Common Emitter Configuration:

Transcription

1 Hw wll measure the cmmn emtter mde nput, utput & transfer charactarstcs f a n-p-n transstr? Sketch the crcut dagram and charactarstcs. mmn mtter nfguratn: Fr the cmmn-emtter cnfguratn f Fg. 7.21a, the nput termnals are the ase and emtter termnals, ut the utput set s nw the cllectr and emtter termnals. n addtn, the emtter termnal s nw cmmn etween the nput and utput prts f the amplfer. Susttutng the re equvalent crcut fr the npn transstr wll result n the cnfguratn f Fg Nte that the cntrlled-current surce s stll cnnected etween the cllectr and ase termnals and the dde etween the ase and

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3 Questn: Sketch the crcut dagram f a cmmn emmtter f npn transstr & hence explan the mechancs f current flw thrugh the dfferent parts. mtter current: The ave fgure shws a ased transstr the mnus (-) sgn represents free electrns n the emtter regn. The heavly dped emtter emts r njects free electrn n the ase when external vltage surce. frward ases the emtter ase junctn. y cnvectnal flw f current, the emtter current flws n the ppste drectn f electrn flw whch has shwn n the prevus crcut. ase urrent: When the frward as s appled t the emtter ase junctn the electrn n the emtter wll nt reach the ased regn untl the appled vltage vercmes the emtter ase ptental arrer.

4 Once when s the greater than the emtter ase arrer ptental electrns wll reach nt the ase regn. Theretcally these electrns can flw n ether r tw drectns. 1. They can flw t the left and ut f the ase thrugh t the pstve termnal f surce vltage. 2. The free electrns can flw nt the cllectr regn. llectr urrent: lmst all the electrns g nt the cllectr as shwn ave. Once they are n the cllectr, the free electrns flw thrugh the cllectr & thrugh untl they reach pstve termnal f cllectr supply. The cnventnal flw f current has shwn ave. Questn: Otan expressn fr current gan, vltage gan, pwer gan, nput resstance & utput resstance fr the cmmn emtter cnfguratn f a transstr. The current flwng etween emtter and cllectr f a transstr s much greater than that flwng etween ase and emtter. Thus a small ase current s cntrllng the emtter cllectr current. The rat f the tw currents, / s cnstant, prvded that the cllectr emtter vltage s cnstant. Therefre, f the ase current rses, s des cllectr current. Ths rat s the UNT GN f the transstr and s gven the syml h fe. farly lw gan transstr mght have a current gan f 20 t 50, whle a hgh gan type may have a gan f 300 t 800 r mre. The spread f values f fr any gven transstr s qute large, even n transstrs f the same type and atch. haracterstc curves (graphs) can e drawn t shw ther parameters f a transstr and are used th t detal the perfrmance f a partcular devce and as an ad t the desgn f amplfers.

5 ltage Gan: urrent Gan: result n : crcut wll t thenput - dvder rule pplyngthe current. t permt theapprxmatn ften tclse t s f The magntude v e e β and fr theapprxmatn r gves ) β(r ng susttut β β β

6 mmn-ase nput esstance : pply test current, wth lad resstr Present at utput.

7 Questn: What s D. lad lne? Hw wll draw a D. lad lne n the utput characterstcs curve f a transstr & mprtance. (sat) = /( + ) D Lad Lne (m) (ff) = The straght lne s knwn as the D lad lne ts sgnfcance s that regardless f the ehavr f the transstr, the cllectr current and the cllectr-emtter vltage must always le n the lad lne, depends ONLY n the, and (.e. The dc lad lne s a graph that represents all the pssle cmnatns f and fr a gven amplfer. Fr every pssle value f, and amplfer wll have a crrespndng value f.) t must e true at the same tme as the transstr characterstc. Slve tw cndtn usng smultaneus equatn graphcally Q-pnt!!

8 mprtance Of D. lad lne: Lad lne s mprtant tls n electrncs ecause t cntans every pssle peratng pnt fr the transstr crcut. Stated anther way when the ase resstance vares frm zer t nfnte, t causes t vary that makes & t vary ver entre ranges. f we plt the & a values fr every pssle value. We ll get the lad lne. Therefre the lad lne s a vsual summery f all the pssle transstr peratng pnts. Questn: What s peratng pnt? Hw t s determned? What s saturatn pnt & ut-ff pnt? xplan refly. The "Q pnt" fr a transstr amplfer crcut s the pnt alng ts peratng regn n a "quescent ", where n nput sgnal gets amplfed. The currents and vltages n an electrnc crcut when the nput sgnal s replaced y ts average value, s that all currents and vltages can e apprxmated y seres expansns arund ths pnt. ls knwn as Q pnt. When the sgnal f appled varatn f cllectr current and cllectr emtter vltage take place aut ths pnt that s : why t s called slent pnt. Saturatn Pnt: The pnt at whch the lad lne ntersects the characterstc curve near the cllectr current axs s referred t as the saturatn pnt. t ths pnt f tme, the current thrugh the transstr s maxmum and the vltage acrss cllectr s mnmum fr a gven value f lad. Therefre saturatn current fr the fxed as crcut, c (sat) =cc/c utff Pnt: The pnt where the lad lne ntersects the cutff regn f the cllectr curves s referred as the cutff pnt (.e. end f lad lne). t ths pnt, cllectr current s apprxmately zer and emtter s grunded fr fxed as crcut. Therefre, ce (cut) = c = cc

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