Is current gain generally significant in FET amplifiers? Why or why not? Substitute each capacitor with a

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1 FET Sall Snal Mdband Mdel Ntatn: C arables and quanttes are enerally desnated wth an uppercase subscrpt. AC arables and quanttes are enerally desnated wth a lwercase subscrpt. Phasr ntatn wll be used when phase shfts ccur. dentfy each equatn r quantty: A (3.1) Z n Z (3.) Z ut Z (3.3) where: s current an enerally snfcant n FET aplfers? Why r why nt? Fr the dband AC equalent crcut f a FET aplfer: Substtute each capactr wth a All pwer supply cnnectns are cnnected t Replace the FET wth Why s a ltae cntrlled current surce (CS) used n the FET AC del? (3.4) d s 008 Straneway, Lkken, Petersen, and Gassert pae 1

2 What s? Hw can t be deterned fr the FET characterstc cures? (3.5) d s d s (3.6) where t s understd that d and s are the f the phasr AC dran current and ate-t-surce ltae (f phase shfts are present, then full phasr ntatn shuld be used). Nte: s setes ndcated wth the sybl y fs n specfcatn sheets. Anther ethd t deterne s t utlze equatn: 1 P (3.7) Why s the derate taken n the next equatn? Explan each step that fllws: d 1 d 1 P P d (3.8) d d d du du nu dx dx d 1 d P 1 d P d n n1 (3.9) P P P P (3.10) 008 Straneway, Lkken, Petersen, and Gassert pae

3 Why s the neate sn n frnt requred? (Hnt: cnsder the sns f r P ) Nte: Often, the antudes f and P are used and the nus sn s drpped. Are the quanttes utsde f the parentheses n Eq. Errr! Reference surce nt fund. cnstants r arables fr a en FET? Explan. f the ate-t-surce ltae s zer, what s the alue f? 1 1 P P P (3.11) Nte: the sns f and P are always the sae. s transcnductance a cnstant wth respect t bas cndtns? es chane f the dran current chanes? es depend n the Q-pnt? s Q-pnt stablty crucal n FET aplfers? Hence, s a bas stablzatn ethd (such as self-bas) necessary? What s a typcal alue f the nput pedance f a FET? Hence, the nput pedance wll be apprxated n the AC del wth Ntes: The utput pedance s ften ~10 s f kl-hs, s practcally t can be nred. 1 n specfcatn sheets, the utput cnductance s usually en the sybl s r y s. s s Fure 3.1 AC del fr the FET 1 Check ths specfcatn fr the partcular FET n use t ay hae a alue that cannt be nred. 008 Straneway, Lkken, Petersen, and Gassert pae 3

4 Cn Surce Aplfers: Sall Snal Mdband Analyss Hw s the dband AC analyss f a FET aplfer perfred n eneral? Exaple 3.1 Gen: esred: Stratey: eterne the dband AC an, nput pedance, and utput pedance fr the JFET aplfer shwn n Fure 3.. The transstr specfcatns are en belw. 9A, P 5, ys(ax) 50 S dband AC an, nput pedance, and utput pedance (Fll n.) s Fure 3. FET crcut fr Exaple Straneway, Lkken, Petersen, and Gassert pae 4

5 s Fure 3.3 FET dband AC del fr Exaple 3.1 Slutn: (Explan each step.) P 5 R S R 1000 A R S B R S P P C P 1 B B 4A C A crtern: reject a slutn f: S1 1 R 1 1 R S B B 4A C A S R 1 R S R S S S ) - s reater than - ) - s reater than -P 3) - s reater than - R S transcnductance: P P 1 P Straneway, Lkken, Petersen, and Gassert pae 5

6 Bas cndtns: 4.47 A, 1.48, 9.05,.54 S S The axu utput cnductance s specfed t be 50 μs. Can t be nred? (justfy) A A Z s R R L RRL R R s L , ,000 R 500 k G Z R R 909 L eterne the ltae an f the surce resstr s nt bypassed by C S. Fure 3.4: AC equalent Crcut wth C S reed. 008 Straneway, Lkken, Petersen, and Gassert pae 6

7 R R L A R S (3.1) A R R L 1 R S (3.13) Hw des the surce-bypass capactr affect the an f a FET CS aplfer? 008 Straneway, Lkken, Petersen, and Gassert pae 7