Copyright 2004 by Oxford University Press, Inc.

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1 JT as an Amplfer &a Swtch, Large Sgnal Operaton, Graphcal Analyss, JT at D, asng JT, Small Sgnal Operaton Model, Hybrd P-Model, TModel. Lecture # 7 1 Drecton of urrent Flow & Operaton for Amplfer Applcaton on ; on 0.5 Typcally, 0.7 on ; on 0.5 Typcally, 0.7 0n ; on 0.4 Typcally, 0.3 0n ; on 0.4 Typcally, 0.3 e S T e S T opyrght 2004 by Oxford Unersty Press, nc. 2 1

2 Large Sgnal qualent Model S e T S T e T Se 1 A T Se 1 A ro e S A T ro e S A T opyrght 2004 by Oxford Unersty Press, nc. 3 bers-moll Model D S ( e T 1) D S ( e T 1) T T S ( e 1) S ( e 1) D F S S S S F J Area J Area S D opyrght 2004 by Oxford Unersty Press, nc. 4 2

3 Operaton n Saturaton Mode on ; on 0.5 Typcally, n; on 0.4 Typcally, sat on ; on 0.5 Typcally, n; on 0.4 Typcally, sat sat forced forced eta forced means that the transstor s operatng n saturated mode. The rato of eta forward and the forced beta s called oerdre factor, the more transstor goes nto saturaton the oltage wll also be lower. F opyrght 2004 by Oxford Unersty Press, nc. 5 Transfer haracterstcs of Amplfer e O S T e S T e S T O The resstance has two functons, frstly, to establsh the desred D bas oltage at the collector and second conertng the current sgnal nto oltage. sat sat d A d O A (a) asc common-emtter amplfer crcut. (b) Transfer characterstc of the crcut n (a). The amplfer s based at a pont Q, and a small oltage sgnal s supermposed on the dc bas oltage. The resultng output sgnal o appears supermposed on the dc collector oltage. The ampltude of o s larger than that of by the oltage gan A. T opyrght 2004 by Oxford Unersty Press, nc. 6 3

4 xample 5.2 =1mA = A = -272 / o =1.36 =12m = m A? T ˆ A, 0 opyrght 2004 by Oxford Unersty Press, nc. 7 xercse 5.19 A T Negate Swng nput Ampltude A opyrght 2004 by Oxford Unersty Press, nc. 8 4

5 Graphcal Analyss 1 ths shows a lneae relatonshp between &. Ths lnear relatonshp can be represented by a straght lne. opyrght 2004 by Oxford Unersty Press, nc. 9 Graphcal Analyss 1 opyrght 2004 by Oxford Unersty Press, nc. 10 5

6 as Pont Locaton Graphcal determnaton of the sgnal components be, b, c, and ce when a sgnal component s supermposed on the dc oltage (see Fg. aboe). opyrght 2004 by Oxford Unersty Press, nc. 11 as Pont Locaton ffect of bas-pont locaton on allowable sgnal swng: Load-lne A results n bas pont Q A wth a correspondng whch s too close to and thus lmts the poste swng of. At the other extreme, load-lne results n an operatng pont too close to the saturaton regon, thus lmtng the negate swng of. opyrght 2004 by Oxford Unersty Press, nc. 12 6

7 xercse 5.20 opyrght 2004 by Oxford Unersty Press, nc. 13 Operaton as a Swtch To use t as swtch we utlze the cutoff and saturaton modes of operatons. A smple crcut used to llustrate the dfferent modes of operatons of the JT., 0.4 so J s reersed based as long as ths holds. ncreasng beyond saturaton but ( OS ) ( OS ) ( OS ) sat 0.3 ( OS ( OS ) ) when we assume sat dge of saturaton ( OS ) 0.7 (where dres the JT to edge of saturaton). ncrease the base current whch send JT deeper nto only changeslghtly, as t should be noted that n saturaton one can force JT to operate at desred, whch s called forced ( OS ) forced. sat oer dre factor opyrght 2004 by Oxford Unersty Press, nc. 14 7

8 xample 5.3 sat ( OS ) ( OS ) sat sat 0.2 mn sat oer dre factor (sat) =9.8mA (OS) = ma =1.96mA =2.2K opyrght 2004 by Oxford Unersty Press, nc. 15 xercse 5.21 opyrght 2004 by Oxford Unersty Press, nc. 16 8

9 9 xample 5.4 xample opyrght 2004 by Oxford Unersty Press, nc. 17 xample 5.5 xample opyrght 2004 by Oxford Unersty Press, nc. 18

10 xample 5.6 opyrght 2004 by Oxford Unersty Press, nc. 19 xercses 5.22, 5.23 & 5.24 opyrght 2004 by Oxford Unersty Press, nc

11 xample opyrght 2004 by Oxford Unersty Press, nc. 21 xercses 5.25 & 5.26 opyrght 2004 by Oxford Unersty Press, nc

12 xample 5.8 opyrght 2004 by Oxford Unersty Press, nc. 23 xercses ma ange of 0.3 ( ) opyrght 2004 by Oxford Unersty Press, nc

13 xample 5.9 opyrght 2004 by Oxford Unersty Press, nc. 25 xample ( 1 2) 1 [ /( 1)] opyrght 2004 by Oxford Unersty Press, nc

14 xercse 5.28 wrte n terms of % hange ma opyrght 2004 by Oxford Unersty Press, nc. 27 xample 5.11 opyrght 2004 by Oxford Unersty Press, nc

15 xercses 5.29 & 5.30 opyrght 2004 by Oxford Unersty Press, nc. 29 xample 5.12 opyrght 2004 by Oxford Unersty Press, nc

