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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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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
extending the upper cut off
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.
common base configuration
Common-Base Configuration

Common-emitter amplifier

Common-base amplifier

common base quiescent conditions
Common-Base Quiescent Conditions

i.e. exactly the same as common emitter amplifier.

common base voltage gain
Common-Base Voltage Gain

i.e. same as C-E but non-inverted.

high frequency effects
High Frequency Effects
  • Neither CBC or CBE connects vin to vout.
  • There is, therefore, no Miller effect.
  • Cin = CBE
  • Cout = CBC
c b vs c e comparison
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
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 ?