Fig. 1: TL loop employing CCII’s and diodes The proposed TL loop can be extended to third order by including two additional diodes and two additional CClIs. This extended TL loop is shown in Fig. 2. Using the V-I relationship of the diodes and the voltage tracking properties of the CCII, the loop formed by the diodes D1- D6 results in the following relation: Here, DTL circuits can implement differential functions. Using the DTL principle, a 2"'-order low-pass filter can be realized as shown in Fig. 3. Fig. 3: The 2" order LPF using extended TL loop Applying the DTL principle to analyze the circuit shown in Fig. 3 and using Equation 2, we can derive the transfer function of the LPF as follows: Fig. 5: LPF-LPF topology to implement a 4""-order filter Table 1: Transfer functions of the 4'"-order filter A fourth-order filter was also realized using two gat order LPF’s in cascade and applying feedback to demonstrate the application of the CCII-based TL loop in filter design. The resulting 4"-order is depicted in Fig. 5. Table | gives the three transfer functions. Square-root, cube-root and cube characteristics were also obtained by varying the values of the relevant current sources depending on the function being realized. Simulations of these are shown in figures 8, 9 and 10. Geometric mean of I, and I, (set to either lu or 4u) is depicted in Fig. 11. Fig. 6: Static multiplication/division behavior Fig 12: Magnitude response of the LPF An AC analysis of the LPF (Fig. 3) was carried out with I;, =luAg. + Ige keeping the value of other current sources at 1 uA and the capacitances at 30pF. The magnitude and phase response of the LPF are shown in Fig 12 and 13 respectively. The maximum operating frequency was found to be 30 MHz. The power consumed by the LPF was found to be 250uW at the aforesaid bias conditions. A transient analysis of the LPF (Fig. 3) was carried out at 1MHz frequency with J,,=luAg. + 0.1uA (signal peak value), L=L=I,=luA; Cl = C2 = 10pF and is shown in Fig. 14. Total harmonic distortion was found to be under 0.5% up to 1MHz. gresvzom rrRar sm: ae 1 e 441 Fig 16: LPF gain tuning by varying I, Fig 15: LPF frequency tuning by changing I, Fig 13: Phase response of the LPF Simulation of the 4th order LPF (Fig. 5) was carried out with I3/l2 = Ih3/l2 = 3.1; C=C pyp=C =Cr= 10pF while keeping the input bias the same as for LPF (Fig 3). The high-pass, band-pass and low-pass characteristics are shown in Fig. 17. The quality factor was found to be 1.43 and the power dissipation amounts to 500uW.
