The two main types of FETs are the junction field effect transistor (JFET) and the metal oxide field effect transistor (MOSFET).

Transcription

1 Mcrelectrncs Chapter three: Feld Effect Transstr sall snal analyss Intrductn: Feld-effect transstr aplfers prde an excellent ltae an wth the added feature f hh nput pedance. They are als lw-pwercnsuptn cnfuratns wth d frequency rane and nal sze and weht. Just as there are npn and pnp bplar transstrs there are n-channel and p-channel feld effect transstrs. Hweer t s prtant t keep n nd that the BJT transstr s a bplar dece the prefx b ndcates that the cnductn leel s a functn f tw chare carrers electrns and hles. The FET s a unplar dece dependn slely n ether electrn (n - channel) r hle (p -channel) cnductn. Whereas a BJT dece cntrls a lare utput (cllectr) current by eans f a relately sall nput (base) current the FET dece cntrls an utput (dran) current by eans f a sall nput (ate-ltae) ltae. In eneral therefre the BJT s a current-cntrlled dece and the FET s a ltae-cntrlled dece. In bth cases hweer nte that the utput current s the cntrlled arable. Because f the hh nput characterstc f FETs the ac equalent del s sewhat spler than that eplyed fr BJTs. Whereas the BJT has an aplfcatn factr β (beta) the FET has a transcnductance factr. The tw an types f FETs are the junctn feld effect transstr (JFET) and the etal xde feld effect transstr (MOSFET). Th-Qar Unersty/En. Cllae/BME. EPT. Lectures are prepared by Nadee Kharan

2 Mcrelectrncs 3.2 JFET Lw Frequency SMLL-SIGNL MOEL Befre exann the lw frequency sall snal del we recall the JFET paraeters. We knw that dran t surce current f JFET s cntrlled by ate t surce ltae. The chane n the dran current due t chane n ate t surce ltae can be deterned usn the transcnductance factr " ". It s en by: I GS ran current ( I fllws: ) can be deterned fr any GS f GS ( ff ) and I SS are knwn as I GS ISS (1 ) GS ( ff ) 2 Where: I SS s dran t surce current wth ate shrt GS ( ff ) s the cutff ltae that akes I apprxately zer. We knw that n BJT the relatn between an utput and nput quantty s en by aplfcatn factr β whereas n JFET ths relatn s en by transcnductance factr " ". The prefx trans - n the ternly appled t reeals that t establshes a relatnshp between an utput and an nput quantty. The rt wrd cnductance was chsen because s deterned by a current-t-ltae rat slar t the rat that defnes the cnductance f a resstr 1 I G. R In addtnal I can be calculated usn the fllwn frula: GS GS 0(1 ) GS ( ff ) When a alue f 0 s nt aalable yu can calculate t usn alues f I SS and GS ( ff ) Th-Qar Unersty/En. Cllae/BME. EPT. Lectures are prepared by Nadee Kharan

3 Mcrelectrncs 0 2I SS GS ( ff ) nther prtant paraeter f JFET s dran resstance "r d ". It s en by: S rd I It deternes the utput pedance f the JFET aplfer. JFET C Equalent Crcut Nw that the prtant paraeters f an ac equalent crcut hae been ntrduced and dscussed a del fr the JFET transstr n the ac dan can be cnstructed. The cntrl f I d by s s ncluded as a current surce s cnnected fr dran t surce as shwn n Fure 3.1. The current surce has ts arrw pntn fr dran t surce t establsh a 180 phase shft between utput and nput ltaes as wll ccur n actual peratn. Fure 3.1: Internal FET equalent crcut. Th-Qar Unersty/En. Cllae/BME. EPT. Lectures are prepared by Nadee Kharan

4 Mcrelectrncs In part (a) the nternal resstance r s appears between the ate and surce. ls the nternal dran t surse resstance r ds s ncluded. In part (b) a splfed deal del s shwn. The resstance rs s assued t be nfntely lare s that there s an pen crcut between the ate and surce. ls rds s assued lare enuh t nelect. JFET Cn Surce aplfer 1. JFET Fxed Bas cnfuratn The fxed-bas cnfuratn f fure 3.2 ncludes the cupln capactrs C 1 and C 2 whch slate the dc basn arraneent fr the appled snal and lad; they act as shrt crcut equalents fr the ac analyss. Fure 3.2: JFET fxed-bas cnfuratn. Once the leels f s deterned fr the dc basn arraneent specfcatn sheet r characterstcs the ac equalent del can be substtuted between the apprprate ternals as shwn n fure 3.3. Nte that bth capactrs hae the shrt-crcut equalent because the reactance X C 1 2fc s suffcently sall cpared t ther pedance leels f the netwrk and the dc batteres GG and are set t 0 by a shrt-crcut equalent. Th-Qar Unersty/En. Cllae/BME. EPT. Lectures are prepared by Nadee Kharan

