A New Current-mode Pseudo-exponential Circuit with an N-th Order Approximation

An original implementation in CMOS technology of the exponential function will be presented. In order to improve the circuit frequency response, only MOS transistors working in saturation are used and a current-mode operation is chosen. The compatibility to VLSI designed is achieved by using a FGMOS (Floating-Gate MOS) transistor for reducing the circuit complexity. The approximation error caused by a limited Taylor series expansion is strongly reduced considering a th n − order approximation of the exponential function. Additionally, the ) exp(x function approximation does not depend on technological parameters. The circuit is implemented in m 35 0 μ . CMOS technology on a die area of about m 20 m 16 μ μ × for the second-order approximation and of m 35 m 20 μ μ × for the th 5 − order approximation, the resulting approximation error having very small values.