The foundation of the book ¿Microelectronics¿ is the study of the most essential topics in electronics. The operational amplifiers which are one of the basic electronic devices are studied at the beginning of the book. Although its complex internal circuit incorporates 20 or more transistors, its terminal behavior is almost ideal. Therefore it is treated as a single circuit element for designing powerful circuits, without any knowledge of its internal construction. The circuit constructions, characterizations, operations and applications of the basic semiconductor devices, such as the diode, the MOS transistor, the bipolar transistor are described in the following chapters. This book also presents the MOS and bipolar transistors for designing the practical amplifier circuits which can be fabricated in integrated-circuit (IC) form. The important circuit building blocks for designing integrated circuit amplifiers are biasing the basic gain cell with passive and active loads, cascoding to transform the load resistance in order to achieve maximum signal gain, differential pair configuration, current mirrors, IC biasing, amplifier output stage.