This chapter reviews the two-port circuit equations, parameters, and equivalent circuits. The two-port circuit description naturally leads to the representation of the four ideal amplifier types, namely, ideal voltage, current, transresistance, and transconductance amplifiers. The chapter shows how a practical amplifier can be made to approach any of these ideal types by appropriate negative feedback connections. It discusses the ideal operational amplifier, in both the noninverting and inverting configurations, in terms of negative feedback and signal-flow diagrams. The relationships between the variables in a feedback system can be conveniently represented diagrammatically by means of a signal-flow diagram. If the sign of the loop gain is positive, the system is a positive feedback system, the disturbance will be reinforced if the magnitude of the loop gain exceeds unity, leading to instability.