## 1 Introduction  
Current-mode active elements have attracted the attention of analogue circuit designers because of their advantages over conventional operational amplifiers, which include higher accuracy, wider frequency response, larger dynamic range, greater linearity, lower power consumption and simpler implementation [1; 2]. Thus, applications and advantages of various active filter transfer functions that use different active elements have been studied intensively [3; 4; 5; 6; 7; 8; 9; 10; 11; 12]. Depending on the nature of input and output signals, filters are classified as current-mode (CM), voltage-mode (VM), transadmittance-mode (TAM) and transimpedance-mode (TIM) filters. In a VM structure, both input and output signals are voltages while in a CM structure, both input and output signals are currents. The TAM and TIM structures can function as bridges for transferring VM to CM and vice versa. Hence, mixed-mode circuits are worthy of study. Many types of mixed-mode circuits have been developed [13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27], utilizing assorted types of active elements such as conventional second-generation current conveyors (CCIIs) [13; 14], current feedback operational amplifiers (CFOAs) [15], differential voltage current conveyors (DVCCs) [16], differential difference current conveyors (DDCCs) [17], fully differential current conveyors (FDCCIIs) [18], four-terminal floating nullors (FTFNs) [19], operational transconductance amplifiers (OTAs) [20; 21; 22], current controlled current conveyors (CCCIIs) [23; 24; 25], and current controlled current conveyor transconductance amplifiers (CCCCTAs) [26; 27]. Highperformance electronically tunable active components have also received much attention. In CCCII and OTA, X-port parasitic resistance \(\mathrm{R_{X}}\) and transconductance gain \(\mathrm{g_{m}}\), that are electronically controllable by the input bias currents \(\mathrm{I_{S}}\) and \(\mathrm{I_{B}}\), respectively, have become the most promising active elements in the field of analogue circuit design. Thus, several electronically tunable mixed-mode biquadratic filters have been presented in the literature [20; 21; 22; 23; 24; 25]. In the literature, circuits with different filter responses are realised according to the selected input signals, and they usually require two/three voltage signals for realising bandstop/allpass (BS/AP) filters and hence other active elements are needed to duplicate the input voltage signals. An attractive feature of multifunction active biquadratic filters is that lowpass (LP), bandpass (BP) and highpass (HP) outputs are simultaneously available in various circuit modes. These additional outputs can be used in systems that employ more than one filter function. Circuits that simultaneously use LP, BP and HP filters have applications in crossover networks used in three-way high-fidelity loudspeakers, touch-tone telephone systems and phase-locked loop frequency modulation stereo demodulators [12]. Simultaneously obtaining various filter functions in the same circuit topology increases the flexibility and versatility of practical applications. Although several electronically tunable mixed-mode multifunction biquads with multiple-output current controlled conveyors (MOCCIIs) and two grounded capacitors have been proposed [23; 24; 25], these circuits [24; 25] suffer from the high-input impedance terminal in both VM and TAM. One interesting solution is the electronically tunable mixed-mode circuit [26]. Three CCCCTAs and two grounded capacitors were used to realise LP, BP and HP responses in all four modes in the same configuration. Both BS and AP responses can be realised in CM and TAM, but the AP response in CM and TAM requires a component-matching condition. Another valuable electronically tunable mixed-mode universal biquadratic filter structure with three CCCCTAs and two grounded capacitors was proposed in [27].