SFG Synthesis of General High-Order All-Pass and All-Pole Current Transfer Functions Using CFTAs

Worapong Tangsrirat

doi:10.1155/2014/271926

Abstract

An approach of using the signal flow graph (SFG) technique to synthesize general high-order all-pass and all-pole current transfer functions with current follower transconductance amplifiers (CFTAs) and grounded capacitors has been presented. For general nth-order systems, the realized all-pass structure contains at most n + 1 CFTAs and n grounded capacitors, while the all-pole lowpass circuit requires only n CFTAs and n grounded capacitors. The resulting circuits obtained from the synthesis procedure are resistor-less structures and especially suitable for integration. They also exhibit low-input and high-output impedances and also convenient electronic controllability through the g m-value of the CFTA. Simulation results using real transistor model parameters ALA400 are also included to confirm the theory.

Highlights

Introduction and MotivationIn 2008, the conception of the current follower transconductance amplifier (CFTA) has been introduced [1]
The transconductance gain of the CFTA is directly proportional to the external bias current Io, which is approximately equal to gm and VT ≅ 26 mV at 27∘C
Using the current and voltage relations of the CFTA given in (1), we find that these two subgraphs can be realized using CFTA by the subcircuits as shown in Figures 4(b) and 4(d), respectively

Summary

Introduction and Motivation

In 2008, the conception of the current follower transconductance amplifier (CFTA) has been introduced [1]. The signal flow graph (SFG) procedure is applied to synthesize the high-order, all-pass, and all-pole current transfer functions using CFTAs as active elements together with grounded capacitors as passive elements. The approach is based on drawing a signal flow graph directly from the given transfer function and obtaining, from the graph, the active-C filter involving CFTAs. The design procedure shows that the resulting structures are canonical in the number of active components, n + 1 CFTAs for realizing nth-order all-pass circuit and n CFTAs for realizing nthorder all-pole circuit. The circuits have low sensitivity characteristics, and exhibit electronic controllability of important filter coefficients via transconductance gains (gm) of CFTAs. To demonstrate the proposed approach, the third-order current-mode all-pass filter and Butterworth. All-pole lowpass filter were designed and simulated using PSPICE program

Basic Concept of the CFTA

SFG Synthesis of General High-Order All-Pass Current Transfer Functions

SFG Synthesis of General High-Order All-Pole Current Transfer Functions

Computer Simulations and Results

Conclusions