TABLE OF CONTENTS In the previous chapter we considered the MOS transistor with terminal voltages undergoing variations with time. No restrictions were placed on the magnitude of these variations. In this chapter we will consider the case where the terminal voltage variations are sufficiently small so that the resulting small current variations can be expressed in terms of them using linear relations. We will derive such linear relations and develop linear circuits to represent them. These circuits will be called small-signal equivalent circuits. When excited by voltages equal to the small variations of the actual terminal voltages, these circuits will produce currents equal to the small variations of the actual transistor currents. Such models find wide use in analog circuit design, and have been studied since the early days of MOS transistors. [1] [2] [3] [4] [5].
In most of this chapter, we will concentrate on the intrinsic part of the transistor (Fig. 7.1). We will first develop a small-signal model valid when the voltage and current variations are so slow that charge storage effects can be neglected. Then we will develop a small-signal model valid at medium speeds, assuming quasi-static operation. Such models [1]- [5], [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] are sufficient for many applications. (More...
