Fully Differential Operational Amplifier and its Application in a Digitally Programmable VGA

The design of high-accuracy analogue circuits is becoming a difficult task with the scaling down of supply voltages and transistor channel lengths of the current mixed-signal integrated circuits. Most of this kind of circuits require the use of the highestperformance active cell: the operational amplifier (op amp). Designers are continuously working toward tradeoff solutions between gain, input/output swings, speed, power dissipation and noise. Basically, the principal topologies of op amps are based on the telescopic cascode, folded cascode, two-stage or gain boosting schemes. Op amps with active cascode circuits achieve to increase the open-loop gain without adding cascade stage and cascode devices. In this way, high speed circuits with low headroom can be obtained. This paper presents an op amp that combines the high-speed, low-power consumption of the telescopic structure with the high-swing capability of the twostage architecture. High performance is achieved by assuring high enough common mode and power supply rejection ratios for mixed-signal applications such as the digitally programmable variable gain amplifier proposed in this work.