Lect_10 - ECE!442 Solid"State!Devices!&!Circuits

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1 Jose E. Schutt " Aine " ECE 442 ECE 442 Solid " State Devices & Circuits 10. Frequency Response of Amplifiers Jose E. Schutt-Aine Electrical & Computer Engineering University of Illinois jschutt@emlab.uiuc.edu
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2 Jose E. Schutt " Aine " ECE 442 Low-Pass Circuit In frequency domain: 1 1 i o V V j C R jC ! "# $ 1 11 io ov i VV VA j RC V j RC !! " % " " $$ 2 1 1 / v A j RC jf f ""
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3 Jose E. Schutt " Aine " ECE 442 2 11 22 f RC & &’ "" 2 time constant RC " " Low-Pass Circuit
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4 Jose E. Schutt " Aine " ECE 442 High-Pass Circuit 11 1 ii o V R V V R j C j RC !! "" $$ 2 1 1/ 1 2 o v i V A V jf f j fRC & " " " ( ( 2 1 2 f RC "
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5 Jose E. Schutt " Aine " ECE 442 If f 2 = 2f 1 , then f 2 is one octave above f 1 If f 2 = 10f 1 , then f 2 is one decade above f 1 22 2 10 11 # log 3.32log ff of octaves "" 2 10 1 # log f of decades f " 2 GHz is one octave above 1 GHz 10 GHz is one decade above 1 GHz Octave & Decade
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6 Jose E. Schutt " Aine " ECE 442 3dB points are points at which the magnitude is 1/ 2 20log(1.414) 3 dB A dB "" 1 2 oo VV j " $ that at midband frequencies. 3 dB Definition From which Power is halved. Voltage is scaled as:
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7 Jose E. Schutt " Aine " ECE 442 Amplifier has intrinsic gain A o 1 1/ hi jf f $ Low-pass characteristics is: High-pass characteristics is: Overall gain A(f) is: / lo lo jf f jf f $ /1 () 1 / 1 / lo o lo hi jf f A f A jf f jf f " # # $$ Gain
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8 Jose E. Schutt " Aine " ECE 442 At very high frequencies, STC goes as 2 20log 1 ( / ) o G !! " ( $ 20log( / ) 20 log( / ) oo G X where X ! ! ( " ( " ! Slope of curve is –20; so if X=1 ( ! =10 o ), decrease is –20 dB ! -20 dB/decade Gain
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