5 Mcrelectrncs Fure 3.3: Substtutn the JFET ac equalent crcut unt nt the netwrk Nte the defned plarty f s whch defnes the drectn f s. If s s neate the drectn f the current surce reerses. The appled snal s represented by and the utput snal acrss R by Input pedance ( n ): n R G Output pedance ( ): R ltae an ( ): s R But s S that: R Th-Qar Unersty/En. Cllae/BME. EPT. Lectures are prepared by Nadee Kharan

6 Mcrelectrncs Phase Relatnshp The neate sn n the resultn equatn fr clearly reeals a phase shft f 180 between nput and utput ltaes. Exaple 3.1: fr the crcut shwn belw deterne )nput pedance ) utput pedance ) ltae an Slutn: R M ) 1 n G R k ) 5.1 ) R Th-Qar Unersty/En. Cllae/BME. EPT. Lectures are prepared by Nadee Kharan

7 Mcrelectrncs 2. JFET wth Self-Bas (Bypassed R S ): The fxed-bas cnfuratn has the dstnct dsadantae f requrn tw dc ltae surces. The self-bas cnfuratn f fure 3.4 requres nly ne dc supply t establsh the desred peratn pnt. Fure 3.4: Self-bas JFET cnfuratn. Under ac cndtns the capactr assues the shrt-crcut state and shrt crcuts the effects f R S. The JFET equalent crcut s establshed n Fure 3.5: 3.5: JFET ac equalent crcut. Input pedance ( n ): n R G Th-Qar Unersty/En. Cllae/BME. EPT. Lectures are prepared by Nadee Kharan

8 Mcrelectrncs Output pedance ( ): R ltae an ( ): R Phase Relatnshp The neate sn n the slutns fr aan ndcates a phase shft f 180 between and. Th-Qar Unersty/En. Cllae/BME. EPT. Lectures are prepared by Nadee Kharan

9 Mcrelectrncs. JFET wth Self-Bas (Unbypassed R S ) If C S s reed fr F 3.4 the resstr R S wll be part f the ac equalent crcut as shwn n F In ths case there s n bus way t reduce the netwrk t lwer ts leel f cplexty. In deternn the leels f and ne ust be ery careful wth ntatn and defned plartes and drectn. Intally the resstance r d wll be left ut f the analyss t fr a bass fr cparsn. Fure 3.6: Self-bas JFET cnfuratn ncludn the effects f R S Input pedance ( ): R G Output pedance ( ): pplyn Krchhff s current law results n Wth ( I I ) R s s I I I s S that I I ( I I ) R I R I R s s s I 1 R s I 1 R s nd I I (the cntrlled current surce s = 0 fr the appled cndtns) Th-Qar Unersty/En. Cllae/BME. EPT. Lectures are prepared by Nadee Kharan

10 Mcrelectrncs Snce I R Then ( I ) R I R I R ltae an ( ): Fr the netwrk f F. 3.6 applcatn f Krchhff s ltae law t the nput crcut results n 0 s R s R (1 R ) s s s s s Snce the utput ltae s en by: I R R s R 1 R S Phase Relatnshp The neate sn n last equatn aan reeals that a 180 phase shft wll exst between and. Exaple 3.2: fr the crcut shwn belw deterne ) nput pedance ) utput pedance ) ltae an take 1.58 S Th-Qar Unersty/En. Cllae/BME. EPT. Lectures are prepared by Nadee Kharan

11 Mcrelectrncs Slutn: R M ) 1 ) 2.2 ) G R k 3 3 R ( )(2.210 ) Rs 1 ( )(110 ) Th-Qar Unersty/En. Cllae/BME. EPT. Lectures are prepared by Nadee Kharan

