption than the voltage-mode counterparts, for example, operational amplifiers [1,2]. Simultaneously realized filter functions are applied in phase-locked loop frequency modulation stereo demodulators, touch-tone telephone tone decoders, and crossover networks used in three-way highfidelity loudspeakers [3,4].
Several single-input and multioutput current-mode universal biquadratic filters were presented in [5][6][7][8][9][10][11][12] but these configurations employ at least three current conveyors. In 2009, Horng et al. [13] proposed a novel current-mode universal filter with five inputs and two outputs using two multioutput second-generation current conveyors and five passive elements. However, one of the output currents is not at high impedance output terminal. In the same year, Yuce and Minaei [14] proposed another current-mode universal filter with a single input and three outputs. The resonance angular frequency ( ) and the quality factor ( ) of the filter cannot be orthogonally controllable. In 2012, a versatile current-mode universal filter was proposed in [15]. This circuit employs three multioutput differential voltage current conveyors (MO-DVCCs). A current-mode universal biquadratic filter employing two plus-type dual current output terminals differential voltage current conveyors (DO-DVCCs), two grounded capacitors, and two grounded resistors was presented in [16]. The circuit can realize the lowpass (LP), bandpass (BP), highpass (HP), bandreject (BR), and allpass (AP) transfer functions by choosing the appropriate input and output terminals but it needs passive componentmatching conditions. Moreover, the and parameters of the filter cannot be orthogonally controllable.  This paper presents a new and better versatile tunable current-mode universal filter employing only two MO-DVCCs. Either two-input three-output or three-input singleoutput current-mode universal biquadratic filter can work in the same configuration. The proposed circuit enjoys the following advantages: (i) the employment of only two MO-DVCCs, (ii) the employment of only grounded capacitors, (iii) the employment of MOSFET-based electronic resistors that act as three grounded resistors at terminals of MO-DVCCs suitable for electronic tunability with control voltage applied to the gate and absorbing series parasitic resistances at terminals of MO-DVCCs, (iv) simultaneous realization of BP, BR, and HP filtering responses with two-input and threeoutput while the AP filtering response can be easily obtained by connecting appropriated output current directly without using additional stages, (v) realization of LP, BP, HP, BR, and AP filtering responses with three-input and single-output by selecting different three-input current signals, (vi) highoutput impedance good for cascade ability for the currentmode circuits, (vii) no capacitor is connected at the port of the current conveyors, (viii) no need to employ invertingtype input current signals, (ix) orthogonal controllability of and , and (x) low active and passive sensitivity performances. To the best knowledge of the author, any versatile tunable current-mode universal biquad employing only two MO-DVCCs has not been reported in the literature earlier. In Table 1, the main features of the proposed new circuit are compared with recently reported DVCC-based current-mode universal function filters. It can be seen that the proposed circuit enjoys all the advantages of the recently reported in [15] and employs one fewer active component.

Circuit Description
As a current-mode active device, the DVCC has the advantages of both second-generation current conveyor (such as large signal bandwidth, great linearity, and wide dynamic range) and the differential difference amplifier (such as high input impedance and arithmetic operation capability) [17]. This element is a versatile building block whose applications exist in the literature [15][16][17][18][19][20][21][22][23]. The terminal characteristic of  the DVCC can be given by 1 The proposed versatile tunable current-mode universal biquadratic filter with high output impedance is shown in Figure 1. It is based on two MO-DVCCs, two grounded capacitors, and three grounded resistors by MOSFET-based electronic resistors. As shown in Figure 1, each resistor ( = 1, 2, 3) is realized by two MOS transistors (MR 1 and MR 2 ) that can be adjusted with their corresponding control voltages and − . The two MOS transistors are operated in saturation region. The matched transistors MR 1 and MR 2 are diode connected. Using the square law characteristic, the drain currents in MR 1 and MR 2 can be expressed as [23,24] where is carrier mobility, is the gate capacitance per unit area, is the threshold voltage, and and are the channel length and width, respectively. A voltage in is applied to the central node of the electronic resistor circuit, developing a current in into the node. Using Kirchhoff 's current law with (1), a simple algebraic expression can be obtained for the resistance as Equation (2) is applicable when both MOS transistors remain in the saturation region, which is true if | in | < ( − ). Straightforwardly analyzing the filter in Figure 1, the following three-output current signals can be derived as: ( ) is given by Depending on the status of the biquad input four currents, 1 , 2 , 3 , and 4 , numerous filtering functions are obtained. There are two cases shown as follows.
Thus, the circuit realizes an inverting BP response at 1 , a noninverting BR response at 2 , and an inverting HP signal at 3 . A current-mode AP response is easily obtained by connecting the two currents 1 and 2 , yielding a transfer function as follows: Because the output impedances of the currents 1 , 2 , and 3 are very high, the three-output terminals, 1 , 2 , and 3 , can be directly connected to the next stage, respectively. The two grounded resistors by MOSFET-based resistors are connected to the two terminals of the two MO-DVCCs, respectively. This design offers another feature of a direct incorporation of the parasitic resistance at the terminal of the MO-DVCC ( ) as a part of the main resistance. From (7) to (9), it can be seen that the proposed circuit is a two-input three-output CM multifunction biquadratic filter. (i) if 1 = 2 = 0 (opened), and 3 = in is given by the input current signal, an LP filter can be obtained at 1 (ii) if 1 = 3 = 0 (opened), and 2 = in is given by the input current signal, a BP filter can be obtained at 1 (iii) if 2 = 3 = 0 (opened), and 1 = in is given by the input current signal, a HP filter can be obtained at 1 (iv) if 2 = 0 (opened), and 1 = 3 = in is given by the input current signal, a BR filter can be obtained at 1 (v) if 1 = 2 = 3 = in is given by the input current signal, an AP filter can be obtained at 1 .
From (3), it can be seen that the gain constants for the LP, BP, HP, BR, and AP responses are equal to unity in magnitude. The circuit does not need critical passive component-matching conditions and inverting-type current input signals to realize the above five generic filtering responses in the design. Obviously, it is a three-input and single-output universal current-mode biquadratic filter.
From the denominator polynomial of the transfer functions given in (6), the and of the filter are expressed as = √ 1 It should be noted that the parameter can be adjusted by changing the resistor 3 without disturbing the parameter . Also, the passive and sensitivities are equal to or smaller than unity. Taking into account the nonidealities of a DVCC, namely, The active and passive sensitivities of the filter parameters and are shown as which are equal to or less than unity in magnitude. Thus, the proposed filter of Figure 1 has small component sensitivities.

