Novel Electronically Tunable Biquadratic Mixed- Mode Universal Filter Capable of Operating in MISO and SIMO Configurations

In this paper, a novel electronically tunable biquadratic universal mixed-mode filter is presented. The filter is based on extra X current conveyor transconductance amplifier (EXCCTA), recently introduced by authors. The proposed filter employs two EXCCTAs, two capacitors, a switch, and four resistors. The filter can work in both multi-input-single-output (MISO) and single-input-multi-output (SIMO) configurations without change in its structure. The filter provides all five responses in voltage-mode (VM), current-mode (CM), transimpedance-mode (TIM), and transadmittance-mode (TAM). The attractive features of the filter include (i) ability to operate in both MISO and SIMO configurations in all four modes, (ii) no requirement of capacitive matching, (iii) tunability of quality factor (Q) independent of natural frequency (ω0) in MISO & SIMO configurations and (iv) no requirement for double/negative input signals (voltage/current) in MISO configuration. The non-ideal gain and sensitivity analysis is also carried out to study the effects of process variations and passive components spread on filter performance. The filter is designed in Cadence Virtuoso using Silterra Malaysia 0.18μm PDK. The complete layout of the EXCCTA is designed and the parasitic extraction is done. The filter is tested at a supply voltage of ±1.25 V and the obtained results validate the theoretical findings.


Introduction
The design and development of frequency filters is an important field of communication engineering and research. The filters are an integral part of almost every electronic system [1][2][3]. The universal filter structure is the most versatile and sought-after filter configuration as it provides all five generic filter responses namely, lowpass (LP), high-pass (HP), band-pass (BP), band-reject (BR), and all-pass (AP) from same configuration. It serves as a stand-alone solution for all filtering requirements. They are employed in data acquisition systems as analog front-end, in communication systems, biomedical signal processing, instrumentation, and oscillator design, etc. [3][4][5][6][7][8][9][10][11][12][13][14][15]. Owing to their wide bandwidth, high slew-rate, simple circuit, good linearity, and better performance under low-voltage low-power (LVLP) environment currentmode (CM) active building blocks (ABBs) are preferred for designing analog filters [2, 5,6]. The most popular CM ABBs are the second-generation current conveyor (CCII) [1][2][3][4][5][6], current feedback operational amplifier (CFOA) [16], fully differential current conveyor (FDCCII) [18], differential voltage current conveyor (DVCC) [20], current controlled current conveyor transconductance amplifier (CCCCTA) [21], differential difference current conveyor (DDCC) [23], etc. In present day complex signal processing systems the need for interaction between currentmode and voltage-mode (VM) circuits arises often. This requirement can be met by employing transadmittancemode (TAM) and transimpedance-mode (TIM) circuits to facilitate distortion free interfacing between CM and VM units [7][8][9][10][11]23]. Although several TAM and TIM filter structures have been proposed, but a single topology providing the CM, VM, TAM, and TIM responses will be an added advantage in terms of area and power requirements. Numerous mixed-mode universal filters can be found in the open literature  that were designed to cater to the above-mentioned requirements. The filter structures can be classified in three basic groups such as single-input-multi-output (SIMO), multi-input-multioutput (MIMO), and multi-input-single-output (MISO). The comparison between the filter structures can be done based on following important criteria: A detailed comparison of the state-of-the-art mixed-mode filters with the proposed design is presented in Table 1.
It can be inferred from the table that the filter structures [16, 19-26, 28-30, 32, 34] employ three or more ABBs for the design. The designs in [16,28,29,33,34] utilize seven or more passive components. The design in [29] requires capacitive matching, which is undesirable in today's submicron technologies. In filters [16-17, 21, 22, 26, 28-30, 32] the quality factor cannot be controlled independent of the natural frequency. The filter structures [17-22, 25, 27, 34] are not truly universal mixed-modes since they cannot realize all five filter responses in VM, CM, TAM and TIM operation. None of the above mixed-mode filters except [20] is designed at natural frequency higher than 4 MHz. The filter structures [16-18, 20, 23, 27-29, 33, 34] lack inbuilt tunability. None of the existing filters (with the exception of [33]) can work in both MISO and SIMO configurations and provide all five filter responses in all Four modes of operation. In addition, some other drawbacks of the design [33] are: (i) the design is not modular as it uses two different ABBs, namely FDC-CII and DDCC, also it requires five input voltages and six input currents in MISO configuration, (ii) both capacitors are connected to X terminals which is undesired as it effects the high frequency performance as shown in [35], (iii) use negative and double inputs in MISO configuration, and (iv) lack of built-in tunability. The literature survey points out that although many exemplary mixed-mode filter designs exists, the research in the mix-mode filter design is still limited and newer designs need to be developed to cater to increasing demand of mixed-signal processing systems. In context, this paper aims to introduce a novel mixed-mode filter structure composed of two extra X current conveyor transconductance amplifier (EXCCTA), one switch, two ca-pacitors, and four resistors, which employs only three input current/voltage signals in MISO operation and is free from the above drawbacks of [33]. The striking features of the proposed filter are: (i) provides all five filter responses in all four modes of operation, (ii) it can work in both MISO and SIMO configuration without change in topology, (iii) it has inbuilt tunability, and (iv) the filter exhibits low active and passive sensitivities to passive elements. Beside these the filter enjoys all the properties mentioned in Table 1. The precise design, layout and simulation of the EXCCTA, is done in Cadence Virtuoso using Silterra Malaysia 0.18µm PDK. The layout verification and parasitic extraction is carried out using Mentor Graphics Calibre. The post layout results bear close resemblance with the theoretical predictions.

