Using the Hybrid- p Model

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# Using the Hybrid- p Model - PowerPoint PPT Presentation

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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Presentation Transcript

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
• 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-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.

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
• 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 ?