# Energy-Efficient Amplifiers Based on Quasi-Floating Gate Techniques
## 1 Introduction  
Today, we are facing significant challenges in the design of electronic circuits. Many emerging wireless connectivity and Internet of Things edge computing applications require ultra-low-power wireless devices providing high performance in both indoor and outdoor environments. Energy efficiency is mandatory in such applications in order to increase battery lifetime. In fact, due to maintenance costs, battery replacement or manual recharge becomes impractical or even unfeasible in several scenarios with hard-to-reach wireless nodes (e.g., large civil infrastructures, vast natural ecosystems or implantable medical devices), requiring energy-autonomous wireless devices with batteries lasting several years. In some cases, the limited and often intermittent residual energy harvested from the environment (light, thermal gradients, vibrations, etc.) must be enough to operate the wireless device [1]. Aside from this, modern nanometer integrated circuit (IC) processes pose further challenges since device scaling directly impacts electronic design. Intrinsic gain reduction degrades DC gain. Supply voltage lower than 1 V becomes commonplace to reduce power consumption and to avoid gate oxide breakdown of nanometer devices. To complicate things, transistor threshold voltage cannot scale at the same rate as the supply voltage to keep subthreshold leakage currents low, so the available voltage swing reduces [2]. Preserving circuit performance in these demanding scenarios of energy scarcity and very low-voltage operation is a real challenge, and conventional circuit design techniques are often no longer valid in this framework. Innovative design techniques are required to meet these new demands. A critical block in modern mixed-signal ICs is the amplifier,which typically accounts for a significant percentage of the IC power budget [3]. Hence, designing high-gain wideband amplifiers operating with low supply voltage and low power requirements has become a major research topic. Conventional class A amplifiers are usually impractical under these constraints, since their maximum load current is limited by their bias current, so dynamic performance can only be improved at the expense of increased static power. These days, class AB amplifiers are preferred for low-voltage, low-power applications, since they allow maximum dynamic currents not bound by the quiescent currents [4]. However, providing class AB operation to the amplifier may degrade other characteristics, such as minimum supply voltage requirements, tolerance to process, supply voltage or temperature (PVT) variations, stability or noise performance. Likewise, to achieve energy efficiency, it is important that the large dynamic currents provided by the class AB circuits are generated in the output branch, without requiring internal copies of these large currents. Therefore, careful design of class AB amplifiers is mandatory. This paper deals with the design of low-voltage class AB CMOS amplifiers able to maximize performance, preserving, at the same time, energy efficiency. It will be shown that quasi-floating gate (QFG) transistors [5] represent an excellent choice to achieve these goals. The QFG technique allows independent control of static and dynamic operation of the transistor by using separate input terminals for DC bias and AC signals. This allows overcoming the tradeoff between dynamic performance and quiescent power consumption of class A topologies. Very low and accurately controlled quiescent currents can be achieved by properly setting the DC input level. At the same time, large dynamic currents can be provided, since signals are capacitively coupled to the high impedance gate node.