When we consider amplifiers, we are usually dealing with signals that vary in time. Generally speaking, an amplifier takes a small time-dependent signal as input and usually transforms it into a larger replica of the signal at the output. In transistor amplifier design, first a DC bias point is usually established. This point consists of the DC voltages and currents that exist within the amplifier when no input signal is applied. When a signal is applied to the input, the internal voltage levels depart from their DC operating point. The variation from this DC level gives rise to the amplification process. Simple amplifiers can be made from a single bipolar junction transistor (BJT). To understand how to do this, we will first examine the DC characteristics of a BJT and then investigate how the change from these DC conditions gives rise to amplification. To implement these circuits, you will use the NPN 2N3904 BJT, which is a general purpose transistor.