An Improved Circuit of Super Source Follower Based on g_m-boosting Technology

Abstract

An improved circuit of super source follower (SSF) based on gm-boosting technology is proposed in this paper. The traditional SSF circuit is used to reduce the output impedance of the source follower, to provide stronger load capacity as the output stage of the whole circuit, or to push the non-dominant poles in the multi-stage amplifier circuit to a higher frequency position. In the paper, based on the above SSF circuit, an operational amplifier is connected in parallel between the gate and drain of the Nmos as the feedback loop, which takes the drain as the positive input and the gate as the output to form the gm-boosting structure. The advantage of this structure is to further reduce the output resistance through the voltage-voltage feedback characteristics of the gm-boosting structure. Because two loops are formed after adding the structure, as long as the open-loop gain of the operational amplifier is high enough, the loop formed by the operational amplifier will occupy a dominant position, thus shielding the influence of the source follower on the overall output resistance. The output resistance can change with the operational amplifier gain. The simulation results show that the output impedance of the circuit is only about 92 Ω, which is much lower than 1 kΩ of SSF structure.