Adaptive cancellation linearisation and its application to wide‐tunable Gm‐C filter design

Abstract

This paper presents an adaptive g m3 cancellation for linearisation of operational transconductance amplifier (OTA) and its application to design of a wide tunable Gm-C filter. g m3 cancellation through paralleling triode-and subthreshold-mode transconductors makes good linearisation, but only in a limited range of tuning voltage. An auxiliary circuit is employed to adaptively change the operating point of subthreshold-mode transconductor such that to keep linearisation throughout the tuning range. By this way, the linearity of OTA is significantly improved in overall transconductance tuning range from 18 to 289 μA/V. While by applying 0.6 Vpp input voltage at 1 MHz the total harmonic distortion (THD) of conventional OTA lies between −75.1 and −44.4 dB, it reduces to lie between −83.3 and −57.4 dB in the proposed OTA. Using the proposed OTA, a third-order low-pass Gm-C filter is designed in 0.18 μm CMOS technology. The cutoff frequency of filter is tunable from 1.73 to 27.25 MHz, thus, can be applicable to support WCDMA, WLAN (IEEE 802.11a/b/g/n) and WiMAX standards in multi-mode direct conversion receivers. The third-order intercept point of filter is between 7.1 and 13.7 dBm in different tuning conditions.

Ladder-Type Gm -C Filters With Improved Linearity

“…The idea of linearized transconductors with degenerated input pair and loop control is discussed in [32]. Several solutions based on transistor in the triode region [33], [34], or floating-gate [35], body-driven [36] and cross-coupled cells [37] have been also reported and combined with efficient techniques such as adaptive biasing [38]- [42], master-slave automatic tuning [43], and cancellation of the nonlinear terms [44], [45]. Most of the above methodologies, however, establish inevitable trade-offs between the power, area and noise parameters and the DR.…”

A class-AB linear transconductor with enhanced linearity

“…adaptive biasing [13]- [21], floating-gate devices [22]- [23], and nonlinearity cancellation techniques [24]- [27]. Sometimes two or more techniques are used together to improve linearity.…”