1. Using the Hybrid-  Model

3.  r bb and r o are omitted (insignificant)  R B represents parallel combination of R B1 and R B2  At high frequencies C 1, C 2 and C 3 approximate short circuits.  Problem : C BC influences the input and output halves of the circuit

4. The Miller Effect (input capacitance)

5. The Miller Effect (output capacitance)

6. The Miller Effect – Summary

7. Using the Miller Effect

8. Extending the Upper Cut-Off Use a different transistor – lower C BC. Reduce the gain; C IN is proportional to gain. Reduce the source resistance. Eliminate the Miller effect – use a different amplifier configuration.

9. Common-Base Configuration Common-emitter amplifier Common-base amplifier

10. Common-Base Quiescent Conditions i.e. exactly the same as common emitter amplifier.

11. Common-Base Voltage Gain i.e. same as C-E but non-inverted.

12. Common-Base Output Resistance i RC ieie

13. Common-Base Input Resistance

14. High Frequency Effects Neither C BC or C BE connects v in to v out. There is, therefore, no Miller effect. C in = C BE C out = C BC

15. C-B vs. C-E Comparison Identical quiescent conditions Identical voltage gain (except C-E inverts) Identical output resistance Common-Base input impedance is very low Common-Emitter suffers Miller effect

16. Summary Common-emitter upper cut-off frequency is disappointingly low due, mainly, to the Miller effect. Common-base configuration does not suffer Miller effect but has impractically low input impedance. Solution : combine the two ?