Common Collector & Common Base Amplifier Circuits

Transcription

1 xprimnt (6): ommon ollctor & as Amplification ircuit xprimnt No. (6) ommon ollctor & ommon as Amplifir ircuits Study Objctiv: (1) To comput and masur th basic charactristics of & amplification. (2) To distguish btwn th & amplifirs. Introduction: 1. ommon as () Amplifir (A) asic knowldg of amplifir Th common-bas () amplifir provids high voltag ga with a maximum currnt ga of 1. Sc it has a low put rsistanc, th amplifir is th most appropriat typ for crta application whn sourc tnd to hav vry low-rsistanc outputs such as Antnnas and Transmission Ls. Th basic circuit of amplifir is shown Figur (1). As th bas is th common trmal btwn th V i and V O. this circuit is thus calld common-bas () amplifir. c i c c V V V Figur (1) () D bias dsign for amplifir Sc th abov Figur has two D powr supplis( V and V ), it is mor practical us th circuit Figur (2) which is has on D powr supply(v). Th bas is viwd as short-circuit for A signal. Th bas thus sms to b groundd for V i and V O. Th D bias is analyzd as follows: 2 V V 1 2 V V V I I I I V V V I 1

2 xprimnt (6): ommon ollctor & as Amplification ircuit 1 V I I V c 2 I V V 2 1 V Figur (2) () A analysis for amplifir Th circuit shown Figur (3a) can b rprsntd by th quivalnt circuit of Figur (3b). c i c o V r αi V V V (a) Figur (3) (b) From this circuit w can fd out that th amplifir has th followg charactristics: 1. Voltag Ga: Th voltag ga from mittr to collctor is dvlopd as follows (V =v, =v c ) Vo vc i Av V v ir sc 1, thn: Av r Notic that th givn xprssion is th sam for th amplifir. Howvr, thr is no phas vrsion from mittr to collctor. 2. Input sistanc: Th rsistanc appars at th mittr (put) is: V v i r r I i i Sc th put rsistanc of amplifir ( r ) is dpnds on th put currnt, i, r 26mv / I c 2

3 xprimnt (6): ommon ollctor & as Amplification ircuit th larg put signal suffrs from distortion gratr than th small signal. This is th rason that amplifir oprats only on th small signal and known as a small signal amplifir 3. Output sistanc: Lookg to th collctor (output), th output rsistr is simply th collctor rsistor: o 4. urrnt Ga: Th currnt ga is th output currnt dividd by th put currnt. i c is th ac output currnt, and i is th ac put currnt. Scic i, th currnt ga is approximatly 1. A 1 i 2. ommon ollctor () Amplifir (A) asic knowldg of amplifir ommon-collctor () or mittr followr amplifir is an important small signal amplifir configuration. It is vry usful on bcaus it has vry high put rsistanc, high currnt ga, vry small output rsistanc, and approximatly unity voltag ga. Th high put rsistanc and low output rsistanc mak th mittr followr an idal buffr btwn a high impdanc sourc and a low impdanc load. A buffr is any circuit that kps th sourc from bg affctd by a load. amplifir is shown Figur(4), you can sn that th put is from th bas and th output from mittr as wll as th V will b short-circuit with rspct to A signal ( th quivalnt circuit), is th common trmal btwn V and. In othr words, is groundd. V V I I V c c V V I Figur(4) Figur(5) () D bias dsign for amplifir Th mittr-fdback bias circuit is shown Fig. 2. whr V I V I I I I V (1 ) I V V ( 1 ) I I (1 ) I I and V I 3

4 xprimnt (6): ommon ollctor & as Amplification ircuit () A analysis for amplifir Th followg small-signal A quivalnt circuit (Figur(6b)) can b usd to calculat th voltag ga, put rsistanc, and output rsistanc of th commoncollctor amplifir of Figur(6a). V V S s S Vs βr r ßib βi b c c i V (a) Figur (6) (b) 1. Voltag Ga: V i o V ( ) i r Vo i Av V i ( r ) r If >>r, thn a good approximation is: A v 1 Sc th output voltag is at th mittr, it is phas with th bas voltag, so thr is no vrsion from put to output. caus thr is no vrsion and bcaus th voltag ga is approximatly 1, th output voltag closly follows th put voltag both phas and amplitud; thus th trm mittr-followr. 2. Input sistanc: V ib r ib I i ( r ) If >>r, thn: b 3. Output sistanc: Th output rsistanc, lookg to mittr of, is approximatd as follows: S o ( r ) \ \ r Whr S is th rsistanc of put sourc. 4. urrnt Ga: I i o Ai I i b 4

5 xprimnt (6): ommon ollctor & as Amplification ircuit amplifir is not applicabl voltag amplification, but is suitabl for impdanc matchg. ommon-mittr, ommon-collctor, and ommon-bas amplifir configuration ar summarizd Tabl -1. Tabl -1: lativ omparison of Amplifir onfigurations Voltag Ga, A V urrnt Ga, A i Powr Ga, A p Input sistanc, Stag Output sistanc, o c /r β Vry A i A v Low βr r / c Low 1 β A i β Vry Low r c /r Low 1 A v Vry Low r r c Phas Angl 180 o 0 o 0 o 5

