241 ANALO LTRONI I Lectures 2&3 ngle Transstor Amplfers R NORLAILI MOH NOH
3.3 Basc ngle-transstor Amplfer tages 3 dfferent confguratons : 1. ommon-emtter ommon-source Ib B R I d I c o R o gnal appled to : B Amplfed output taken from : 2
2. ommon-collector (emtter follower) ommon-dran (source follower) R B R L o R L o gnal appled to : B Amplfed output taken from : 3
3. ommon-base ommon-gate R R B o o gnal appled to : Amplfed output taken from : 4
ommon-emtter confguraton Ib (n) (p) B In forward actve : I cisexp Τ R I s c s v Ic I b I I exp βf βf Τ β o (n) F o IR c v I saturated current forward current gan R I exp s Τ When o approaches 0 (.e. ce approaches 0), the -B juncton becomes feedback and the devce enters saturaton. Once the transstor becomes saturated, the output voltage and collector current take on nearly constant values : o (sat) (sat) I c R I 5
o () ut-off regon (sat) (n) Forward-actve regon I b (p) Ic B o (n) at.regon 0.5 1 1.5 () R When o approaches 0 (.e. ce approaches 0), the -B juncton becomes feedback and the devce enters saturaton. Once the transstor becomes saturated, the output voltage and collector current take on nearly constant values : o (sat) (sat) I c R When 0, the transstor operates n the cut off state (as B- and B- junctons are both reverse bas) and no current flows other than leakage current I O. 6
In saturaton : c o I (sat) R (sat) fxed value o () (sat) ) ut-off regon Forward-actved regon at. regon 0.5 1 1.5 When s hgh and > o, the -B juncton s forward bas. Thus, devce s n the saturaton regon. As s hgh, I b s also hgh. Forward current gan β F I c /I b. Forward current gan β F reduces as transstor leaves the forward-actve regon of operaton and moves nto saturaton. () Ib (n) (p) B R I (n) c o In the forward-actve regon, small changes n the nput voltage can gve rse to large changes n the output voltage. The crcut thus provdes voltage gan. When 0, I b 0 and I c 0. I c R o and snce I c 0, o Ths s the devce operatng n the cut off regon.. 7
o () (sat) ut-off regon Forward-actve regon at.regon 0.5 1 1.5 o IR c R I exp s p Τ When, I When, I 0 c o c o () In the forward actve regon, v I cisexp Τ I I c b β F Ib (n) (p) B R Ic (n) o 8
mall-sgnal crcut of ommon-mtter I R (n) (p) B R Ic (n) o v R rπ g m v v 1 1 rο c Rc o vo Input resstance Transconductance, o v r π β o g m v gmv v 1 rπ 1 g o mv g 1 mv R g m v v v g m v 0 o R r 9
o v rπ v 1 g v m 1 rο Rc vo Ro t rο Rc v t Output resstance, Ro R o v t // r o R t v 0 10
Open-crcut or unloaded voltage gan, a v v gmv r v v o 0 //R 1 o o nce 1, then a v -g m r o // R. If R, then I 1 g r η lm a A A v m o T I T R I c dc collector current at the operatng pont T thermal voltage A arly voltage o η kt v gm v rπ v 1 1 q A rο Rc vo 11
lm a 1 v-gmr o- - R η A T represents the max. low-freq. voltage gan obtanable from the transstor. It s ndependent of the bas current for BJT and the magntude s 5000 for typcal npn devces. s/c current gan: a o gmv1 gmr v v π βο 1 o 0 rππ B o v v 1 r π gm1 r o R v o o example on pg. 178. 12
omparson between the MO and BJT small sgnal models : BJT MO 1 r π nput resstance nput resstance from to 2 g m of BJT s hgher than that of MO based wth the same current. Hence, hgh-gan amplfers wth BJTs are easer to obtan than wth MO. B r π v gmv r ο gs g v m gs ro 13
3.3.2 ommon-ource () confguraton R o I I I d I o o v g m ro R vo o () ut-off regon Actve regon 0,I d 0 cutoff operaton o -I d R d Trode regon t t 1 2 3 () 14
I I d R I o o o () t ut-off regon t 1 2 3 Actve regon () Trode regon > t, I d flows actve/sat. operaton o and > - t o I d R For actve devces, I kw 2 ov 2L 2 o kw ov R 2L kμ nox ov t t ov o 15
o () BJT ut-off regon o () MOFT ut-off regon Forward-actve regon Actve regon (sat) at.regon 0.5 1 1.5 () t t 1 2 3 Trode regon () o and < - t trode operaton I kw 2 2L 2 ( t ) 2L Output resstance s low and small-sgnal voltage gan drops dramatcally. The slope of ths transfer characterstc at any operatng pont s the small sgnal voltage gan at that pont. As slope for MO < slope for BJT, a v(mo) <a v(bjt) 16
R o I d I I o o v g m ro R vo o m m g v g v v v o 0 v R R t o v ro R t v 0 m o/c or unloaded voltage gan, ( o ) v g v r R a g r R o m v m o v v o 0. Hence, transconductance of the crcut, m transconductance of the transstor, g m v0 r o R g 0 m t R o 17 v t
a g ( r R ) v m o lm a -g v mr If R o s very large, R whch s the max. possble voltage gan of a one-stage amplfer. g m 2Iμ W nox L gm I r A o 1 λi I r 1 o I a I v I a 1 v I lm a A v-g mr o- For BJT, R T Therefore, max. voltage gan of MOFT s dependent on the dran current whereas max. voltage gan of BJT s ndependent of I. 18
For MO: g W m k L ( ) I kw 2L t ( ) 2 gm 2 2 I t r 1 A o λι Ι ( ) ov Hence, lm a -g r -g - R I t A 2 A v m o m O For BJT: lm Hence, R T 26m O 200 m a v_mo < a v_bjt a A v-gmr o- T (o example page 182) 19
3.3.3 ommon-base confguraton v B Rc vo B rπ gmv v 1 1 rο I/p sgnal appled to. O/p taken from. B ted to ac gnd. The hybrd-π model provdes an accurate representaton of the small-sgnal behavor of the transstor ndependent of the crcut confguraton. However, for the common-b, the hybrd-π model becomes tougher to analyze as the dependent current source s connected between the /p and o/p termnals. 20
To smplfy the analyss of a common-b (B) amplfer, use a T- model nstead. The T-model at low freq: re g v m 1 v 1 Hybrd-π π model at low freq: r r π e 1g r r β ο π g m m π r e r β 1 β α 1g r g 1 g π ο ο ο m π m m gm 1g βο βο m g m 21