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Common Base and Common Collector Amplifiers

Common Base and Common Collector Amplifiers. Reminder: BJT Small Signal Model. B. C. i c. i b. r .  i b. i e. E. Amplifier. Definition of input and output voltage and current directions. i in. i out. v in. v out. Common Base Amplifiers.

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Common Base and Common Collector Amplifiers

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  1. Common Base and Common Collector Amplifiers

  2. Reminder: BJT Small Signal Model B C ic ib r ib ie E

  3. Amplifier Definition of input and output voltage and current directions iin iout vin vout

  4. Common Base Amplifiers • Uses the same stabilised bias scheme as CE configuration.

  5. Common Base Amplifiers • Uses the same stabilised bias scheme as CE configuration. • The base is grounded for a.c. signals using a large capacitor. • Signal input at the emitter (between the emitter and the common base) • Signal output at the collector (between the collector and the common base)

  6. +VCC RE GND Common Base Amplifiers R1 RC RL vout R2 CB vin

  7. We will ignore the a.c. coupled load RL vout vin +VCC RE GND Common Base Amplifiers R1 RC R2 CB

  8. CB Amplifier, a.c. equivalent. iin E C ic= iout ie ib vout vin RC RE r ib B

  9. Common Base Amplifier

  10. Common Base Amplifier

  11. Input impedance to transistor Common Base Amplifier

  12. Common Base Amplifier iin E C ic= iout ie ib vout vin RC vin RE r ib B

  13. Input impedance to stage (whole amplifier) Common Base Amplifier

  14. Common Base Amplifier iin E C ic= iout ie ib vout vin RC RE r ib B

  15. Common Collector Amplifier • Signal input between base and common collector. • Signal output between emitter and common collector.

  16. Common Collector Amplifier VCC R1 R2 vin RE vout

  17. ib io B r RS ib RE vs C CC Amplifier, a.c. equivalent circuit ie E RB = R1//R2 vin vout

  18. Common Collector Amplifier

  19. Common Collector Amplifier • Transistor input impedance =

  20. Common Collector Amplifier

  21. Common Collector Amplifier

  22. To find the output impedance we note that: Hence Common Collector Amplifier

  23. Common Collector Amplifier • The voltage gain is slightly less than unity. • The CC circuit is sometimes known as an emitter follower since the output voltage at the emitter follows the input voltage at the base.

  24. Common Collector Amplifier • The prime function of this circuit is to connect a high resistance source to a low resistance load (“a buffer amplifier”). • Since voltage gain is approximately one this is achievable as the current gain is high.

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