Lecture 10 ANNOUNCEMENTS 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 1 OUTLINE 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 biasing Reading Chapter 5 3 3 5 4 EE105 Fall 2007 Lecture 10 Slide 1 Prof Liu UC Berkeley Biasing of CB Stage RE is necessary to provide a path for the bias current IE to flow but it lowers the input impedance 1 RE 1 g RE Rin RE m 1 gm RE 1 g m RE gm vX Rin RE vin vin Rin RS RE 1 g m RE RS vout vout v X Av vin v X vin vout RE g m RC vin RE 1 g m RE RS EE105 Fall 2007 Lecture 10 Slide 2 Prof Liu UC Berkeley Reduction 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 VT EE105 Fall 2007 Lecture 10 Slide 3 Prof Liu UC Berkeley Creation 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 4 Prof Liu UC Berkeley Example 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 VCC 2 5V IS 5x10 16 A 100 VA 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 Vb and I1 IB e g I1 52 A CB is chosen so that 1 1 1 CB is small compared to 1 gm at the frequency of interest EE105 Fall 2007 Lecture 10 Slide 5 Prof Liu UC Berkeley Emitter Follower Common Collector Amplifier EE105 Fall 2007 Lecture 10 Slide 6 Prof Liu UC Berkeley Emitter 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 7 Prof Liu UC Berkeley Unity 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 V A Av 1 EE105 Fall 2007 Lecture 10 Slide 8 Prof Liu UC Berkeley Small Signal Model of Emitter Follower The voltage gain is less than 1 and positive V A v vin vout v vout KCL at emitter g m v r RE vin vout vout g m vin vout r RE vout 1 RE r 1 1 vin 1 RE gm 1 RE EE105 Fall 2007 Lecture 10 Slide 9 Prof Liu UC Berkeley Emitter Follower as a Voltage Divider V A EE105 Fall 2007 Lecture 10 Slide 10 Prof Liu UC Berkeley Emitter Follower with Source Resistance V A vout RE vin R 1 RS E g m 1 EE105 Fall 2007 Lecture 10 Slide 11 Prof Liu UC Berkeley Input 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 V A vx Rin r 1 RE ix EE105 Fall 2007 Lecture 10 Slide 12 Prof Liu UC Berkeley Effect 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 13 Prof Liu UC Berkeley Emitter 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 Av g m RC Rspeaker EE105 Fall 2007 Rin1 r 2 1 2 Rspeaker Av g m RC Rin1 Lecture 10 Slide 14 Prof Liu UC Berkeley Output 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 1 Rs RE Rout g m 1 EE105 Fall 2007 Lecture 10 Slide 15 Prof Liu UC Berkeley Emitter 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 Av RE rO RS 1 RE rO 1 g m Rin r 1 RE rO Rout EE105 Fall 2007 Rs 1 RE rO 1 g m Lecture 10 Slide 16 Prof Liu UC Berkeley Emitter 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 17 Prof Liu UC Berkeley Supply 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 18 Prof Liu UC Berkeley Summary 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 19 Prof Liu UC Berkeley Amplifier 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 network CE stage Small signal equivalent circuit Simplified small signal equivalent circuit vout R2 RC R1 R1 RS 1 vin RE R1 RS 1 g m EE105 Fall 2007 Lecture 10 Slide 20 Prof Liu UC Berkeley Amplifier Example 2 AC grounding shorting and Thevenin transformation are needed to transform this complex circuit into a simple CE stage with emitter degeneration vout RC R1 RS R1 1 vin R2 R1 RS 1 g m EE105 Fall 2007 Lecture 10 Slide 21 Prof Liu UC Berkeley Amplifier Example 3 First identify Req which is the impedance seen at the emitter of Q2 in parallel with the infinite output impedance of an ideal current source Second use the equations for a degenerated CE stage with RE replaced by Req Rin r 1 r 2 R1 1 R1 Req g m2 1 EE105 Fall 2007 Lecture 10 Slide 22 RC Av 1 1 R1 g m1 g m 2 1 Prof Liu UC Berkeley Amplifier …
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