why common emitter input impedance more than common base config

[Jishnu Rajeev]

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

 

[Venkadesh_M]

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.......

 

[LvW]

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

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.

 

[chuckey]

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

 

[BradtheRad]

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