# Lect_13 - #\$,0!1(2/3*0 456(77,2,8!9(7,20 Jose E Schutt-Aine...

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1 !"#\$ %& ’()*++ " ,-.\$ " %/% 001 !"! ##\$ %&’() " %*+*, -,.(/,0 1 "(2/3(*0 456 -(77,2,8*(+’ 9:;’(7(,20 Jose E. Schutt-Aine Electrical & Computer Engineering University of Illinois [email protected]

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2 !"#\$ %& ’()*++ " ,-.\$ " %/% 001 Background Differential Amplifiers – The input stage of every op amp is a differential amplifier – Immunity to temperature effects – Ability to amplify dc signals – Well-suited for IC fabrication because – (a) they depend on matching of elements – (b) they use more components – Less sensitive to noise and interference – Enable to bias amplifier and connect to other stage without the use of coupling capacitors
3 !"#\$ %& ’()*++ " ,-.\$ " %/% 001 Differential Amplifiers Practical Considerations – Both inputs to a differential amplifier may have different voltages applied to them – In the ideal situation with perfectly symmetric stages, the common-mode input would lead to zero output – Temperature drifts in each stage are often common-mode signals – Power supply noise is a common-mode signal and has little effect on the output signal

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4 !"#\$ %& ’()*++ " ,-.\$ " %/% 001 MOS Differential Pair Assume current source is ideal v ID =v gs1 -v gs2 Output is collected as v D2 -v D1
5 !"#\$ %& ’()*++ " ,-.\$ " %/% 001 - If v ID is positive, v D2 -v D1 is positive v ID >0 ! v gs1 >v gs2 I D1 > I D2 v D1 lower voltage point than v D2 MOS Differential Pair For proper operation, MOSFETS should not enter triode region

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6 !"#\$ %& ’()*++ " ,-.\$ " %/% 001 DC Analysis 1 2 D D DD IR VV !" 2 2 D D DD IR #\$ 2 2 ox D GS T CW I V V L % 2 D I I ! GS T ox LI !& SQ T ox LI ( ! " & ) * + ,
7 !"#\$ %& ’()*++ " ,-.\$ " %/% 001 Incremental Analysis ' 1 2 in o m D v v g R !" Neglecting the body effect ' 2 2 in o m D v v g R ! ' D D out R Rr ! ! ' 21 oo D mD in vv A g R v " !! 1 1 2 !& g cm id v v v 2 1 2 g cm id v v v

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8 !"#\$ %& ’()*++ " ,-.\$ " %/% 001 ' 1 2 high out D f CR - ! Frequency Response When driven by a low-impedance signal source, the upper corner frequency is determined by the output circuit