This chapter addresses the amplifier behavior in the time and frequency domains. The analysis of fundamental patterns in Chap. 3 and exercises in Chap. 4 were carried out by assuming that the by-pass and coupling capacitors treated as short circuits, and the parasitic capacitors as open-circuits. These assumptions are acceptable if the signal frequency is in the amplifier mid-band. In this chapter, attention will therefore be paid to the behavior in the frequency domain near the lower and upper cut-off frequencies, where the by-pass, coupling, and parasitic capacitors should be considered.