General-Purpose Operational Amplifiers Bipolar

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