# High-Input Impedance Voltage-Mode Multifunction Filter
## 1 Introduction  
Due to the flexibility and versatility of high-input impedance single-input and triple-output voltage mode (VM) multifunction biquad filters in practical applications, people are increasingly interested in their design. This type of biquad can be used in some systems that employ multiple filter functions. For examples, the multifunction biquads could be applied to high fidelity three-way speaker systems, touch-tone telephone systems, and phase-locked-loop (PLL) frequency-modulation (FM) stereo demodulators [1].  
Several VM biquads based on active components have been presented in [1, 2, 3, 4, 5, 6, 7]. The applications and advantages in the realization of the VM biquad filters employing the current-feedback amplifier (CFA) as the active building block have also received considerable attention [8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18]. The circuits in [8, 9, 10, 11, 12, 14, 16, 17, 18] propose some CFA-based biquad filters but they cannot simultaneously realize non-inverting low-pass (NLP), inverting band-pass (IBP), and non-inverting band-stop (NBS) voltage responses. The circuit in [19] described a biquad for realizing VM NLP, non-inverting band-pass (NBP), and non-inverting high-pass (NHP) voltage responses simultaneously; but it uses four CFAs, two capacitors, and six resistors. The circuit in [20] described a biquad for realizing VM NLP, NBP, and NHP voltage responses, simultaneously; but it uses three CFAs, three capacitors, and four resistors. The circuit in [21] described another biquad for realizing VM NLP, IBP, NHP, and NBS voltage responses simultaneously; but it uses four CFAs, two capacitors, and four resistors. The circuit in [22] described a biquad that uses three CFAs, two capacitors and four resistors to realize VM inverting low-pass (ILP), NBP, and NHP voltage responses simultaneously. However, the resonance angular frequency (\(\omega_{0}\)) and the quality factor (Q) parameters of this circuit cannot be implemented by orthogonal controllability.  
Recently, a high-input impedance two-input triple-output VM multifunction biquad filter has been proposed in [23], which can simultaneously realize NLP, NBP, and NBS voltage responses at high-input impedance terminal, and the circuit parameters of \(\omega_{\rm o}\) and Q offer orthogonal controllability. In this paper, the authors propose another high-input impedance single-input triple-output VM multifunction biquad filter. The proposed circuit employs three CFAs, two grounded capacitors, and three resistors with high-input impedance. It can simultaneously realize NLP, IBP, and NBS responses at high-input impedance terminal and still enjoys the orthogonal controllability of \(\omega_{\rm o}\) and Q. The comparison between the proposed biquad and the previous CFA-based biquads is summarized in Table 1.  
CFA is equivalent to a positive current conveyor (CCII) cascaded with a unity gain voltage follower (VF) [17; 18]. According to the datasheet [24], the integrated circuit (IC) of the commercial AD844 comprises of a CCII cascaded with a unity gain VF. The voltage on the non-inverting input port (V+) is transferred to the inverting input port (V\(-\)), and the current flowing to the inverting input port is replicated to the port Tz, and the voltage on the port Tz is transferred to the port W of unity gain VF. Hence, the proposed circuit is implemented using the commercial AD844, which results in high-input impedance, high current drive capability, simplicity, and low cost.