Basic IC Output Stages

Abstract

In this chapter, the performance is studied of typical output stages for analog integrated circuits. In general, the output stage should be able to drive heavy loads, resistive and capacitive. This implies that the output stage should have a very low output resistance. At the input, the input resistance should be large so that the loading on a previous stage is as small as possible. Voltage gain, per se, is usually provided by the preceding ‘gain’ stages and is not necessary in the output stage. However, power gain (and thus current gain) is required. Of course, the transfer characteristic of the stage should be as linear as possible in order not to introduce harmonic distortion. All of these considerations point to the use of a feedback amplifier. In particular, a series-shunt combination is appropriate for the high input resistance, the low output resistance, the linearity, and reduced voltage gain. The simplest form of such amplifiers are the ‘follower’ stages, e.g., an emitter follower (EF) for bipolar transistors and the source follower (SF) for MOS devices. (The follower stages can be considered as forms of two two-port combinations of the series-shunt variety. Note that a C-E or C-S stage with resistance in the common input-output lead is a series-series configuration.) Finally, any output stage should convert the dc power from the bias sources into ac power as efficiently as possible. Therefore, power conversion efficiency must be given attention.