Linearity improvement technique of transconductance amplifier for Gm-C filter

Abstract

The voltage to current transconductor, what is called transconductance amplifier, has been used as a basic building block of analog circuits such as analog filter and variable gain amplifier. Generally, the transconductance amplifier dominates the total linearity characteristic of analog circuits. However, as device sizes and power supply voltages are scaled down, analog circuit design is getting difficult to guarantee the required linearity. Various efforts to improve the linearity of transconductor have been developed. Moreover, the tremendous demands of multi-mode communication terminal accelerate the research on the multi-standard transceiver. Recently, one of issues in continuous-time analog filter design is the possibility of reconfigurable function adapting to the various channel bandwidth and group delay specification. According to the trend, the linearity improvement technique of transconductor is proposed in this paper. The proposed high linear transconductor constitutes a $5^{th}$ elliptic Gm-C filter, which is applicable to various standards by implementing various cutoff frequency There are several methods to increase the linearity of transconductor; one of the efficient and easy ways of improving linearity is source degeneration technique. The degree of degeneration ratio in source degeneration transconductor determines the total transconductance curve. If the degeneration ratio dynamically varies with the change of input voltage, we can get flat transconductance curve. The proposed linearization technique is based on the typical source degeneration of differential amplifier. By attaching a non-linear resistor composed of two NMOS transistors the degeneration ratio changes with input level. This proposed transconductor is used in constructing a multi-bandwidth Gm-C filter which provides 5MHz, 10MHz, and 20MHz cutoff. This filter is fabricated in 1P6M 0.18um CMOS process. The measured IIP3 and SFDR are 15.5dBm and 43.87dB while consuming 4.3mW p...