Biasing

Inverter amplifiers have traditionally been biased using a constant voltage replica biasing technique. The replica is typically an equal sized inverter with input and output shorted. This method of biasing ensures that the inverters are biased at their maximum transconductance (g_m) and that the input and output common mode voltages remain equal at V_M. Unfortunately, this method of replica biasing has its limitations as the bias point is directly affected by PVT variations. In fact, using this technique the effective transconductance can vary ≈ 40 % with PVT variations impacting bandwidth, stability and gain. To solve this problem we introduce three techniques, semi-constant current biasing (SCCB), constant current biasing and constant gm biasing. We have verified the semi-constant current biasing technique using multiple fabricated designs, while the constant current and gm biasing technique have only been verified using circuit simulations. Therefore, we first introduce the semi-constant current biasing technique and evaluate its performance. This is followed by constant current and gm biasing techniques.