16 xercse 5.31 opyrght 2004 by Oxford Unersty Press, nc. 31 asng JT Amplfer rcuts Two obous schemes for basng the JT: (a) by fxng ; (b) by fxng. oth result n wde aratons n and hence n and therefore are consdered to be bad. Nether scheme s recommended. opyrght 2004 by Oxford Unersty Press, nc

17 asng JT Amplfer rcuts 1 [ /( 1)] lasscal basng for JTs usng a sngle power supply: (a) crcut; (b) crcut wth the oltage dder supplyng the base replaced wth ts Théenn equalent. The small changes n the are swept away by large alue of.also 1 opyrght 2004 by Oxford Unersty Press, nc. 33 xample 5.13 opyrght 2004 by Oxford Unersty Press, nc

18 xercse 5.32 opyrght 2004 by Oxford Unersty Press, nc. 35 asng JT Amplfer rcut wth 2 PS asng the JT usng two power supples. esstor s needed only f the sgnal s to be capactely coupled to the base. Otherwse, the base can be connected drectly to ground, or to a grounded sgnal source, resultng n almost total -ndependence of the bas current. opyrght 2004 by Oxford Unersty Press, nc

19 xercse 5.33 opyrght 2004 by Oxford Unersty Press, nc. 37 asng JT Amplfer rcut wth Feedback esstor (a) A common-emtter transstor amplfer based by a feedback resstor. (b) Analyss of the crcut n (a). opyrght 2004 by Oxford Unersty Press, nc

20 xercse 5.34 opyrght 2004 by Oxford Unersty Press, nc. 39 asng usng onstant urrent Source Ths has adantage that emtter current s ndependent of beta and base resstance, so base resstance can be made large to hae hgh nput resstance wth out affectng the bas stablty. t s more smplfed desgn. The crcut n (b) utlzes a matched transstors Q1 and Q2 (Q1 s connected as a dode), both transstors hae hgh beta alues, f we neglect both base current the current through Q1 s: ( ) F As both transstors hae same so ther collector current wll be equal; = F. y keepng greater than (- + ), we can guarantee that Q2 remans n acte regon and the current remans constant. (a) A JT based usng a constant-current source. (b) rcut for mplementng the current source. opyrght 2004 by Oxford Unersty Press, nc

21 xercse 5.35 opyrght 2004 by Oxford Unersty Press, nc. 41 Small Sgnal Operaton So we frst set be to zero, and followng alues of current and oltage are determned for D. e T S be ollector urrent & Transconductance e T S e e T be T S be T e ( be ) T S f we suppose e be be 1 T be c be T T g m be T ths s only aldf c 10m be (a) onceptual crcut to llustrate the operaton of the transstor as an amplfer. (b) The crcut of (a) wth the sgnal source be elmnated for dc (bas) analyss. opyrght 2004 by Oxford Unersty Press, nc

22 22 Transconductance Transconductance opyrght 2004 by Oxford Unersty Press, nc. 43 Lnear operaton of the transstor under the small-sgnal condton: A small sgnal be wth a trangular waeform s supermposed on the dc oltage. t ges rse to a collector sgnal current c, also of trangular waeform, supermposed on the dc current. Here, c = g m be, where g m s the slope of the cure at the bas pont Q. ase, mtter current and Gan ase, mtter current and Gan b be T c nput resstance lookng nto base and emtter s; Small sgnal 1 c c c e e b e e T e e be e e c c T b be r r r r r r r r r r ) ( ) ( 1 & as & We can fnd relatonshp between another alternate relatonshp resstance between base & emtter lookng nto emtter s; Small sgnal another alternate relatonshp opyrght 2004 by Oxford Unersty Press, nc. 44 m be be m be c be m c c g g A g oltage The smallsgnal

23 xercse 5.37 & 5.38 opyrght 2004 by Oxford Unersty Press, nc. 45 Hybrd-π Model The most wdely used model. Two slghtly dfferent ersons of the smplfed hybrd- model for the small-sgnal operaton of the JT. The equalent crcut n (a) represents the JT as a oltage-controlled current source (a transconductance amplfer), and that n (b) represents the JT as a current-controlled current source (a current amplfer). opyrght 2004 by Oxford Unersty Press, nc

24 The T Model Two slghtly dfferent ersons of what s known as the T model of the JT. The crcut n (a) s a oltage-controlled current source representaton and that n (b) s a current-controlled current source representaton. These models explctly show the emtter resstance r e rather than the base resstance r featured n the hybrd- model. opyrght 2004 by Oxford Unersty Press, nc. 47 xample 5.14 (a) crcut; (b) dc analyss; (c) small-sgnal model. opyrght 2004 by Oxford Unersty Press, nc

25 xample 5.15 opyrght 2004 by Oxford Unersty Press, nc. 49 xample 5.16 (a) crcut; (b) dc analyss; (c) small-sgnal model; (d) small-sgnal analyss performed drectly on the crcut. opyrght 2004 by Oxford Unersty Press, nc

26 xample 5.16 Dstorton n output sgnal due to transstor cutoff. Note that t s assumed that no dstorton due to the transstor nonlnear characterstcs s occurrng. nput and output waeforms for the crcut of Fg Obsere that ths amplfer s non-nertng, a property of the common-base confguraton. opyrght 2004 by Oxford Unersty Press, nc. 51 Small Sgnal Model to Account for arly ffect We hae seen that the collector current not only depend on but also on, so the dependence on can be modelled by assgnng a fnte output resstance to the controlled current source n the hybrd p model. ( A ro ) A g o mbe( o r ) The hybrd- small-sgnal model, n ts two ersons, wth the resstance r o ncluded. opyrght 2004 by Oxford Unersty Press, nc

27 xercse 5.40 opyrght 2004 by Oxford Unersty Press, nc