Since their inception, many operational amplifier designs have used bipolar transistors as the differential amplifier pair at the operational amplifier inputs. Because these input transistors operate from constant-current sources, an additional pair of matched transistors are used to obtain closely matched base-emitter voltages for a predictable ratio of currents for the constant-current generators. Phase-shift is controlled by frequency compensation that is internal to the amplifier. Amplfier phase-shift must be less than 135o at the frequency where the open-loop gain curve and closedloop gain curve intersect. In bipolar operational amplifiers, the phase-shift is typically set with an internal capacitance of approximately 30 pF. The output stage should be designed to have a wide range of voltage swing with medium current capability.
The bipolar operational amplifier is usually operated in a class-B configuration. The key features of a bipolar operational amplifier are as follows:
Table 2-1 is a selection guide showing the major parameters to be considered in choosing bipolar operational amplifiers for a particular circuit design.
Table 2-1. Bipolar Operational Amplifier Comparison Chart