Techniques for the Design of Low Voltage Power Efficient Analog and Mixed Signal Circuits

Emerging applications in various fields, such as Ambient Intelligence scenarios or remote biomedical monitoring, currently demand wireless sensor networks with transceivers having extremely low power consumption requirements. This is a key issue in order to decrease battery weight and size and to increase the lifetime of the battery, which usually in these sensing nodes is not replaceable. To achieve these strict power requirements, several solutions have been proposed at various layers. At the physical layer, savings in power consumption are achieved by low- voltage operation and optimized power-to-performance ratio. Supply voltages of IV (or less) are anyway mandatory in modern deep submicron technologies to operate reliably due to the extremely thin oxide. Furthermore reduction of the supply voltage (even of not required) strongly reduces power consumption in digital circuits since it scales with supply voltage. Although this is not so simple in analog circuits, they should operate at the same supply voltage than the digital part in mixed-mode systems to avoid the complexity involved in generating various supply voltages. The canonic way of designing analog circuits consist in using high-gain amplifiers with passive components in negative feedback loops, both in continuous-time or discrete-time form. Sometimes amplifiers are operated in open loop (e.g. Gm-C filters, some VGAs, etc.), and in this case a large linear range is required for the amplifier at the expense of gain. In any case, amplifiers play a key role in analog design, and their power consumption directly impacts that of the overall analog system. Such amplifiers usually take the form of Operational Transconductance Amplifiers (OTAs) with high output resistance, typically driving capacitive loads, or operational amplifiers with low output resistance able to drive low resistive loads. Besides low-voltage and power-efficient operation, these amplifiers should feature a fast settling response, not limite- - d by slew rate. Conciliating all these requirements is difficult with conventional class A topologies, since the bias current limits the maximum output current. Hence a trade-off between slew rate and power consumption do exists. To overcome this issue, class AB topologies are often employed. These circuits provide well-controlled quiescent currents, which can be made very low in order to reduce drastically the static power dissipation. However, they automatically boost dynamic currents when a large differential input signal is applied, yielding maximum current levels well above the quiescent currents. Several class AB amplifiers have been proposed. Most of them are based on adaptive biasing techniques, by including extra circuitry that increases quiescent currents (e.g. by increasing tail currents in differential stages). However, often the extra circuits included increase both power consumption and silicon area, and add significant parasitic capacitance to the internal nodes. Also positive feedback is often employed to get boosting of dynamic currents, which makes difficult to guarantee stability considering process and temperature variations. In this work we illustrate the use of new circuit design techniques to achieve low-voltage class AB amplifiers that combine simplicity and power efficiency. These techniques allow introducing class AB operation at the input stage and at the active load of the amplifier with minimum penalty in other performance parameters. The tutorial is divided into several sections. Section 2 presents the concept of Super Class AB amplifiers and various circuit implementations. As an application, a Sample and Hold circuit is described in Section 3. Section 4 covers the design of class AB amplifiers using quasi- floating gate transistors. Their application in a VGA and a sigma-delta modulator for a wearable electroencephalogram monitoring system is described in Section 5.