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isources - c Copyright 2009 W Marshall Leach Jr Professor...

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c ° Copyright 2009. W. Marshall Leach, Jr., Professor, Georgia Institute of Technology, School of Electrical and Computer Engineering. Current Sources Current mirrors are commonly used for current sources in integrated circuit design. This section covers other current sources that are often seen. FET Current Sources Figure 1(a) shows two FET current sources, one which uses an n-channel depletion mode MOSFET and the other which uses an n-channel JFET. The equivalent p-channel sources are shown in Fig. 1(b). Remember that the JFET is a depletion mode device. The analysis for the two sources is the same with the exception that the transconductance parameter is denoted by K for the MOSFET and by β for the JFET. Figure 1: MOSFET (depletion mode) and JFET current sources. (a) n-channel. (b) p-channel. For the n-channel device, the MOSFET drain current and gate-source voltage are given by I D = K ( V GS V TO ) 2 V GS = I S R S = I D R S The object is to solve for R S for a desired drain current. When the equation for V GS is substituted into the equation for I D , we obtain I D = K ( I D R S V TO ) 2 = K ( I D R S + V T O ) 2 This can be solved for R S to obtain R S = p I D /K V TO I D where V TO < 0 . Note that K = K 0 (1 + λV DS ) . If V DS is not speci fi ed, an often used approximation is K ' K 0 . For the n-channel JFET, the drain current is given by I D = β ( V GS V TO ) 2 It follows that the MOSFET solution for R S can be used with the substitution of β for K to obtain R S = p I D V TO I D 1
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The output resistance is a fi gure of merit for a current source. Ideally, it should be in fi nite. The output resistance is the resistance seen looking into the drain of each source. It is given by r out = r id = r 0 μ 1 + R S r s + R S r s = 1 g m r 0 = λ 1 + V DS I D where g m = 2 KI D for the MOSFET and g m = 2 βI D for the JFET.
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