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ECE 442 Solid-State Devices & Circuits 9. BJT Amplifiers

ECE 442 Solid-State Devices & Circuits 9. BJT Amplifiers. Jose E. Schutt-Aine Electrical & Computer Engineering University of Illinois jschutt@emlab.uiuc.edu. Small-Signal Model. What is a small-signal incremental model?

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ECE 442 Solid-State Devices & Circuits 9. BJT Amplifiers

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  1. ECE 442 Solid-State Devices & Circuits 9. BJT Amplifiers Jose E. Schutt-Aine Electrical & Computer Engineering University of Illinois jschutt@emlab.uiuc.edu

  2. Small-Signal Model • What is a small-signal incremental model? • Equivalent circuit that only accounts for signal level fluctuations about the DC bias operating points • Fluctuations are assumed to be small enough so as not to drive the devices out of the proper range of operation • Assumed to be linear • Derives from superposition principle

  3. Hybrid-p Incremental Model for BJTs rp: input resistance looking into the base rx: parasitic series resistance looking into base – ohmic base resistance gm: BJT transconductance ro=rce: output collector resistance related to the Early effect

  4. Hybrid-p Parameters Can show that

  5. Common Emitter Configuration The emitter current IEcan be approximated as: An incremental conductance ge can be defined as From which we get: Emitter resistance

  6. Common Emitter (CE) Amplifier Bias: Choose R1 & R2 to set VBVE is then set. Choose RE to set IE~IC. Quiescent point of Vout will be determined by RC. Emitter is an AC short.

  7. Incremental Model for CE Amplifier Hybrid-p model (ignoring rx)

  8. CE Amplifier gain from base to collector

  9. CE Amplifier Open-circuit voltage gain:

  10. CE Amplifier Output Impedance It can be seen that if Rsig>> rp, the gain will be highly dependent on b. This is not good because of bvariations

  11. CE with External Resistors

  12. CE with External Resistors

  13. CE with External Resistors RE and RB degrade the gain

  14. CE with External Resistors The gain can be written as:

  15. Example Given VBEON=0.6V, find the gain for the circuit shown

  16. Example (Cont’) AC analysis: RE2 is shorted and RE=RE1=100W. Since b is not known, use:

  17. Emitter Follower circuit Incremental model

  18. Emitter Follower Emitter follower has unity voltage gain

  19. Emitter Follower – Input Impedance

  20. Emitter Follower – Output Impedance

  21. Output Impedance (cont’) If we neglect RB

  22. Common Base Configuration

  23. Common Base Configuration

  24. BJT Topologies - Summary CE EF CB 1 Avo Rin Rout

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