Simulation Results
To verify the theoretical study, the behavior of the proposed versatile tunable current-mode universal filter has been verified by using TSMC 0.18 m 1P6M CMOS process technology HSPICE simulations. The CMOS implementation of the MO-DVCC is shown in Figure 2 [15]. The dimensions of MOS transistors used in implementation of the MO-DVCC are given in Table 2. The supply voltages were DD = − SS = 0.9 V, and the biasing voltages were 1 = −0.1 V and 2 = −0.38 V. The layout of the entire schematic was done using Cadence's Virtuoso tool. Figures 3 and 4 show the overall chip layout and the detail layout of the filter core, respectively. The layout floorplan is shown in Figure 5  MHz and a quality factor of = 1 with 1 = 2 = 3 = 1 V in Cases 1 and 2, respectively. In Figure 7, the magnitude of input currents 1 , 2 , and 3 can be chosen as in Table 3. Showing the frequency tunability of the proposed filter by selecting equal control voltages of with different values of 0.95 V, 1 V, 1.1 V, and 1.2 V results in filter responses with a quality factor of = 1 and resonance frequencies of ≈ 3.78, 4.76, 6.32, and 7.47 MHz, respectively. Thus we obtain different resonance frequencies at 1 output terminal with 2 = 0 and 1 = 3 = in for the BR responses as shown in Figure 8. To show the quality factor tunability of the proposed filter, by keeping the values of 1 = 2 = 0.9 V and varying only 3 , the can be controlled by the following setting: 3 = 0.9 V to realize = 1, 3 = 1 V to realize = 1.78, 3 = 1.2 V to realize = 2.81, and 3 = 1.65 V to realize = 3.94. Thus we obtain different s at 1 output terminal with 2 = 0 and 1 = 3 = in for the BR responses as shown in Figure 9. It is observed from Figures 6-9 that the filter postlayout simulation performs all the filter functions well, but the small departures filter responses mainly stems from the parasitic impedance effects and nonideal gains of MO-DVCC. To test the input dynamic range of the filter, the postlayout simulation has been repeated for a sinusoidal input signal at = 2.72 MHz. Figure 10 shows the input dynamic range of the inverting BP response at 1 output terminal with 1 = 2 = 3 = 0.9 V, 1 = 3 = 0, and 2 = in , which extends up to amplitude of 40 A (peak to peak) without signification distortion. The total power dissipation is found to be 0.438 mW. The chip area without pads is only 167.5 × 85.76 m 2 .

Conclusion
In 2012, a versatile current-mode universal filter was proposed in [15]. This circuit employs three MO-DVCCs. In      this paper, the author also presents a versatile current-mode universal biquadratic filter. The new circuit employs only two MO-DVCCs, two grounded capacitors, and three grounded resistors by MOSFET-based electronic resistors. Either a twoinput and three-output circuit or a three-input and singleoutput circuit current-mode universal biquadratic filter can work in the same configuration. The proposed filter can be used for realizing all of the standard filtering functions without inverting-type current input signals. The parameters and are orthogonally controllable. The postlayout simulations were carried out to verify the functionality of the design. The filter chip operates to 7.47 MHz and the power dissipation is 0.438 mW. The specifications of our analog filter chip are summarized in Table 4.