Extra X current conveyor transconductance amplifier (EXCCTA)
The EXCCTA is a versatile electronically tunable ABB carrying features of extra X current conveyor (EXCCII) [13] and operational transconductance amplifier (OTA) [14] in one compact integrated circuit implementation. The EXCCTA provides two independent low impedance current input terminals X P,N together with a high impedance voltage input terminal Y. It also has OTA at the output stage imparting tunability to the structure. The block diagram and voltage-current relations of the EXCCTA are given in Figure 1 and Equation (1), respectively. The complete CMOS implementation [15] is presented in Figure  2. The class AB output stage is utilized in the first stage to minimize supply voltage and power dissipation.

Proposed electronically tunable mixed-mode universal filter
The proposed mixed-mode universal filter is presented in Figure 3. The filter employs four resistors, two capacitors, and two EXCCTAs. The filter can work in both SIMO and MISO configurations by adding a single pole double throw (SPDT) switch. The operation and features of the filter in each configuration are discussed below.

SIMO configuration
In SIMO configuration, the currents I 1 to I 3 and input voltages V 1 to V 3 are set to zero. This grounds all the passive components except R 3 as can be inferred from Figure 3(b). In addition, in SIMO configuration no switch is needed for generating filter responses in all four modes. In SIMO configuration the filter has following attributes: (i) inbuilt tunability, (ii) use of grounded capacitors and no capacitive matching requirement, (iii) high input impedance in CM and TIM, (iv) CM and TAM output available form explicit high impedance nodes, (v) tunability of Q independent of ω 0 , (vi) AP gain tunability in VM and TIM, and (vii) availability of all filter function in all four mode.

SIMO voltage-mode and transadmittance-mode operation
To obtain VM and TAM responses, the input current I in is set to zero and the input voltage V in is applied as shown in Figure 3(b). The routine analysis of the circuit leads to the transfer functions as given in Equations (2-6). The VM responses are obtained from terminals V out1(SIMO) to V out4(SIMO) as follows: ( ) To obtain unity gain AP response a simple resistive matching of R 1 = R 3 is required and the response is obtained across resistor R 4 .
If the O2-terminal is disconnected from the resistor R 4 , Equation (5) turns to: and a BR response is obtained.
The TAM responses are obtained from high impedance I out1(SIMO) to I out3(SIMO) terminals. The transfer functions are given in Equations (7-11).
In TAM, the BR and AP responses can be obtained by appropriately connecting the HP, LP and BP currents.
It must be pointed out that, if the filter is designed to work in SIMO configuration then there is no need for the SPDT switch.

