15V 10 k 250 F Vout Rsig = Rig = 200 k , 100 F S19 V. 10 M 100 F sig 1 mA 15V So

Question

15V 10 k 250 F Vout Rsig = Rig = 200 k , 100 F S19 V. 10 M 100 F sig 1 mA 15V

Explanation / Answer

. The gate (G) of the MOSFET plays a similar role to the base (B) of the BJT. Similarly, the drain (D) can be likened to the collector (C), and the source (S) to the emitter (E). As in the case of BJTs, the current directions and voltage polarities of one device are opposite to those of the other. In fact, the n- and the p-channel devices are said to be complementary to each other, and when they are fabricated on the same substrate, the resulting technology is referred to as complementary MOS, or CMOS for short. Unlike the BJT, which requires a base current to go on, the FET has iG = 0. Consequently, for FETs we have iS = iD. MOSFETs possess a fourth terminal called the body (B), which is internally tied to the substrate. Though this terminal is not used on purpose, it must be properly biased to prevent unintentional effects. In the n-channel MOSFET the body forms a pn junction both with the source and the drain regions, with B acting as the anode. To avoid inadvertently turning on either of these junctions, we must at all times hold B at the most negative voltage (MNV). Likewise, in the p-channel MOSFET, B acts as the cathode, so it must be held at the most positive voltage (MPV). When physically possible, B is tied to S, and the above constraints are satisfied automatically. When an n-channel MOSFET (nMOSFET) is biased in the pinchoff region, also called saturation region or active region, and characterized by the conditions vGS Vtn (1a) vDS vGS Vtn (1b) its drain current iD depends on the applied gate-source voltage vGS and the operating drain-source voltage vDS as iD = ( )( ) 2 1 2 n GS tn n DS k vV v + (2) where • kn is a scale factor known as the device transconductance parameter, in A/V2 • Vtn, known as the threshold voltage, is the value of vGS at which the MOSFET starts conducting • n, whose dimensions are V-1, is called the channel-length modulation parameter (n is the reciprocal of the Early voltage VAn of an npn BJT, or n = 1/VAn). For a low-power nMOSFET, kn is typically in the range of 102 µA/V2 , Vtn is in the range of 100 V, and n is on the order of 10-2 V-1. If Vtn > 0, the nMOSFET is said to be of the enhancement type; if Vtn < 0, it is said to be of the depletion type. Moreover, the extrapolated value of iD in the limit vDS 0 is iD = (kn/2)(vGS Vtn) 2 . The device transconductance parameter kn depends on device geometry as ' n n n n W k k L = (3) where • ' n k , also in A/V2 , is called the process transconductance parameter; • Wn and Ln, both in µm, are the channel width and channel length of the nMOSFET Similar considerations hold for the p-channel MOSFET (pMOSFET), provided we reverse all current directions and voltage polarities. Thus, the active-region conditions of Eq. (1) become, for a pMOSFET, vSG Vtp (4a) vSD vSG + Vtp (4b) Similarly, Eq. (2) is rephrased as iD = ( )( ) 2 1 2 p SG tp p SD k vV v + + (5)

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