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Using the Hybrid- p Model

Using the Hybrid- p Model. r bb and r o are omitted (insignificant) R B represents parallel combination of R B 1 and R B 2 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.

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Using the Hybrid- p Model

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  1. Using the Hybrid-p Model

  2. rbb and ro are omitted (insignificant) • RB represents parallel combination of RB1 and RB2 • At high frequencies C1, C2 and C3 approximate short circuits. • Problem : CBC influences the input and output halves of the circuit

  3. The Miller Effect (input capacitance)

  4. The Miller Effect (output capacitance)

  5. The Miller Effect – Summary

  6. Using the Miller Effect

  7. Extending the Upper Cut-Off • Use a different transistor – lower CBC. • Reduce the gain; CIN is proportional to gain. • Reduce the source resistance. • Eliminate the Miller effect – use a different amplifier configuration.

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

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

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

  11. Common-Base Output Resistance iRC ie

  12. Common-Base Input Resistance

  13. High Frequency Effects • Neither CBC or CBE connects vin to vout. • There is, therefore, no Miller effect. • Cin = CBE • Cout = CBC

  14. 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

  15. 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 ?

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