SIMO current-mode and transimpedance-mode operation
To obtain CM and TIM response, input voltage V in is set to zero and the input current I in is applied to the filter.
In CM operation all passive elements are grounded. The CM responses are available from high impedance terminals I out1(SIMO) to I out3(SIMO) and TIM responses are obtained from terminals V out1(SIMO) to V out4(SIMO) . The CM filter transfer functions are given in Equations (12)(13)(14)(15)(16). In CM, the BR and AP responses can be obtained by appropriately summing the output currents (I HP , I LP , I BP ).
Note that to obtain unity gain AP response a simple resistive matching of R 1 = R 3 is required and the response is obtained across resistor R 4 : Subsequently, the expression for natural frequency and Q of the SIMO mixed-mode filter are:

MISO configuration
In MISO configuration, the input current I in and input voltage V in are set to zero. The input currents I 1 to I 3 and input voltages V 1 to V 3 are applied to obtain the required filter responses. In this configuration only three resistors are employed, resistor R 4 is not required and can be eliminated as shown in Figure 3(c). The attractive features of the filter include: (i) low output impedance for VM and TIM, (ii) high output impedance explicit current output for CM and TAM, (iii) no requirement for double/negative input signals (voltage/current), (iv) tunability, (v) simultaneous availability of VM and TIM/ CM and TAM responses from same input sequence, and (vi) filter is cascadable in all four modes. The operation of the filter is described below.

MISO voltage-mode and transadmittance-mode operation
To obtain VM and TAM responses, the input voltage V 1 to V 3 are applied according to the Table 2 and the SPDT switch is connected to point B. The output responses are obtained from low impedance terminal V out(MISO)(VM-Mode) and high impedance terminal I out(MISO)(TAM-Mode) . The transfer functions for VM and TAM modes are given as: while f 0 and Q correspond to Equations (22) and (23), respectively.

MISO current-mode and transimpedance-mode operation
To obtain CM and TIM responses, the input voltages V 1 to V 3 are set to zero, the SPDT switch is connected to point A, and input current signals I 1 to I 3 are applied according to Table 3.
Note that except for AP there is no requirement for matching passive components. In case of HP response, the value of transconductance g m1 should be adjusted to achieve g m1 R 2 = 1, which can be easily accomplished by adjusting the bias current I bias of the first EXCCTA.
As a brief conclusion it must be emphasised that the proposed filter can realize SIMO (all modes) and MISO (VM and TAM) responses without requiring any switch. The switch is only required to obtain MISO (CM and TIM) responses.

Non-Ideal and sensitivity analysis
The non-ideal model of the EXCCTA is presented in Figure 4. As can be deduced, the various parasitic resistances and capacitances appear in parallel with the input and output nodes of the device. The low impedance X node has a parasitic resistance and inductance in series with it. The other non-ideal effects that influences the response of the EXCCTA are the frequency dependent non-ideal current (α P , α N ), voltage (β P , β N ), and OTA transconductance transfer (γ, γ') gains. These gains cause a change in the current and voltage signals during transfer leading to undesired response. Taking into account the non-ideal gains the V-I characteristics of the EXCCTA in (1) will be modified as follows: As a result of component tolerance and non-idealities in EXCCTA the response of the practical filter deviates from the ideal one. To get a measure of the deviation, the relative sensitivity is applied. Mathematically, relative sensitivity is defined as , where x is the component that is varied and y is the ω 0 and Q in our case.
The sensitivities of ω 0 and Q with respect to the nonideal gains and passive components are given below.
The sensitivities are low and have absolute values not higher than unity.

Simulation results
To validate the proposed mixed-mode filter, the EXC-CTA is designed in Cadence Virtuoso software using 0.18µm PDK provided by Silterra Malaysia. The widths and lengths of the MOS transistors are given in Table 4. The supply voltage is set to ±1.25 V and the bias current of th OTAs is set to120µA resulting in transconductance of g m1 = g m2 = 1.0321 mS. The complete layout of the EXCCTA is designed as presented in Figure 5. The lay-out verification and parasitic extraction are done using Mentor Graphics Calibre verification tool. The high performance nhp and php MOSFETs from the PDK library are employed in the design. The EXCCTA occupied a total chip area of (52.78×22.085)µm 2 .

