To determine the biasing conditions needed to obtain a specific gain each stage must be considered.

Transcription

1 PHYSIS 56 Experiment 9: ommon Emitter Amplifier A. Introdution A ommon-emitter oltage amplifier will be tudied in thi experiment. You will inetigate the fator that ontrol the midfrequeny gain and the low-and high-break frequenie. Although a ommon-emitter amplifier i in priniple a imple deie it neerthele utilize a number of direte omponent for proper operation. Below i a ummary of the indiidual omponent and their purpoe, and the ymbol onention. ) i the output reitane of ignal oure. ) i a oupling apaitor whih pae A ignal from the oure to amplifier input but blok D offet from the oure o that it doe not affet the quieent ondition of the tranitor. ) i a oupling apaitor, whih pae the amplified A ignal while preenting oillation in the external load ( ) from affeting the D ondition. 4) fixe the D potential at Bae. The oltage drop aro uually will be extremely mall for a FET, but an be ubtantial for a BJT. The polarity of bae oltage (I b ) will be the ame a the bia oltage V n. 5) determine the quieent urrent flowing through the tranitor. 6) i a by-pa apaitor. It at like a wire to onnet emitter to ommon for all frequenie of interet. 7) i load reitor, whih onnet olletor to power upply o that an output oltage an be reated by hanging the urrent through the tranitor. To determine the biaing ondition needed to obtain a peifi gain eah tage mut be onidered. Step : A tranitor an be repreented a an ideal urrent oure with an equialent P i i i r o i r r o i o i reitor r in parallel. i the olletor reitor for a BJT and the drain reitor for a FET. The output oltage i gien by o =-i. The minu ign our beaue the hange in urrent alway produe a oltage hange at olletor that i oppoite to the input oltage hange.

2 o ( r ) The oltage gain i gien by = A =. i rm r m an be alulated by Eber-Moll equation 5( Ω ma) rm = + IΩ I Step The ignal amplitude from a oure depend on the urrent that the oure mut proide. r i i equialent reitane of oure. r i peifie the urrent that flow in to bae. The equialent iruit how that a imple oltage diider relation an be ued to alulate the dereae in ignal amplitude. ri = r + r ri ro Gain A = = r + r i r m i

3 Step Signal are amplified for a purpoe, to be applied to ome load. P r o i r Sine i in parallel with and r o gain would be modified a r, i r0 ri ro A = = r + ri rm r + r ro + rm ri r0 A = r + ri rm ro + ri A= ( a) r + r r0 + We ee that the ignal V S i attenuated at the input, inerted and amplified by the deie with gain a, and attenuated at the output by the effet of load. learly gain ha three part. ri ) Input attenuation (oltage diider): r + ri ro ) Deie gain a = rm ) Output attenuation (oltage diider): r + o

4 +5V=V p 0.µF V V r b =K 470µF 4 Fig V=V n. For the iruit gien in figure 0. the total gain i gien by β IH A= = β β ( a ) β r r β β it i ih m b o oh + it 0 + < βoh are higher frequeny attenuation term and βi βo < are low frequeny attenuation term. So we an ee that final gain expreion ha eeral omponent: ) Deie gain ) Frequeny independent input and output attenuation ) ow frequeny attenuation 4) High frequeny attenuation The mid-frequeny gain of the tranitor by itelf i alled the deie gain. a m = = b r m ro + (without the external load reitor) r o The input ( ) and output ( ) reitane reult in ignal attenuation. The apaitor it produe frequeny dependent attenuation term β that hae the following form: β = [ + ( f / f) ] / β = [ + ( f / f ) ] H H /

5 . Break Frequenie: The oupling apaitor and aue low break at the input and output. f = [ π ( r + )] i it f = [ π ( r + )] o o The by-pa apaitor at the emitter alo aue a low break. f = [ π ( r + ), ] b m 4 Stray apaitane and apaitane inide of the tranitor aue high break at the input and output. f = [ π ( r, )] ih i it f = [ π ( r, )] oh o o Notie that the reitor are in erie for the low break and in parallel for the high break.. BJT Parameter 5( Ω ma) r = + Ω m I ro = ( r, ) r = 4000( rm + ) it = (, rb ) r = h ( r + ) b fe m 4. FET Parameter r VT = ( r, ) m = o d II d d r 50 K ( Ω ma) / I d = (0.) it V = V ( I / I ) r g T d d d