12 Mcrelectrncs JFET SOURCE-FOLLOWER (COMMON-RIN) CONFIGURTION The JFET n fure 3.7 s the surce-fllwer cnfuratn. Nte that the utput s taken ff the surce ternal and when the dc supply s replaced by ts shrt-crcut equalent the dran s runded (hence the ternly cn-dran). Fure 3.7: JFET surce-fllwer cnfuratn. Substtutn the JFET equalent crcut results n the cnfuratn f F Fure 3.8: Netwrk f F. 3.7 fllwn the substtutn f the JFET ac equalent del. The cntrlled surce and the nternal utput pedance f the JFET are ted t rund at ne end and R S n the ther wth acrss R S. Snce s and R S are cnnected t the sae ternal and rund they can all be placed n parallel as shwn n F The current surce reersed drectn but s s stll defned between the ate and surce ternals. Th-Qar Unersty/En. Cllae/BME. EPT. Lectures are prepared by Nadee Kharan

13 Mcrelectrncs Fure 3.9 Netwrk f F. 3.8 redrawn. Input pedance ( ): R G Output pedance ( ): : Settn = 0 wll result n the ate ternal ben cnnected drectly t rund. The fact that s and are acrss the sae parallel netwrk results n s. I I I s s I s R s Snce s I R 1 I ( ) R s s 1 1 Rs // I 1 1 R 1 s Th-Qar Unersty/En. Cllae/BME. EPT. Lectures are prepared by Nadee Kharan

14 Mcrelectrncs Therefre R //(1/ ) s ltae an ( ) The utput ltae s deterned by s Rs nd applyn Krchhff s ltae law arund the pereter f the netwrk f F. 3.9 results n S that ( ) R s s R R s Or s s R R nd 1 S that s s Rs 1 R s Phase Relatnshp snce s a pste quantty; and are n phase fr the JFET surce-fllwer cnfuratn. Th-Qar Unersty/En. Cllae/BME. EPT. Lectures are prepared by Nadee Kharan

15 Mcrelectrncs Exaple 3.3: fr the crcut belw 2.5 S. Calculate and Slutn: R 1 M G 1 1 Rs // 3.3 k // S Rs R s Th-Qar Unersty/En. Cllae/BME. EPT. Lectures are prepared by Nadee Kharan

16 Mcrelectrncs JFET COMMON-GTE CONFIGURTION The last JFET cnfuratn t be analyzed n detal s the cn-ate cnfuratn f F FIG. 3.10: JFET cn-ate cnfuratn. Substtutn the JFET equalent crcut results n F Nte the cntnun requreent that the cntrlled surce s be cnnected fr dran t surce. The slatn between nput and utput crcuts has busly been lst snce the ate ternal s nw cnnected t the cn rund f the netwrk and the cntrlled current surce s cnnected drectly fr dran t surce. In addtn the resstr cnnected between nput ternals s n lner R G but the resstr R S cnnected fr surce t rund. Nte als the lcatn f the cntrlln ltae s and the fact that t appears drectly acrss the resstr R S. Fure 3.11: Netwrk f F fllwn substtutn f JFET ac equalent del Input pedance ( ): It s en by R // s 1 I s s R s // 1 Th-Qar Unersty/En. Cllae/BME. EPT. Lectures are prepared by Nadee Kharan

17 Mcrelectrncs Output pedance (): Substtutn = 0 n F wll shrtut the effects f R S and set s t 0. The result s s = 0. Therefre R ltae an ( ): Fure 3.11 reeals that s nd I R I s R S that I R R R We bsere that there are n phase shft between nput and utput n cn ate aplfer. Th-Qar Unersty/En. Cllae/BME. EPT. Lectures are prepared by Nadee Kharan

18 Mcrelectrncs Exaple 3.5: Fr the crcut belw 2.8 S. Calculate and Slutn: 1 1 Rs // 1 k // 1000// S 0 R 5.1 k R 2.8S 5.1 k Exaple 3.6: The crcut shwn belw has 2.5 L ltae an lare. S S. Calculate f the crcut. ssue all capactrs t be extreely Th-Qar Unersty/En. Cllae/BME. EPT. Lectures are prepared by Nadee Kharan

19 Mcrelectrncs Slutn: Fure belw shws the sall snal equalent crcut fr the aplfer shwn n fure abe. S S R s L Snce s R L RL ( RL // R ) S RG R r G s RG R r S G s ( RL // R ) RG 2.5 S (28 k // 3 k )1M R r 1M 50k S G s Th-Qar Unersty/En. Cllae/BME. EPT. Lectures are prepared by Nadee Kharan