Common-base-input operational amplifiers

A common-base-input operational amplifier is useful when the common-mode input voltage lies close to or beyond either one of the DC supply rails. Input impedance and gain-bandwidth product are comparable with those for a conventional common-emitter-input amplifier which uses similar transistors. However, a CB-input amplifier can be all-npn, in which case its non-dominant poles are further out than for a circuit which requires lateral pnps. Offset voltage referred to the input is also comparable with a CE-input amplifier, but offset current is worse - perhaps 2% of the first-stage quiescent emitter current. Therefore offset-to-signal ratios for CB-input and CE-input amplifiers are comparable, provided the source resistance is not too large. Four practical applications are given for CB-input operational amplifiers inside high-speed analogue integrated circuits.