6 5. Nonlinearity The I-V relation for tranitor i nonlinear, hene the hange in urrent i larger when V be i inreaed rather than dereaed. I i the quieent tranitor-urrent. I = I I = I e ) ( Vb /5mV MEASUEMENTS: The ame iruit will be ued throughout the experiment, but the omponent will be hanged to emphaize the ariou fator that affet the gain. Arrange the iruit imilar to the diagram on page to aoid onfuion, and make it eay to hange omponent. The reitane i inluded to repreent the effet of the ignal-oure reitane. In thi repreentation the ignal obered at point A i the ideal. omponent alue are gien at the bak of thee intrution. D indiate a diret onnetion in pleae of a omponent, and X mean the omponent i not preent.. The emitter reitor will be zero until tep 8. b ) B. Midfrequeny Gain and High-Frequeny Attenuation ) When referring to oltage refer ue peak-to-peak oltage. will be kept relatiely mall to minimize non-linearity. Monitor the input ignal when the frequeny i hanged (refer to GI etion 4.) For I = ma, meaure the midfrequeny gain ( / at 0kHz. Obere that the gain i ontant in the midfrequeny region by arying the ignal frequeny from khz to 00kHz (i.e. the amplitude of hould be ontant when b i ontant). Meaure the output highbreak-frequeny f (refer to GI etion 5.6, 5.6A). Meaure the gain at oh f 5 0 oh. epeat the oberation for I = ma. (You annot obere the ignal at 0 f oh for I 5 beaue that i aboe the upper limit of the ignal generator.) = ma alulate the midfrequeny gain and the output high-break frequeny for I = ma and 5 ma. The meaured break frequenie may not agree with the alulated alue from tep, beaue that alue depended on the aumed alue of (0 pf). Howeer, the hange in break frequeny aued by the hange in o hould agree with the alulation. f ) alulate from the meaured o oh. The ope probe ued for the meaurement add about 0pf to o. The peifiation for the N904 etimate b a 4pf. Etimate the tray apaitane ( ) of your iruit from the meaured o.( = ) i relatiely high in the plug-in hai. o b ope

7 . Input Attenuation at Mid- and High-Frequeny ) Ue =0mV. Meaure the midfrequeny gain, and obere that it i / ontant from khz to 50kHz. 4) Obere and and meaure the break frequeny introdued by. Why i i thi meaurement not affeted by the output high-break-frequeny? Sine = 0 in the remainder of the experiment, there will be no high-frequeny attenuation at the input. D. Output Attenuation 5) Ue =0mV. Meaure Am = /, and obere that it i ontant in the frequeny range from approximately khz to 00kHz. E. ow Frequeny Attenuation The three low break frequenie will be inetigated. Initially you will alulate all three frequenie to ee whih i dominant and hek by meaurement. Then the omponent will be hanged to remoe the higher break o that the lower break an be obered. 6) Ue = 0mV. Obere to meaure the low break frequeny that mark the end of the midfrequeny region (i.e. the highet of the three break). 7) eplae by a diret onnetion. Obere to meaure the middle lowbreak-frequeny. 8) emoe. Obere top meaure the lowet break frequeny. F. Tranitor Nonlinearity Tranitor nonlinearity will be inetigated in thi part. 9) onnet the ope probe to the olletor and ue D oupling at the ope input. Adjut the ertial poition ontrol o the trae (whih repreent the quieent V ) i in the enter of the ope. Now apply input ignal of 0, and 40m (p-p) with f=0khz. Meaure the peak alue of V relatie to the quieent line. The nonlinearity hould be ery eident, but it may be maller than alulated in tep 4. 0) Inreae to obere the limit and ompare to the alulation in 6. You hould notie that you reah the negatie limit firt a indiated in alulation 4. ) Meaure a m and ompare. You hould alo obere with an amplitude of approximately 5V (p-p) to ee that it i muh more ymmetri around the quieent ondition than it wa in tep 5. No report required.

8 G. ommon-soure Amplifier The BJT will be replaed by a FET in thi etion. The pin order i not the ame for the two tranitor (GI-0.) Vg i poitie; hene the polarity of the oure by-pa apaitor mut be reered. ) Meaure the quieent oltage at the drain and oure. Meaure the deie gain at 0kHz and obere that it i ontant in the frequeny range from khz to 00kHz. It hould be eident that the gain potential of an FET i muh maller than it i for a BJT.

9 Phyi 56 Experiment 0: omponent (A) Aume η =4000, D=Diret onnetion, X=No onnetion Step I 4 - D 0K 4K D X D X 5 D K 5.K D X D X -4 5 D K 5.K K X D X 5 D 0K 4K D 0K D 0.0µf 6 D 0K 4K D 0K 0.05µf 0.0µf 7 D 0K 4K D 0K D 0.0µf 8 D 0K 4K D X D X 9-0 D 0K 4K D X D X 40Ω 0K 4K D X D X? D 4.7K 0K D X D X Initial omponent Tranitor: N904 apaitor: 0. µf, 470 µf eitor: K, 0K, 4K Pleae pik up the following omponent later if omeone i waiting: Tranitor: MPF0 apaitor: 0.0 µf, 0.05 µf eitor: 40Ω, of K, 4.7K, 5.K, 0K