## 1 Introduction  
The current conveyor transconductance amplifier (CCTA) was introduced in [1, 2]. This active building block incorporates the advantages of the second-generation current conveyor (CCII) [3], such as wide signal bandwidth, high linearity, wide dynamic range, and the advantages of the operational transconductance amplifier (OTA), such as electronic tuning capability, and its circuit is easy to implement as a single building block. However, the CCII, which is placed in the first stage of CCTA, has a single voltage input y, which requires additional passive resistors if addition/subtraction of voltage signals are required. Usually, to realize analog circuits such as filters or oscillators, feedforward and feedback techniques are used to create closed loop systems, that require an addition/subtraction function, especially for voltage-mode circuits. To address this limitation, a differential difference current conveyor transconductance amplifier (DDCCTA) was introduced [4]. This device has a differential difference current conveyor (DDCC) in the first stage, which is cascaded by OTA in the second stage. Thus, the DDCCTA has the voltage addition/subtraction property of the DDCC at the input (i.e., \(V_{x}=V_{y1}-V_{y2}+V_{y3}\)) and the transconductance property of the OTA at the output (i.e., \(I_{o}=g_{m}V_{x}\)). DDCCTA has been used to realize a number of analog circuits such as universal filters [4, 5, 6, 7, 8], sinusoidal wave oscillators [9, 10], wave generator [11], memristor emulator [12] and many other. Unfortunately, the DDCCTAstructures applied in these applications are not designed to operate at low supply voltage and low power consumption, for example, the voltage supply is 2.5 V in [5], 4 V in [5; 6; 10], 1.8 V in [7; 8], 3 V in [9]. Therefore, in this paper, it is interesting to present a DDCCTA that is able to operate with much lower supply voltage and low power consumption. The mixed-mode universal filters are the circuits that can realize multiple modes of operation, i.e., voltage-mode (VM), current-mode (CM), transmittance-mode (TAM) and transimpedance-modes (TIM), from the same topology, under appropriate conditions. If the transfer function is a voltage ratio, it can be classified as a VM filter, if the transfer function is a current ratio, it can be classified as a CM filter, if the input signal is a voltage and the output signal is a current, it operates as a TAM filter, and if the input signal is a current and the output signal is a voltage, it operates as a TIM filter. Therefore, the mixed-mode universal filter can be a useful interface between the voltage- and the current-mode blocks of an electronic system, without the need for additional voltage-to-current converter (V/I) and current-to-voltage converter (I/V) converters [13]. For full versatility of a mixed-mode universal filter, each mode should provide five standard filter responses, namely, a low-pass (LPF), high-pass (HPF), band-pass (BPF), band-stop (BSF), and all-pass (APF) responses. Thus, a mixed-mode universal filter can provide a total of 20 transfer functions in the same topology. In addition, it is desirable to have both non-inverting and inverting filter responses in each mode, which avoids additional inverting amplifiers from being required. Therefore, the versatile universal filter will provide 40 filter responses. A number of versatile mixed-mode filters have been presented in the literature, based on various active devices, such as DDCC [13; 14; 15], four-terminal floating nullor (FTFN) [16], fully differential current conveyor (FDCCII) [17; 18], differential voltage current conveyors (DVCC) [19; 20], or current feedback operational amplifiers (CFOA) [21; 22]. However, the natural frequency and/or quality factor of these filters cannot be electronically controlled, as it is required to compensate for process and temperature variations and is available for many other universal mixed-mode filters in the literature [23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46].