6 xprimnt (6): ommon ollctor & as Amplification ircuit xprimnt quipmnts: (1) KL-200 Lar ircuit Lab. (2) xprimnt Modul: KL (3) xprimnt Instrumnt: 1. Multimtr or digital multimtr. 2.Two channl Oscilloscop. (4) Matrials: As dicatd th KL xprimnt Itms: Itm On (1) xprimnt for amplifir. xprimnt Procdurs: (1) Insrt th short-circuit clips by rfrrg to Figur(7), onnct th D +12V without connctg put signal (Us transistor 2N3904 ). (2) Adjust V2 (10kΩ), so that V = 1/2V, thn rcord V, I, I and I as shown tabl (2a) (3) onnct signal gnrator to th put trmal V (IN) and (hannl 1) oscilloscop (D position. Thn connct (hannl 2) oscilloscop (D position) to th output trmal (OUT). (4) Adjust th signal gnrator frquncy to l khz and Thn adjust signal gnrator such that th oscilloscop can display maximum non-distortd output viw V and, ( apply Vs = 300mV P-P,350 mv P-P,400 mv P-P,410 mv P-P,420 mv P-P,450 mv P-P, 500 mv P-P,800 mv P-P ). s th output (Vo) at ach valu. (5) Us oscilloscop to masur v, v a and v o rspctivly, thn calculat A v as shown tabl (2b) (6) Mov th Q pot location up and down till th output is distortd and sktch th output ach cas. Va Vi Figur(7) Tabl -2: (a) D rsults V V I I I β 6V V (D) 6V v S (p-p) V a(p-p) v (p-p) v o(p-p) A v (b) A rsults Phas shift btwn Vi and Vo 6

7 xprimnt (6): ommon ollctor & as Amplification ircuit Itm Two (2): xprimnt for amplifir xprimnt Procdurs: (1) Insrt th short-circuit clips by rfrrg to Figur(8), onnct th D +12V without connctg put signal (Us transistor 2N3904). (2) Adjust V2 (10kΩ), so that V = 1/2V, thn rcord V, I and I as shown tabl (3 ) (3) onnct signal gnrator to th put trmal V (IN) and (hannl 1) oscilloscop (D position. Thn connct (hannl 2) oscilloscop (D position) to th output trmal (OUT). Thn adjust signal gnrator such that th oscilloscop can display maximum non-distortd output, ( apply Vs = 2V P-P,4V P-P, 8 V P-P,9V P-P,10V P-P,12V P-P, 20V P-P ). s th output (Vo) at ach valu. Sktch V and at maximum non-distortd output voltag. (4) Us th oscilloscop to masur v a(p-p), v (p-p) and v o(p-p) as tabl (3b) (5) Mov th Q pot location up and down till th output is distortd and sktch th output ach cas 7

8 xprimnt (6): ommon ollctor & as Amplification ircuit xprimnt sults: (a)tabulat your rsults as shown Tabl (3a,b). (b)graphs of stps 3 and 5. Tabl -3: (a) D rsults V V I I I β 6V V = 6V V S(p-p) v a(p-p) v (p-p) v o(p-p) i (p-p) i b(p-p) A v A i A p i vo, v vo AV, (b) A rsults va v v ib,, i b i Ai, AP AV Ai. i b 8

9 xprimnt (6): ommon ollctor & as Amplification ircuit Discussion: 1. Why th buffr is usd to match btwn high rsistanc sourc and low rsistanc load?. xpla why w can us as a buffr. 2. xpla why th known as small signal amplifir? 3. Match th followg gnralizd charactristic with th appropriat (.,.,.) amplifir configuration: a. Good currnt ga, unity voltag ga, high put rsistanc. b. Good currnt ga, good voltag ga, low put rsistanc. c. Unity currnt ga, good voltag ga, low put rsistanc. 4. xpla why th amplifir is calld mittr-followr?. 5. hoos th corrct answr: (a) A small signal Amplifir 1. Gos to saturation onc on ach put cycl. 2. Uss only a small portion of its load l. 3. Is always a common-mittr amplifir. (b) Among all configurations of th transistor amplifirs, which on has th largst of voltag ga and currnt ga? 1. configuration. 2. configuration. 3. configuration. (c) In a crta mittr-followr circuit, th currnt ga is 50. Th powr ga is AV Answrs 1 and 2. (d) Th ma disadvantag of th amplifir compard to th and amplifir 1. has low put rsistanc. 2. its powr ga A V. 3. has low currnt ga. () If on stag of amplifir is connctd btwn two stags of amplifirs, what is th function of this amplifir? 1. Voltag amplification. 2. Impdanc matchg. 3. urrnt amplification. 6. Writ a conclusion for this xprimnt. 9