Electronic Circuits. BJT Amplifiers. Manar Mohaisen Office: F208 Department of EECE

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Tamsin Jody Watts
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1 Elctronic Circuits BJT mplifirs Manar Mohaisn Offic: F208 Dpartmnt of EECE

2 viw of th Prcdnt Lctur Explain th DC Oprating Point Explain th Voltag-dividr Bias Othr Bias Mthods Discussions

3 Class Objctivs Explain th mplifir Opration Explain th BJT C Modls Th Common-mittr mplifir Th common-bas amplifir Th common-collctor amplifir Multi-stag amplifirs

4 mplifir Opration Quantitis DC Quantitis Upprcas italic lttrs Upprcas subscripts I B, V C C Quantitis Upprcas italic lttrs Upprcas subscripts I b, V Instantanous Quantitis Upprcas italic lttrs Upprcas subscripts b i, v c sistancs C DC sistanc, c C rsistanc, r intrnal ac rsitanc.

5 mplifir Opration contd. Th Linar mplifir Coupling capacitors block th DC quantitis. Prvnts th chang of th DC bias voltags at th bas and collctor. Collctor voltag is invrsly proportional to I c. It has a 180 phas diffrnc with I c.

6 mplifir Opration contd. Graphical prsntation C load lin. Diffrnc bt. DC and C load lin will b rvisitd in Chaptr 7.

7 mplifir Opration contd. Exampl 6.1

8 Transistor C Modls contd. r Param. r Paramtrs α ac β ac Dscription ac alpha ac bta r ac mittr rsistanc r b ac bas rsistanc r c ac collctor rsistanc r b (small) can b nglctd (short) r c (high) can b nglctd (opn)

9 Transistor C Modls contd. Dtrmining r by a Formula r 25 mv I E Not that Emittr currnt changs with tmpratur. lso, in th drivation, it was considrd that thr is an abrupt junction btwn th n and p rgions. Howvr, this approximation is valid for analysis.

10 Transistor C Modls contd. Comparison btwn th C Bta and th DC Bta

11 Transistor C Modls contd. h (hybrid) Paramtrs Supplid by th manufacturr. H Paramtr Dscription Condition h i Input impdanc (rsistanc) Output shortd h r Voltag fdback ratio Input opn h f Forward currnt gain Output shortd h o Output admittanc (conductanc) Input opn Configuration Dscription Common-mittr h i, h r, h f, h o Common-bas h ib, h rb, h fb, h ob Common-collctor h ic, h rc, h fc, h oc α h ac β r ac h h fb hf r o r 1 c h + ho r r h r i h (1 + h ) b h f o

12 Th Common-mittr mplifir mplifir DC nalysis & C nalysis Common-mittr rfrs to th C opration.

13 DC nalysis Q-point > 10 2, so it is nglctd. Th Common-mittr mplifir contd. β (150)(560 Ω ) 84 kω IN(BSE) DC E V B 12V 2.83 V k V Ω CC kΩ I E V V V 2.83V 0.7V 3.80m E B BE E E 560Ω V V I 8.2V C CC C C V V V 6.07V CE C E

14 C nalysis C1, C2, and C3 ar rplacd by shorts. Th Common-mittr mplifir contd. Thir valus ar slctd so that thir impdancs ar ngligibl. Th DC sourc is rplacd with ground.

15 Th Common-mittr mplifir contd. C nalysis Th ac input rsistanc is sn in paralll with 1 & 2. in( tot) 1 2 in( bas) V b in( tot) in( tot) + s V s

16 Input sistanc looking in at th Bas Th Common-mittr mplifir contd. in( bas) V in Iin Ir b b β I / β ac V I ac r Output sistanc looking in at th collctor out r C c C

17 Exampl 6-3 Find V b. I E 3.8 m. Th Common-mittr mplifir contd. r 25 mv 25 mv 6.58 Ω I 3.8m E β 160(6.58 ) 1.05k in( bas) ac r Ω Ω 873 Ω in( tot) 1 2 in( bas) V b mV(rms) 7.44 mv(rms) in( tot) Vs s in( tot)

18 C Voltag Gain atio btwn Output Voltag and Input Voltag. Th Common-mittr mplifir contd. V I I c c C C C v V b Ir Ir r If you would lik to gt th ovrall amplifir voltag gain v V V V V V c b Vc in( tot) s s b + in( tot) s v

19 Th Common-mittr mplifir contd. Effct of th Emittr Bypass Capacitor On Voltag Gain To obtain a short circuit by th capacitor C2 X C 2 10 E Exampl 6-4 Opration frquncy rang: 200 Hz ~ 10 khz X 56 C 2 10 E Ω C μf π fx 2 C 2

20 Voltag Gain without th Bypass Capacitor Th Common-mittr mplifir contd. v r C + E Exampl 6-5 r 6.58 Ω Gain without Bypass Capacitor v 1000 C r E 1.76 Gain with Bypass Capacitor v r 6.58 C

21 Effct of Load on th Voltag Gain In prsnc of a load Th load is connctd in Paralll with C. Thrfor, Th Common-mittr mplifir contd. v r c C L r

22 Effct of Load on th Voltag Gain contd. Exampl 6-6 L 5 kω, r 6.58 Ω, C 1 k Ω In Prsnc of Load Without Load Th Common-mittr mplifir contd. v 833 Ω c C L r r 6.58 Ω v c C Ω 152 r r 6.58 Ω

23 Stability of th Voltag Gain Th Common-mittr mplifir contd. Stability is a masur of how wll an amplifir maintains its dsign valus for a chang in tmpratur and β. Swamping r duc its ffct. v r E 1 c + E 1 c, for > 10 r E 1 Both E1 and E2 affct th DC bias Only E1 affcts th C voltag gain.

