The MOS transistor as a small-signal amplifier

Under the usual conditions of operation the MOS transistor operates in saturation and is a square-law device: the characteristic curve of the drain current as a function ofthe gate voltage is parabolic in shape (see the preceding article [11, fig. 2b). However, if the MOS transistor is biased to bring the operating point on to the slope of the parabola, then the amplification will be practically linear for small signals, since a small portion of the slope of the parabola near the operating point approximates to a straight line. . The very high input impedance ofthe MOS transistor makes it an attractive device for use in amplifier circuits, especially in the input stages. It has applications both in the audio frequency range [21 and at high frequencies [31. For these applications it is necessary to know what gain the MOS transistor will give at these frequencies, and also its noise characteristics. This information can be obtained from a model of the MOS transistor in the form of a network of electrical elements (fig. 1). For a fairly wide range of frequencies the network can be greatly simplified to give a useful equivalent circuit. This can be used as the basis for designing the circuit in which the MOS transistor is to be included. For more general calculations of the maximum available gain of the MOS transistor, this equivalent circuit can be reduced to a representation of the MOS transistor as a linear four-terminal network (or two-port), characterized by four complex quantities. The general theoryof linear four-terminal networks can then be applied. The noise generated in aMOS transistor is mainly thermal noise, originating in the conducting channel. The magnitude ofthis noise can readily be derived from the elements of the equivalent circuit or from the equivalent four-terminal network. This is not the case for flicker noise, which is predominant at low frequencies and appears to be connected with the behaviour of the charge carriers at the interface between the silicon and the oxide surface layer. The level of the flicker noise is inversely proportional to the frequency, and it is therefore known as I/lnoise. We shall give an approximate expression for the magnitude of this III noise, which