Lecture 10Biasing of CB StageReduction of Input Impedance Due to RECreation of VbExample of CB Stage with BiasEmitter Follower (Common Collector Amplifier)Emitter Follower CoreUnity-Gain Emitter FollowerSmall-Signal Model of Emitter FollowerEmitter Follower as a Voltage DividerEmitter Follower with Source ResistanceInput Impedance of Emitter FollowerEffect of BJT Current GainEmitter Follower as a BufferOutput Impedance of Emitter FollowerEmitter Follower with Early EffectEmitter Follower with BiasingSupply-Independent BiasingSummary of Amplifier TopologiesAmplifier Example #1Amplifier Example #2Amplifier Example #3Amplifier Example #4Amplifier Example #5EE105 Fall 2007 Lecture 10, Slide 1 Prof. Liu, UC BerkeleyLecture 10OUTLINE•BJT Amplifiers (cont’d)–CB stage with biasing–Emitter follower (Common-collector amplifier)–Analysis of emitter follower core–Impact of source resistance–Impact of Early effect–Emitter follower with biasingReading: Chapter 5.3.3-5.4ANNOUNCEMENTS•Alan Wu will hold an extra lab session tomorrow (9/28), 2-4PM•The post-lab assignment for Experiment #4 has been shortened!•2 pgs of notes (double-sided, 8.5”×11”) allowed for Midterm #1EE105 Fall 2007 Lecture 10, Slide 2 Prof. Liu, UC BerkeleyBiasing of CB Stage SEmEECminoutinXXoutinoutvRRgRRRgvvvvvvvvA1 inSEmEEinSininXvRRgRRvRRRv1•RE is necessary to provide a path for the bias current IE to flow, but it lowers the input impedance.EmEEmEmEminRgRRgRgRgR111||1EE105 Fall 2007 Lecture 10, Slide 3 Prof. Liu, UC BerkeleyReduction of Input Impedance Due to RE•The reduction of input impedance due to i1 is undesirable because it shunts part of the input current to ground instead of to Q1 (and RC). Choose RE >> 1/gm , i.e. ICRE >> VTEE105 Fall 2007 Lecture 10, Slide 4 Prof. Liu, UC BerkeleyCreation of Vb•A resistive voltage divider lowers the gain.•To remedy this problem, a capacitor is inserted between the base and ground to short out the resistive voltage divider at the frequency of interest.EE105 Fall 2007 Lecture 10, Slide 5 Prof. Liu, UC BerkeleyExample of CB Stage with Bias Design a CB stage for Av = 10 and Rin = 50. •Rin = 50≈ 1/gm if RE >> 1/gm Choose RE = 500•Av = gmRC = 10 RC = 500•IC = gm·VT = 0.52mA•VBE=VTln(IC/IS)=0.899V•Vb = IERE + VBE = 1.16V• Choose R1 and R2 to provide Vband I1 >> IB, e.g. I1 = 52A•CB is chosen so that (1/(+1))(1/CB) is small compared to 1/gm at the frequency of interest. VCC = 2.5VIS = 5x10-16 A= 100VA = ∞EE105 Fall 2007 Lecture 10, Slide 6 Prof. Liu, UC BerkeleyEmitter Follower (Common Collector Amplifier)EE105 Fall 2007 Lecture 10, Slide 7 Prof. Liu, UC BerkeleyEmitter Follower Core•When the input voltage (Vin) is increased by Vin, the collector current (and hence the emitter current) increases, so that the output voltage (Vout) is increased.•Note that Vin and Vout differ by VBE.EE105 Fall 2007 Lecture 10, Slide 8 Prof. Liu, UC BerkeleyUnity-Gain Emitter Follower•In integrated circuits, the follower is typically realized as shown below.–The voltage gain is 1 because a constant collector current (= I1) results in a constant VBE; hence Vout = Vin .1vAAVEE105 Fall 2007 Lecture 10, Slide 9 Prof. Liu, UC BerkeleySmall-Signal Model of Emitter Follower•The voltage gain is less than 1 and positive. mEEEinoutEoutoutinmoutinEoutmoutingRRRrvvRvvvgrvvRvvgrvvvv11111 :emitterat KCL AVEE105 Fall 2007 Lecture 10, Slide 10 Prof. Liu, UC BerkeleyEmitter Follower as a Voltage DividerAVEE105 Fall 2007 Lecture 10, Slide 11 Prof. Liu, UC BerkeleyEmitter Follower with Source Resistance11SmEEinoutRgRRvvAVEE105 Fall 2007 Lecture 10, Slide 12 Prof. Liu, UC BerkeleyInput Impedance of Emitter Follower•The input impedance of an emitter follower is the same as that of a CE stage with emitter degeneration (whose input impedance does not depend on the resistance between the collector and VCC).ExxinRrivR )1(AVEE105 Fall 2007 Lecture 10, Slide 13 Prof. Liu, UC BerkeleyEffect of BJT Current Gain•There is a current gain of (+1) from base to emitter.•Effectively, the load resistance seen from the base is multiplied by (+1).EE105 Fall 2007 Lecture 10, Slide 14 Prof. Liu, UC BerkeleyEmitter Follower as a Buffer •The emitter follower is suited for use as a buffer between a CE stage and a small load resistance, to alleviate the problem of gain degradation.speaker221)1( RrRin 1inCmvRRgA speakerRRgACmvEE105 Fall 2007 Lecture 10, Slide 15 Prof. Liu, UC BerkeleyOutput Impedance of Emitter Follower•An emitter follower effectively lowers the source impedance by a factor of +1, for improved driving capability. •The follower is a good “voltage buffer” because it has high input impedance and low output impedance.EsmoutRRgR ||11EE105 Fall 2007 Lecture 10, Slide 16 Prof. Liu, UC BerkeleyEmitter Follower with Early Effect•Since rO is in parallel with RE, its effect can be easily incorporated into the equations for the voltage gain and the input and output impedances. OEmsoutOEinmSOEOEvrRgRRrRrRgRrRrRA||||11||111||||EE105 Fall 2007 Lecture 10, Slide 17 Prof. Liu, UC BerkeleyEmitter Follower with Biasing•A biasing technique similar to that used for the CE stage can be used for the emitter follower. •Note that VB can be biased to be close to VCC because the collector is biased at VCC.EE105 Fall 2007 Lecture 10, Slide 18 Prof. Liu, UC BerkeleySupply-Independent Biasing•By putting an independent current source at the emitter, the bias point (IC, VBE) is fixed, regardless of the supply voltage value.EE105 Fall 2007 Lecture 10, Slide 19 Prof. Liu, UC BerkeleySummary of Amplifier Topologies•The three amplifier topologies studied thus far have different properties and are used on different occasions. •CE and CB stages have voltage gain with magnitude greater than one; the emitter follower’s voltage gain is at most one.EE105 Fall 2007 Lecture 10, Slide 20 Prof. Liu, UC BerkeleyAmplifier Example #1•The keys to solving this problem are recognizing the AC ground between R1 and R2, and using a Thevenin transformation of the input
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