# Universal Voltage Conveyor and its Novel Dual-Output Fully-Cascadable VM APF Application
## 1 Introduction  
All-pass filters (APFs) are widely used in signal processing to correct the phase shifts caused by analog filtering operations without changing the amplitude of the applied signal. Moreover, they are with advantage used for the design of high-\(Q\) frequency-selective circuits or quadrature/multiphase oscillators [1; 2; 3; 4]. In the open literature, a large number of voltage-mode (VM) APFs using various active building blocks (ABBs) exist; however, most of them are single-output realizations (i.e., provide only inverting or non-inverting phase response at voltage-mode output). Note that we do not rule out the importance of such circuits; however, in this study we solely focus on VM APFs that provide both inverting and non-inverting voltage-mode outputs simultaneously [5; 6; 7; 8; 9; 10; 11; 12; 13; 14]. In general, the desired features of VM APFs are:  
1. Provide both inverting and non-inverting voltage-mode outputs simultaneously from the same configuration [5; 6; 7; 8; 9; 10; 11; 12; 13; 14],
2. High-impedance character of voltage input terminal and low output impedance at both voltage-mode outputs, which is needed for easy cascading [12], Figure 2b in [13], [14],
3. All passive components are grounded, which is advantageous for monolithic integration [9],
4. No complex passive and/or active matching constraints are required [5; 6; 7; 8; 9; 10; 11; 13; 14]. 5. Use of single ABB in order to avoid increasing the circuit complexity [7; 8; 9; 10; 12; 13; 14]. Table 1 summarizes the advantages and disadvantages of previously reported VM APFs which satisfy feature (i), and it provides their fair performance comparison. Among these filtering topologies, the [5, 6] are based on a fully-differential operational amplifier (FD-OPA) and second-generation currentconveyor (CCII). Utilization of a positive-type inverting second-generation current conveyor (ICCII+) for VM APF design is demonstrated in [7]. The literature offers various types and generations of CCs, such as fully differential second-generation current conveyor (FDCCII) or differential voltage dual-X second-generation current conveyor (DV-DXCCII) and their dual-output VM APF realizations [8; 9]. One of the most recently published VM APFs with attractive performance based on a voltage differencing inverting buffered amplifier (VDIBA) was reported in [10]. The VDIBA is a recently introduced four-terminal active device with an input/output stage that is easily implemented by a differential-input single-output operational transconductance amplifier (OTA) and a unity-gain inverting voltage buffer (IVB), respectively. As it is known, the OTA converts the input voltage to output current while its intrinsic transconductance can easily be tuned via external bias voltage or current. This favorable feature was advantageously used for pole frequency tuning of VM APF in [11]. In the filter topology, except for a single OTA, an additional ABB called a universal voltage conveyor (UVC) is employed. In general, the concept of voltage conveyors (VC) was defined based on the duality principle to current conveyors in 1981 [15]. Since then, different generations (first, second, third) and types (non-inverting, inverting, positive, negative) of voltage conveyors have been introduced in the open literature. Among the special types of VCs is the plus -type differential current voltage conveyor (DCVC+) [16], the concept of which is equivalent to the current differencing buffered amplifier (CDBA) [17; 18]. All of the VCs listed above can be realized using a single UVC by suitable interconnection or grounding of its ports [19; 20; 21; 22]. Hence, the UVC is a \"universal\" active element which has one voltage input X, two difference current inputs (YP, YN), two mutually inverse voltage outputs (ZP, ZN), and one auxiliary terminal W, which determine the generation of the voltage conveyor. Moreover, the UVC is a readily available chip which was produced in cooperation with ON Semiconductor Czech Republic, Ltd. under designation UVC-N1C 0520 [23; 24; 22].