A Highly Linear Modified Super Source Follower-Based Active Feedback Wideband LNA for Sub-GHz Wireless IoT Application

Abstract

This paper presents a highly linear modified super source follower (SSF)-based active feedback wideband low noise amplifier (LNA) for sub-GHz wireless Internet-of-Things (IoT) applications. To further increase the loop gain and improve both the noise figure (NF) and linearity of the LNA, it is necessary to reduce the output impedance of the voltage follower in the feedback path so that it behaves more like an ideal voltage buffer. To achieve this, the proposed wideband LNA employs an advanced source follower (modified SSF) by combining a flipped voltage follower (FVF) with an SSF, forming a voltage buffer within the feedback path. Two feedback paths are utilized to increase the loop gain and enhance the effective transconductance of the input transistor in the advanced SSF. Additionally, to further increase the loop gain, the LNA output signal is applied to the gate of transistor, which serves as the current source device in the modified SSF within the feedback path. This approach directly reduces the output impedance while improving loop gain, noise performance, and linearity. The proposed LNA was designed using a 130-nm CMOS technology. Experimental results show that the input return loss (S11) is less than −10 dB up to 2 GHz, the power gain (S21) is 21 dB, and the NF is 1.6 dB at 900 MHz. The measured 3-dB bandwidth exceeds 2 GHz, and the input third-order intercept point (IIP3) ranges from −6 dBm to −4 dBm with a two-tone spacing of 20 MHz. The total power consumption is approximately 12.5 mW from a 1.2 V supply.