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Transistor CE configuration-input impedance

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guruteja

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Why the input impedance is more for CE configuration transistor?
 

guruteja said:
Why the input impedance is more for CE configuration transistor?
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More than what? Such a formulation always needs a comparison between two alternatives.
 

The CE amplifier will only have a high input impedance if you insert a degeneration resistor (RE), not-bypassed.

Thus, input impedance with bias resistors (R1 & R2) added:

\[R_{i} = R_{1}||R_{2}||(r_{\pi}+(\beta+1)R_{E})\]

Without or bypassed RE:

\[R_{i} = R_{1}||R_{2}||r_{\pi}\]

The equations can be derived from your small signal analysis.
 

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