Chapter 1 Small Signal Analysis

In various engineering problems, there can be a small amplitude AC signal accom-panied with a much larger amplitude DC signal. For example, the DC source (a battery) can be used to bias the transistors of a radio. In addition to the DC volt-ages, the signal received from the antenna having a range on the order of milivolts also appears in the system. The design goal of such a system can be the amplification of the signal of interest to a reasonable voltage level. In this section, we present the small signal approximation for resistive circuits. This topic is particularly important for the amplifier design applications. A very good engineering question is how we define the smallness. It should be remembered that, a small voltage for an high voltage engineer is any voltage less than 1000 V; while any voltage greater than 1 V can be beyond the reach of an electronic engineer. Therefore smallness and largeness is only applicable with in a context and we should always keep in mind that the adjectives such as small, large, fast, efficient are used for a comparison with something else. So whenever somebody tosses you an adjective, ask him or her with respect to what! For our purposes, a signal is considered to be small if the accompanying DC signal is at least an order of magnitude larger. This is just a rule of thumb. A small signal example can be given as vs(t) = 3 + 0:1 cos(!t) V. Figure 1.1 shows how this signal should like on an oscilloscope screen.