| Operational amplifiers are a basic building block used in system and board level designs in many different applications. One of the most challenging applications is in the portable equipment environment. Consumer products designed for this market must be capable of operating at low voltages and consume little current to reduce power consumption from batteries. Design techniques for amplifiers which sustain or even improve on the performance of circuits that operate at these lower voltage supplies are needed to meet the demands of this market. This research discusses the design of operational amplifier circuits which require high performance, even as the supply voltages decrease. The content of the research will concentrate on key issues in biasing, input stage design, switched capacitor amplifiers, and output stage design that have been developed in a commercial design environment. Design methods used in bipolar, CMOS, and BiCMOS process flows will be compared to look at trade-offs in the choices of circuit topologies. Then, applying these techniques, the design of a single supply, high speed operational amplifier will be examined in detail. The motivations for selecting this particular type of amplifier and its target specifications are discussed. A slew rate enhancement circuit will be introduced which brings elements of the current mode type of amplifier circuit to the well known voltage mode input stage, retaining the desirable qualities of both. A new high speed output stage will then be shown which allows increased dynamic range while also increasing the capacitive drive capability of the amplifier. Measured results will be shown, along with the finished layout. The amplifier attains 50MHz unity gain bandwidth performance at supply voltages of 2V. |
