Fully‐differential second‐order tunable bandstop filter based on source follower

Abstract

A new design method for a second-order bandstop filter is presented. The proposed filter is based on the source follower configuration with a partial positive feedback. The positive feedback is used to increase the quality factor of the filter, and the feed-forward paths are used to create complex zeros. The proposed bandstop filter has electronically tunable centre frequency, bandwidth and attenuation with low power consumption. The proposed design has been simulated in TSMC 180 nm CMOS technology. The filter achieves 5.8 nV/ Hz √ input-referred noise density. For a 2 GHz input signal, the filter achieves a signal-to-noise ratio of 60 dB for 238 mV 0−P, diff input signal amplitude. The simulated results show that the centre frequency tuning range is between 1.6 and 3 GHz and the bandwidth is tunable from 0.6 to 1 GHz. The filter consumes about 2.5 mW for a supply voltage of 1.8 V at a centre frequency of 2.3 GHz. Introduction: Continuous-time filters play an important role in communication systems such as wideband receivers. Bandstop filters are usually required to suppress blockers or interferences [1], which can be implemented using different architectures, such as active RC, Gm-C, or passive LC. Each topology is different in terms of linearity, area and power consumption. An active RC filter has excellent linearity due to the close-loop architecture, but it is not suitable for highfrequency applications due to the limitation of the bandwidth (BW) of the operational amplifiers [2]. Furthermore, the centre frequency of the active RC filter must be lower than the gain BW product of the amplifier, which will consume more power at high frequencies. Gm-C filters offer a higher-frequency operation at lower power consumption. On the other hand, they suffer from low linearity due to the open loop architecture [3]. Recently, filters based on the source follower architecture were proposed to overcome the limitations of conventional topologies that are based on operational amplifiers. This topology is suitable for high-frequency applications with low power consumption, low noise, and high linearity [4-6]. The proposed topology introduces a new bandstop filter, which is based on super source follower [6], positive feedback and feed-forward paths. The parameters of the proposed filter, such as BW, attenuation, and centre frequency are electronically tunable by the bias currents and varactors of the circuit.

Second order universal filter using voltage follower topology

“…The utilization of the voltage follower amplifier in the designing of the analog signal processing is a very interesting technique because it has high-performance properties such as lower power consumption, lesser chip area, higher speed, larger dynamic range and simple circuitry [1]. The voltage follower architecture was brought up to conquer the limitations of conventional topologies that are achieved by using a operational amplifier [2][3][4][5][6][7]. In addition, a filter based on a voltage follower is a type of analog filter, which needs to have electronic tunability to adjust the filter parameters that could be caused due to the variations in process and temperature.…”