The Circuit Designer's Companion - diagramas.diagram...

148 The Circuit Designer’s CompanionChapter 5Analogue integrated circuitsThe operational amplifier is the basic building block for analogue circuits, and progressin op-amp performance is the “litmus test” for linear IC technology in much the sameway as progress in memory devices is for digital technology. This chapter will bedevoted to op-amps and comparators, with a tailpiece on voltage references. This is notto deny the enormous and ever-widening range of other analogue functions that areavailable, but these are intended for specific niche applications and little can begeneralised about them.Volumes have already been written about op-amp theory and circuit design andthese aspects will not be repeated here. Rather, we shall take a look at the departuresfrom the ideal op-amp parameters that are found in practical devices, and survey thetradeoffs − including cost and availability, as well as technical factors − that have to bemade in real designs. Some instances of anomalous behaviour will also be examined.5.1 The ideal op-ampThe following set of characteristics (in no particular order, since they are allunattainable) defines the ideal voltage gain block:• infinite input impedance, no bias current• zero output impedance• arbitrarily large input and output voltage range• arbitrarily small supply current and/or voltage• infinite operating bandwith• infinite open-loop gain• zero input offset voltage and current• zero noise contribution• absolute insensitivity to temperature, power rail and common mode inputfluctuations• zero cost• off-the-shelf availability in any package• compatibility between different manufacturers• perfect reliability.Since none of these features is achievable, you have to select a practical op-amp fromthe multitude of imperfect types on the market to suit a given application. Some basicexamples of tradeoffs are