This chapter focuses on transistors for amplifying small signals, with bipolar junction transistor (BJT) and Field-effect transistors amplifiers integrated together in the discussion as much as possible. The quiescent operating point is stabilized mainly by negative feedback in discrete amplifiers, and by some form of constant current source in Integrated Circuits (IC) amplifiers. Of particular interest in amplifiers is the bandwidth, which is the frequency range of signals that can be amplified with a variation of gain within 3 dB. The frequency response of dc amplifiers, including IC operational amplifiers, extends down to zero frequency. The cascode combining a metal–oxide–semiconductor field-effect transistors (MOSFET) and a BJT clearly has some advantages over an all-BJT or all-MOSFET amplifiers. MOSFET current mirrors are analogous to their BJT counterparts. Using a BJT for the second transistor, rather than a Gate Amplifier MOSFET, gives higher gain, wider bandwidth and a large output resistance.