Explicit design formulas for current-mode leap-frog OTA-C filters and 300 MHz CMOS seventh-order linear phase filter

Abstract

The leap-frog (LF) configuration is an important structure in analogue filter design. Voltage-mode LF OTA-C filters have recently been studied in the literature; however, general explicit formulas do not exist for current-mode LF OTA-C filters and there is also need for current-mode LF-based OTA-C structures for realization of arbitrary transmission zeros. Three current-mode OTA-C structures are presented, including the basic LF structure and LF filters with an input distributor or an output summer. They can realize all-pole characteristics and functions with arbitrary transmission zeros. Explicit design formulas are derived directly from these structures for the synthesis of, respectively, all-pole and arbitrary zero filter characteristics of up to the sixth order. The filter structures are regular and the design formulas are straightforward to use. As an illustrative example, a 300 MHz seventh-order linear phase low-pass filter with zeros is presented. The filter is implemented using a fully differential linear operational transconductance amplifier (OTA) based on a source degeneration topology. Simulations in a standard TSMC 0.18µm CMOS process with 2.5 V power supply have shown that the cutoff frequency of the filter ranges from 260 to 320 MHz, group delay ripple is about 4.5p over the whole tuning range, noise of the filter is 420nA-√Hz, dynamic range is 66 dB and power consumption is 200 mW. Copyright © 2008 John Wiley & Sons, Ltd.