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MFETMirrSu10

# MFETMirrSu10 - c Copyright 2008 W Marshall Leach Jr...

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c ° Copyright 2008. W. Marshall Leach, Jr., Professor, Georgia Institute of Technology, School of Electrical and Computer Engineering. FET Current-Mirror Examples Common-Source Ampli fi er Figure 1 shows a common-source ampli fi er. The active device is M 1 . Its load consists of a current- mirror active load consisting of M 2 and M 3 . The current source I REF sets the drain current in M 3 which is mirrored into the drain of M 2 . Because the source-to-drain voltage of M 2 is larger than that of M 3 , the Early e ff ect causes the dc drain current in M 2 to be slightly larger than I REF . Figure 1: Common-source ampli fi er. The voltage V G is the dc component of the gate input to M 1 . It is a dc bias voltage which sets the dc drain current in M 1 . This current must be equal to the drain current in M 2 in order for the dc output voltage to be stable. In applications, V G would usually be set by feedback. It will be assumed that r 01 = in calculating i o ( sc ) but not in calculating r out . Note that the body e ff ect must be accounted for in M 1 . Assume that I D 1 = I D 2 = I D 3 = I REF . (a) Use the source equivalent circuit to calculate the short-circuit output current. Assume that r 01 = . i o ( sc ) = i 0 d 1 = i 0 s 1 = v i (1 + χ 1 ) r is 1 + R S r is 1 = r s 1 1 + χ 1 = 1 g m 1 (1 + χ 1 ) where g m 1 = 2 p K 1 I REF (b) Calculate the output resistance. Assume that r 01 < . r out = r id 1 k r 02 where r id 1 = r 01 μ 1 + R S r is 1 + R S = r 01 [1 + (1 + χ 1 ) g m 1 R S ] + R S 1

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r 01 = λ 1 + V DS 1 I REF r 02 = λ 1 + V DS 2 I REF (c) Calculate the output voltage and the voltage gain.
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MFETMirrSu10 - c Copyright 2008 W Marshall Leach Jr...

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