Lect_10 - ECE 442 ECE 442 SolidState Devices & Circuits...

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CE 442 ECE 442 Solid State Devices & Circuits 10. Frequency Response of Amplifiers Jose E. Schutt-Aine lectrical & Computer Engineering Electrical & Computer Engineering University of Illinois jschutt@emlab.uiuc.edu ECE 442–Jose Schutt Aine 1
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Low-Pass Circuit In frequency domain: 1 1 i o V V j C R C  jC 1 11 io ov i VV VA j RC V j RC    1 / v A R C j f f  ECE 442–Jose Schutt Aine 2 2 jR
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Low-Pass Circuit 2 11 22 f RC   2t i m e c o n s t a n t RC ECE 442–Jose Schutt Aine 3
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High-Pass Circuit 11 ii o VR V V  / o v V A j f f 1 R jC jR C  2 1 1/ 1 2 i Vj j fRC 1 ECE 442–Jose Schutt Aine 4 2 2 f RC
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Octave & Decade 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 f 22 21 0 11 #l o g 3.32log ff of octaves 2 10 1 og f o fd ecades f GHz is one octave above 1 GHz 2 GHz is one octave above 1 GHz 10 GHz is one decade above 1 GHz ECE 442–Jose Schutt Aine 5
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3 dB Definition 3dB points are points at which the magnitude is 1/ 2 that at midband frequencies. ower is halved Voltage is scaled as: oo VV Power is halved. Voltage is scaled as: 1 2 j rom which 20log(1.414) 3 B Ad B  From which ECE 442–Jose Schutt Aine 6 dB
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Gain Amplifier has intrinsic gain A o Low-pass characteristics is: / 1/ lo lo jf f jf f 1 igh ass characteristics is: hi jf f High-pass characteristics is: Overall gain A(f) is: / 1 () lo o lo hi jf f Af A jf f jf f   ECE 442–Jose Schutt Aine 7
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t very high frequencies STC goes as Gain At very high frequencies, STC goes as 0l 1 ( / ) 2 20lo g1 ( / o G   20log( / ) 20 log( / ) oo GX w h e r e X  Slope of curve is –20; so if X=1 ( =10 o ), decrease is –20 dB -20 dB/decade ECE 442–Jose Schutt Aine 8
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Three Frequency Bands ECE 442–Jose Schutt Aine 9
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Model for general Amplifying Element C c1 and C c2 are coupling capacitors (large) F nd t re parasitic capacitors (small) F C in and C out are parasitic capacitors (small) pF ECE 442–Jose Schutt Aine 10
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Midband Frequencies - Coupling capacitors are short circuits - Parasitic capacitors are open circuits out in L MB g in out L vR R AA R R R   ECE 442–Jose Schutt Aine 11 in
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Low Frequency Model - Coupling capacitors are present - Parasitic capacitors are open circuits 1 1 1 1() in in in c in ab cg i n g in vR v j CR v j CR R RR C   1 c jC 1 () i n in ab in R R R vv R C R R  ECE 442–Jose Schutt Aine 12 1 gi n i n  
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Low Frequency Model   12 11 2 2 ll out c i n c define f and f R RC R    2 1 L gi 1 / 1/ in l ab in i n l Rj f f vv RR jf f  1 f 2 2 / , l L out ab L out l jf R Similarly v Av j f f  ECE 442–Jose Schutt Aine 13
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Low Frequency Model 2 // ut in l l vR jf f jf f R l l i A 12 1/ out L in g in L out l l Overall gain R R R j f f j f f   out l l vj f f j f f MB in l l A f f j f f
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Lect_10 - ECE 442 ECE 442 SolidState Devices & Circuits...

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