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Lecture 14

Lecture 14. ANNOUNCEMENTS Midterm #1 results (undergrad. scores only): N=74; mean=62.8 ; median=63 ; std.dev.=8.42 You may pick up your exam during any TA office hour. Regrade requests must be made before you leave with your exam.

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Lecture 14

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  1. Lecture 14 ANNOUNCEMENTS • Midterm #1 results (undergrad. scores only): • N=74; mean=62.8; median=63; std.dev.=8.42 • You may pick up your exam during any TA office hour. • Regrade requests must be made before you leave with your exam. • The list of misunderstood/forgotten points has been updated. OUTLINE • Frequency Response (cont’d) • CE stage (final comments) • CB stage • Emitter follower • Cascode stage Reading: Chapter 11.4-11.6 10 38 25

  2. CE Stage Pole Frequencies, for VA<∞ Note that wp,out > wp,in

  3. I/O Impedances of CE Stage

  4. CB Stage: Pole Frequencies • Note that there is no capacitance between input & output nodes  No Miller multiplication effect! CB stage with BJT capacitances shown

  5. Emitter Follower • Recall that the emitter follower provides high input impedance and low output impedance, and is used as a voltage buffer. • Follower stage with BJT capacitances shown • CL is the load capacitance Circuit for small-signal analysis (Av)

  6. AC Analysis of Emitter Follower • KCL at node X: • KCL at output node:

  7. Follower: Zero and Pole Frequencies • The follower has one zero: • The follower has two poles at lower frequencies:

  8. Emitter Follower: Input Capacitance • Recall that the voltage gain of an emitter follower is Follower stage with BJT capacitances shown • CXY can be decomposed into CX and CY at the input and output nodes, respectively:

  9. Emitter Follower: Output Impedance Circuit for small-signal analysis (Rout)

  10. Emitter Follower as Active Inductor • A follower is typically used to lower the driving impedance  RS > 1/gm so that the “active inductor” characteristic on the right is usually observed. CASE 1: RS < 1/gm CASE 2: RS > 1/gm capacitive behavior inductive behavior

  11. Cascode Stage • Review: • A CE stage has large Rin but suffers from the Miller effect. • A CB stage is free from the Miller effect, but has small Rin. • A cascode stage provides high Rin with minimal Miller effect.

  12. Cascode Stage: Pole Frequencies Cascode stage with BJT capacitances shown (Miller approximation applied) Note that

  13. Cascode Stage: I/O Impedances

  14. Summary of Cascode Stage Benefits • A cascode stage has high output impedance, which is advantageous for • achieving high voltage gain • use as a current source • In a cascode stage, the Miller effect is reduced, for improved performance at high frequencies.

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