# Using the Hybrid-  Model.  r bb and r o are omitted (insignificant)  R B represents parallel combination of R B1 and R B2  At high frequencies C.

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Using the Hybrid-  Model

 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

The Miller Effect (input capacitance)

The Miller Effect (output capacitance)

The Miller Effect – Summary

Using the Miller Effect

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.

Common-Base Configuration Common-emitter amplifier Common-base amplifier

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

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

Common-Base Output Resistance i RC ieie

Common-Base Input Resistance

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

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

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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