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ECE 342 Solid-State Devices & Circuits 13. Emitter and Source Followers

ECE 342 Solid-State Devices & Circuits 13. Emitter and Source Followers. Jose E. Schutt-Aine Electrical & Computer Engineering University of Illinois jschutt@emlab.uiuc.edu. Emitter Follower. Emitter Follower High-Frequency . Exact analysis is too tedious  approximate.

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ECE 342 Solid-State Devices & Circuits 13. Emitter and Source Followers

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  1. ECE 342 Solid-State Devices & Circuits 13. Emitter and Source Followers Jose E. Schutt-Aine Electrical & Computer Engineering University of Illinois jschutt@emlab.uiuc.edu

  2. Emitter Follower

  3. Emitter Follower High-Frequency Exact analysis is too tedious  approximate

  4. Emitter Follower

  5. High-Frequency Analysis of Emitter Follower leads to: 

  6. High-Frequency Analysis of Emitter Follower

  7. Source Follower with Active Load • Characteristics • Provides a buffer stage • M1 is amplifying stage • M2 is active load

  8. Source Follower – Incremental Model

  9. Source Follower – High Frequency Since the source-to-substrate voltage for M1 varies, the body effect for that transistor must be included

  10. Source Follower – High Frequency Modified high-frequency model

  11. Source Follower – Frequency Response

  12. Emitter Follower with Active Load (current mirror) Emitter follower can be used to drive a low-impedance load

  13. Emitter Follower with Active Load Midband gain:

  14. Emitter Follower with Active Load • AC Properties • Gain is less than 1 and near 1 for typical element values • Frequency response has one zero and two poles • Exact frequency response is difficult  Use SPICE • Output stage of NPN current mirror serves as high impedance load at emitter of Q1

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