Before revision: There are three main motivations for combining several transistors on one chip and in one package: to save circuit board space and to reduce the board production cost (only one component needs to be populated instead of several) to ensure closely matching parameters between the transistors (which is almost guaranteed when the transistors on one chip are manufactured simultaneously and subject to identical manufacturing process variations) to ensure a closely matching thermal drift of parameters between the transistors (which is achieved by having the transistors in extremely close proximity ) The matching parameters and thermal drift are crucial for various analogue circuits such as differential amplifiers, current mirrors, and log amplifiers.
After revision: There are three main motivations for combining several transistors on one chip and in one package: to on a common substrate, and) to save circuit board space and to reduce the board production cost (only one component needs to be populated instead of several ensure closely matching parameters between the transistors (which is almost guaranteed when the transistors on one chip are manufactured simultaneously and subject to identical manufacturing process variations) to ensure a closely matching thermal drift of parameters between the transistors (which is achieved by having the transistors in extremely close proximity ) The matching parameters and thermal drift are crucial for various analogue circuits such as differential amplifiers, current mirrors, and log amplifiers.