SIMO configuration operation
First of all, the SIMO configuration of the proposed filter is validated. The filter is designed for centre frequency of 7.622 MHz by setting passive components and OTA bias current values as follows: R 1 = 1 kΩ, R 2 = 2 kΩ, R 3 = 1 kΩ, R 4 = 1 kΩ, C 1 = 15 pF, C 2 = 15 pF, and g m1 = g m2 = 1.0321 mS. For the sake of comparison, the EXCCTA based filter responses are plotted along with the ideal filter results obtained using the Matlab software. The VM responses are shown in Figure 6. The AP response is obtained across resistance R 4 . In addition, the gain of the AP response can be tuned through R 4 without affecting other filter parameters as is evident from Figure 7.  To analyse the quality factor tuning, the BP response is plotted for different values of I Bias1 current of OTA 1 . It can be deduced from Figure 8 that the quality factor can be tuned independent of the centre frequency. The signal processing capability of the VM filter is verified by examining the transient response of the filter. A sinusoidal voltage input signal at 7.622 MHz is applied and the observed LP, BP, HP responses are plotted as given in Figure 9. The total harmonic distortion (THD) of the filter for LP, BP, HP and AP responses is plotted for different input signal amplitudes. The THD remains within acceptable limits for large input range as presented in Figure 10.  To study the effect of process variation on the proposed filter Monte Carlo analysis is carried out for 10% variation in both capacitor C 1 and C 2 values for BP response. The analysis is done for 200 runs and the results are presented in Figure 11. The results for CM SIMO filter are presented in Figures  12 and 13. The BR and AP responses are obtained by summing I HP , I LP , and I BP currents appropriately as discussed in section 3. The quality factor variation with OTA1 bias current I Bias1 is depicted in Figure 14.   Figure 16. As can be deduced the mean value of frequency showed a deviation of approximately 6.1% for designed frequency. The THD for LP, HP, and BP responses are presented in Figure 17. The TAM filter responses are given in Figures 18 and  19, which prove that the filter can generate all five responses in this mode. The BR and AP responses can be obtained by summing the I HP , I LP , and I BP currents.  The LP, BP, and HP responses in TIM configuration are shown in Figure 20. The AP response is given in Figure  21. To verify the frequency tunability the LP response is plotted for different values of resistance R 2 . Figure 22 shows that the frequency tuning also effects the Q of the filter, however, it can be adjusted independent of frequency by varying I Bias1 of OTA1.

MISO VM and TAM configuration operation
The filter is designed for f 0 = 7.9577 MHz by setting passive component and OTA bias current values as follows: R 1 = 1 kΩ, R 2 = 1 kΩ, R 3 = 969 Ω, C 1 = 20 pF, C 2 = 20 pF, and g m1 = g m2 = 1.0321 mS. It must be noted that in MISO configuration resistor R 4 is not required and will be removed. The inputs are applied according to conditions outlined in Table 2. The filter provides VM and TAM responses simultaneously from the same input sequence. The VM filter responses are presented Figure  23. The VM AP response is given in Figure 24. The independent tunability of the Q is depicted in Figure 25 for different bias currents I Bias1 of OTA 1 . To check the phase and signal processing accuracy of the filter, transient analysis is done at 7.9577 MHz with sinusoidal voltage input of 200mV (p-p) for BP configuration. Figure 26 validates the correct functioning of the filter.  The TAM responses of the MISO filter are presented in Figure 27. The AP response is given in Figure 28. The VM outputs are obtained from low impedance node and TAM outputs are obtained from explicit high impedance node which make this filter cascadable.

MISO CM and TIM configuration operation
The CM and TAM filter is designed for f 0 = 8.16 MHz by setting passive component and OTA transconductance values as follows: R 1 = 1 kΩ, R 2 = 1 kΩ, R 3 = 969 Ω, C 1 = 20 pF, C 2 = 20 pF, and g m1 = g m2 = 1.0321 mS. In MISO filter there is again no need for R 4 . The inputs currents are applied according to sequence given in Table 3. The filter provides CM and TIM responses simultaneously from the same input sequence. The CM outputs are available from explicit high impedance node and the TIM outputs are available from low impedance node mak-