Lecture 8. OUTLINE BJT Amplifiers (cont d) Common emitter topology CE stage with emitter degeneration Impact of Early effect (r o )

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1 Lectue 8 NNOUNCMNTS suayof fequently isundestood/issed concepts is now posted on the class website, and will be updated egulaly. Gaded HW assignents can be picked up in lab (353 Coy). Please indicate you lab section on you HW assignents! OUTLIN BJT plifies (cont d) Coon eitte topology C stage with eitte degeneation Ipact of aly effect ( o ) eading: Finish Chapte Fall 2007 Lectue 8, Slide 1 Pof. Liu, UC Bekeley

2 itte Degeneation By inseting a esisto in seies with the eitte, we degeneate the C stage. This topology will decease the gain of the aplifie but ipoe othe aspects, such as lineaity, and input ipedance. 105 Fall 2007 Lectue 8, Slide 2 Pof. Liu, UC Bekeley

3 Sall Signal nalysis The gain of a degeneated C stage the total load esistance seen at the collecto diided by 1/g plus the total esistance placed in seies with the eitte. g C C 1 g 1 g g 105 Fall 2007 Lectue 8, Slide 3 Pof. Liu, UC Bekeley

4 itte Degeneation xaple 1 Note that the input ipedance of Q 2 is in paallel with. C 1 π 2 g Fall 2007 Lectue 8, Slide 4 Pof. Liu, UC Bekeley

5 itte Degeneation xaple 2 Note that the input ipedance of Q 2 is in paallel with C. 1 π C 2 g Fall 2007 Lectue 8, Slide 5 Pof. Liu, UC Bekeley

6 Input Ipedance of Degeneated C Stage With eitte degeneation, the input ipedance is inceased fo π to π (β1) a desiable effect. ( V ) x in i π x i x x π (1 β ) i x (ββ 1) 105 Fall 2007 Lectue 8, Slide 6 Pof. Liu, UC Bekeley

7 Output Ipedance of Degeneated C Stage itte degeneation does not alte the put ipedance, if the aly effect is negligible. ( V ) π in 0 π gπ π 0 π x C i x 105 Fall 2007 Lectue 8, Slide 7 Pof. Liu, UC Bekeley

8 Degeneated C Stage as a Black Box ( V i G ) g 1 1 ( π i g 1 g in in g ) If g >> 1, G is oe linea. 105 Fall 2007 Lectue 8, Slide 8 Pof. Liu, UC Bekeley

9 Degeneated C Stage with Base esistance ( V ) in in in π 1 g. βc ( β 1) C B β 1 B 105 Fall 2007 Lectue 8, Slide 9 Pof. Liu, UC Bekeley

10 Degeneated C Stage: Input/Output Ipedances in1 is oe ipotant in pactice, because B is often the put ipedance of the peious stage. ( V ) ( β 1) in1 π in2 B π ( β 1) C 105 Fall 2007 Lectue 8, Slide 10 Pof. Liu, UC Bekeley

11 itte Degeneation xaple ) ( B C 2 2 1) ( 1 g B in β β π 105 Fall 2007 Lectue 8, Slide 11 Pof. Liu, UC Bekeley 1 C

12 Output Ipedance of Degeneated C Stage with V < itte degeneation boosts the put ipedance. This ipoes the gain of the aplifie and akes the cicuit a bette cuent souce. [ 1 g ( )] ( g π 1)( O O O π O [ 1 g ( π )] 105 Fall 2007 Lectue 8, Slide 12 Pof. Liu, UC Bekeley ) π

13 Two Special Cases Stage with explicit depiction of o : 1) ) >> π : O (1 gπ β O 2 ) << π : (1 g ) O 105 Fall 2007 Lectue 8, Slide 13 Pof. Liu, UC Bekeley

14 nalysis by Inspection This seeingly coplicated cicuit can be geatly siplified by fist ecognizing that the capacito ceates an C shot to gound, and gadually tansfoing the cicuit to a known topology. 1 1 [ 1 g ( ] 2 O ) 1 π [ 1 g ( )] 2 π O Fall 2007 Lectue 8, Slide 14 Pof. Liu, UC Bekeley

15 xaple: Degeneation by nothe BJT [ 1 g 1 ( O 2 π 1 )] O 1 Called a cascode, this cicuit offes any adantages that we will study late Fall 2007 Lectue 8, Slide 15 Pof. Liu, UC Bekeley

16 Bad Input Connection Since the icophone has a ey low esistance (connecting the base of Q 1 to gound), it attenuates the base oltage and endes Q 1 with a ey sall bias cuent. 105 Fall 2007 Lectue 8, Slide 16 Pof. Liu, UC Bekeley

17 Use of Coupling Capacito capacito is used to isolate the DC bias netwok fothe icophone, and to shot (o couple ) the icophone to the aplifie at highe fequencies. 105 Fall 2007 Lectue 8, Slide 17 Pof. Liu, UC Bekeley

18 DC and C nalysis The coupling capacito is eplaced with an open cicuit fo DC analysis, and then eplaced with a shot cicuit fo C analysis. in g π ( B C C O O ) 105 Fall 2007 Lectue 8, Slide 18 Pof. Liu, UC Bekeley

19 Bad Output Connection Since the speake has an inducto with ey low DC esistance, connecting it diectly to the aplifie would ~shot the collecto to gound, causing the BJT to go into deep satuation ode. 105 Fall 2007 Lectue 8, Slide 19 Pof. Liu, UC Bekeley

20 Use of Coupling Capacito at Output The C coupling indeed allows fo coect biasing. Howee, due to the speake ss sallinput ipedance, the oeall gain dops consideably. 105 Fall 2007 Lectue 8, Slide 20 Pof. Liu, UC Bekeley

21 C Stage with Voltage Diide Biasing g ( ) in C C 1 2 π O O 105 Fall 2007 Lectue 8, Slide 21 Pof. Liu, UC Bekeley

22 C Stage with obust Biasing V ( ) V in ) 1 g C [ (β 1) ] π 1 C 105 Fall 2007 Lectue 8, Slide 22 Pof. Liu, UC Bekeley 2

23 liination of itte Degeneation fo C Signals The capacito C 2 shots at highe fequencies to eliinate the eitte degeneation. ( ) ) ( V in g π C C Fall 2007 Lectue 8, Slide 23 Pof. Liu, UC Bekeley

24 Coplete C Stage C L 1 1 s g β 1 2 s 105 Fall 2007 Lectue 8, Slide 24 Pof. Liu, UC Bekeley

25 Suay of C Concepts 105 Fall 2007 Lectue 8, Slide 25 Pof. Liu, UC Bekeley