Lecture 8 ANNOUNCEMENTS A summary of frequently misunderstood missed concepts is now posted on the class website and will be updated regularly Graded HW assignments can be picked up in lab 353 Cory Please Pl i di t your lab indicate l b section ti on your HW assignments i t Midterm 1 Review 3 5PM in 306 Soda will be archived online OUTLINE BJT Amplifiers cont d Common emitter topology CE stage with emitter degeneration Impact of Early effect ro Reading Finish Chapter 5 3 1 EE105 Fall 2007 Lecture 8 Slide 1 Prof Liu UC Berkeley Emitter Degeneration By inserting a resistor in series with the emitter we degenerate g the CE stage g This topology will decrease the gain of the amplifier but improve other aspects such as linearity and input impedance d EE105 Fall 2007 Lecture 8 Slide 2 Prof Liu UC Berkeley Small Signal Analysis The gain of a degenerated CE stage the total load resistance seen at the collector divided by 1 gm plus the total resistance placed in series with the emitter g m RC RC Av 1 1 g m RE RE gm EE105 Fall 2007 Lecture 8 Slide 3 Prof Liu UC Berkeley Emitter Degeneration Example 1 Note that the input impedance of Q2 is in parallel with RE RC Av 1 RE r 2 gm1 EE105 Fall 2007 Lecture 8 Slide 4 Prof Liu UC Berkeley Emitter Degeneration Example 2 Note that the input impedance of Q2 is in parallel with RC RC r 2 Av 1 RE g m1 EE105 Fall 2007 Lecture 8 Slide 5 Prof Liu UC Berkeley Input Impedance of Degenerated CE Stage With emitter degeneration the input impedance is increased from r to r 1 RE a desirable effect effect VA vx r ix RE 1 ix vx Rin r 1 RE ix EE105 Fall 2007 Lecture 8 Slide 6 Prof Liu UC Berkeley Output Impedance of Degenerated CE Stage Emitter degeneration does not alter the output impedance if the Early effect is negligible impedance negligible VA v vini 0 v g mv RE v 0 r vx Rout RC ix EE105 Fall 2007 Lecture 8 Slide 7 Prof Liu UC Berkeley Degenerated CE Stage as a Black Box VA iout vin gm 1 r 1 g m RE iout gm Gm vin 1 g m RE EE105 Fall 2007 If gmRE 1 Gm is more linear Lecture 8 Slide 8 Prof Liu UC Berkeley Degenerated CE Stage with Base Resistance V A vout v A vout vin vin v A vout RC vin r 1 R E R B Av RC 1 RB RE gm 1 EE105 Fall 2007 Lecture 8 Slide 9 Prof Liu UC Berkeley Degenerated CE Stage Input Output Impedances Rin1 is more important in practice because RB is often the output impedance of the previous stage stage V A Rin1 r 1 RE Rin 2 RB r 1 RE Rout RC EE105 Fall 2007 Lecture 8 Slide 10 Prof Liu UC Berkeley Emitter Degeneration Example 3 RC R1 Av 1 RB R2 gm 1 Rin r 1 R2 Rout RC R1 EE105 Fall 2007 Lecture 8 Slide 11 Prof Liu UC Berkeley Output Impedance of Degenerated CE Stage with VA Emitter degeneration boosts the output impedance This improves the gain of the amplifier and makes the circuit a better current source R out 1 g m R E r rO R E r R out rO g m rO 1 R E r R out rO 1 g m R E r EE105 Fall 2007 Lecture 8 Slide 12 Prof Liu UC Berkeley Two Special Cases Stage with explicit depiction of ro 1 RE r Rout rO 1 g m r rO 2 RE r Rout 1 g m RE rO EE105 Fall 2007 Lecture 8 Slide 13 Prof Liu UC Berkeley Analysis by Inspection This seemingly complicated circuit can be greatly simplified by first recognizing that the capacitor creates an AC short to ground and gradually transforming the circuit to a known topology Rout R1 Rout 1 EE105 Fall 2007 Rout 1 1 g m R2 r rO Lecture 8 Slide 14 Rout 1 g m R2 r rO R1 Prof Liu UC Berkeley Example Degeneration by Another BJT Rout 1 gm1 rO2 r 1 rO1 Called a cascode this circuit offers manyy advantages g that we will study later EE105 Fall 2007 Lecture 8 Slide 15 Prof Liu UC Berkeley Bad Input Connection Since the microphone has a very low resistance connectingg the base of Q1 to gground it attenuates the base voltage and renders Q1 with a very small bias current EE105 Fall 2007 Lecture 8 Slide 16 Prof Liu UC Berkeley Use of Coupling Capacitor A capacitor is used to isolate the DC bias network from the microphone and to short or couple couple the microphone to the amplifier at higher frequencies EE105 Fall 2007 Lecture 8 Slide 17 Prof Liu UC Berkeley DC and AC Analysis The coupling capacitor is replaced with an open circuit for DC analysis and then replaced with a short circuit for AC analysis Av g m RC rO Rin r RB Rout RC rO EE105 Fall 2007 Lecture 8 Slide 18 Prof Liu UC Berkeley Bad Output Connection Since the speaker has an inductor with very low DC resistance connectingg it directlyy to the amplifier p would short the collector to ground causing the BJT to go into deep saturation mode EE105 Fall 2007 Lecture 8 Slide 19 Prof Liu UC Berkeley Use of Coupling Capacitor at Output The AC coupling indeed allows for correct biasing However due to the speaker However speaker ss small input impedance the overall gain drops considerably EE105 Fall 2007 Lecture 8 Slide 20 Prof Liu UC Berkeley CE Stage with Voltage Divider Biasing Av gm RC rO Rin r R1 R2 Rout RC rO EE105 Fall 2007 Lecture 8 Slide 21 Prof Liu UC Berkeley CE Stage with Robust Biasing VA RC Av 1 RE gm Rin r 1 RE R1 R2 Rout RC EE105 Fall 2007 Lecture 8 Slide 22 Prof Liu UC Berkeley Elimination of Emitter Degeneration for AC Signals The capacitor C2 shorts out RE at higher frequencies to eliminate the emitter degeneration degeneration V A Av g m R C R ini r R1 R 2 R out R C EE105 Fall 2007 Lecture 8 Slide 23 Prof Liu UC Berkeley Complete CE Stage RC RL R1 R2 Av Rs R1 R2 R1 R2 Rs 1 RE gm 1 EE105 Fall 2007 Lecture 8 Slide 24 Prof Liu UC Berkeley Summary of CE Concepts EE105 Fall 2007 Lecture 8 Slide 25 Prof Liu UC Berkeley
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