24 Th Common-mittr mplifir contd. Th Effct of Swamping on th mplifir s Input sistanc β r ( ) ( ) ac + in bas E1

25 Currnt Gain Currnt gain from bas to collctor Th ovrall currnt Gain Th Common-mittr mplifir contd. β ac I I c b i I I c s with I s s Vs + in( tot) Powr Gain p v i with c v Vs V

26 Th Common-mittr mplifir contd. Exampl 6-8 DC analysis C analysis

27 Exampl 6-8 contd. DC nalysis > 10 2, so th amplifir is stiff. Th Common-mittr mplifir contd. β + Ω Ω ( ) 150(940 ) 141 k IN(BSE) DC E1 E2 V B 2 V CC V V V V V E B BE I E V 1.05 V 1.12 m E + E1 E2 940Ω V V I 10V (1.12m)(4.7k Ω ) 4.74V C CC C C V V V V CE C E

28 Exampl 6-8 contd. C nalysis Th Common-mittr mplifir contd. r 25 mv 25 mv 22 Ω I 1.12m E in( bas) Ω Ω β ac ( r + ) 175(492 ) 86.1k E k Ω in( tot) 1 2 in( bas) ( ) attnuation V s + s in tot c 4.27kΩ V 7530 C L v c c 9.09 r + E1 b E1 in( tot) V b 1 v v V 1.08 s V V (8.45)(10mV) 84.5 mv c v s

29 Exampl 6-8 contd. DC & C nalysis Th Common-mittr mplifir contd. Max V V V (84.5 mv)(1.414) 4.86 V cp ( ) cp ( ) C Min V V V 4.74 (84.5 mv)(1.414) 4.62 V C c c V V V 119 mv out( p) c( p) C

30 Th Common-collctor mplifir Emittr-followr mplifir Common-collctor amplifir

31 Voltag Gain Th Common-collctor mplifir contd. v V I V I r out ( in + ) 1 ( r + ) Not Sinc th output follows th input in both phas and amplitud, Th amplifir is rfrrd to as mittr-followr.

32 Input sistanc Th Common-collctor mplifir contd. in( bas) V V I r in b ( + ) Iin I I b b β ( aci r + ) b β ( ac r + ) Ib Thrfor, in( tot) 1 2 in( bas) Output sistanc Without load. out β s ac E

33 Currnt Gain Th Common-collctor mplifir contd. I I i I V / in in in( tot) Powr Gain p v i i

34 Th Darlington Pair Th Common-collctor mplifir contd. I β I 1 ac1 b1 I β I β β I 2 ac2 b2 ac1 ac2 b1 Ovrall currnt gain β β β ac1 ac2 β β in ac1 ac2 E

35 n pplication Th Common-collctor mplifir contd. Emittr-followr is usd as intrfac bt. circuit with high output rsistanc and low-rsistanc load. Exampl Considr a common-mittr amplifir with C 1 kω, L 8 Ω Th loss in th voltag gain du to th load is high. W.o. load v C r 5 w. load c C L 7.94 v 1.59 r r 5 Th solution is by using th Darlington pair.

36 n pplication Intrfac th amplifir and th spakr. Th Common-collctor mplifir contd.

37 Exampl 6-10 Common-mittr: Th Common-collctor mplifir contd. r 5, VCC 12 V, C 1 k Darlington mittr-followr: 1 10 kω, 2 22 kω, E 22Ω, L 8Ω, V CC 12V, β ac β DC 100. V B β 20 12V 8.0V 2 2 DC E V β 2 CC DC E I E V V 2V 6.6V 300 m E B BE E E 22Ω r 25mV 25mV 83m Ω I 300m E

38 Th Common-collctor mplifir contd. Exampl 6-10 contd. Th load of th CE amplifir is in(tot) of th EF. in( tot) 1 2 in( bas) β 2 ( ) 1 2 ac r k Ω 22k Ω 100 (83mΩ Ω ) 860Ω Th voltag gain of th common-mittr amplifir r 5 Th voltag gain of th Darlington EF 5.87 v 0.99 r + 83 mω Th ovrall voltag gain v (0.99)(172) 170 v(ef) v(ce) v c

39 Th Common-collctor mplifir contd. Th Sziklai Pair

40 Th Common-bas mplifir Voltag Gain Notic that thr is no phas invrsion bt. output and input. v V I V I r r out c c c ( ) ( in ) E E

41 Th Common-bas mplifir contd. Input sistanc Th rsistanc, looking in at th mittr ( in V I r ) E V r in( mittr) I I I Output sistanc Th rsistanc, looking in at th collctor in ( ) E Currnt Gain r out c C C I out Ic i 1 I I in

42 Multistag mplifirs Multistag Voltag Gain L v v1 v2 v3 vn

43 Multistag mplifirs contd. Capacitivly-coupld Multistag mplifir 1.63k Ω c in( bas2) I r 25mV Ω r Ω 1.05m, E 23.8, and β 3.57k in( bas2) ac1 IE

44 Multistag mplifirs contd. Capacitivly-coupld Multistag mplifir 68.5 c1 v1 r 4.7kΩ Ω 7 v2 r Th ovrall Voltag Gain (68.5)(197) 13, v v v

45 Multistag mplifirs contd. Dirct-coupld Multistag mplifir dvantags No coupling capacitors Can amplify low frquncis Down to dc (0 Hz). Disadvantag ffctd by th changs du to tmpratur.

46 Class Summary Explaind th mplifir Opration Explaind th BJT C Modls Introducd th Common-mittr mplifir Introducd th common-bas amplifir Introducd th common-collctor amplifir Discussd th opration of multi-stag amplifirs

47 Discussion & Nots K K K K K K K K K K K K

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