A wideband amplifier topology based on positive capacitive feedback

Abstract

We propose a novel bandwidth expansion technique for cascode amplifiers, that exploits positive capacitive feedback (PCF) applied to the cascade of common base and common collector stages to increase the overall amplification at high frequencies. The resulting amplifier has a nearly unchanged input impedance, that we enhance introducing a second PCF loop. We present an ac model of the topology to better understand the behavior of the proposed technique and to compare it with other PCF-based amplifiers. Simulations, carried out using a 230GHz-fT SiGe technology, have shown a 40% bandwidth extension rate with respect to a simple cascode differential pair when a 50Ω source resistance is used, and up to 59% for a 200Ω source resistance. This is a 15–20% more than what can be achieved with other PCF techniques in the literature. The proposed topology is robust against process variations, and retains its advantages when accurate layout parasitics are taken into account.