why common emitter input impedance more than common base config

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Jishnu Rajeev

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Hi,
why common emitter input impedance more than common base config even if input is given across base - emitter junction in both the cases ?


Thanks
Jishnu
 

Its simple..
In common emitter the input in base side which has low current, so high impedance........
In common base the input in emitter side that have a lot current, so low impedance.......
 
Jishnu Rajeev said:
Hi,
why common emitter input impedance more than common base config even if input is given across base - emitter junction in both the cases ?
Thanks
Jishnu
Click to expand...

In common base circuits the input signal is applied to the emitter node. When, for example, the input signal makes the emitter potential somewhat more negative the B-E voltage goes higher and the base current Ib increases.
However, at the same time the emitter current Ie=Ib*(beta+1) also increases drastically. Both currents flow through the emitter node. Thus, the corresponding input resistance is rather low.
 
Or, if you inject say, 1mV of AC into the base of a transistor, this will cause a small current to flow, say 1micro amp into the base, but at the emitter you will get 100 X as much current flowing due to the gain of the transistor. So the input impedance at the base is 1 mV/ 1micro A ~ 1k and at the emitter, 1mV/100 micro amps ~ 10 ohms.
Frank
 
With common base, the input is either a component in the emitter leg, or is applied across a resistor in the emitter leg.

Common base is useful when you have a low-amplitude signal coming from a low impedance source. Also when you want the output polarity to match that of the input.

For example:

* a speaker being used as a microphone

* a thermocouple

* a substance which you wish to test for resistance changes in response to temperature

 
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