Question about Emitter follower

[muoinhohn]

emitter follower working principle

Hi al,
Do you known why the emitter follower uses in power amplifier?
What is the difference about input and impedance between Emitter Follower and Common Emitter?
I've searched some documents, so I do not found the document that explains clearly about the problem.

Regards
MuoiNho

 

[simonkuo]

wide band emitter follower

The output impedance of Emitter follower is small that it have the capabiliity to drive the heavy output loading (say 50 ohm). The output impedance of common emitter is large.

 

[muoinhohn]

Thank for your reponse. Is the input impedance of common emitter smaller than emitter follower?

Regards
MuoiNho

 

[vonleo]

emit follower is a voltage buffer, the input imepdance is big, so he can get most of the voltage in; output impedance is low so he can drive a relative small load.
you can refer to some basic book about this.

 

[aryajur]

The input impedance of an common emitter circuit is beta times re' where re' is the intrinsic emitter base junction resistance roughly given be 25mV/Ie , where Ie is the DC biasing junction current at room temperature. The input impedance of an emitter follower is made many times larger by the emitter degeration resistor put from the emitter to the gnd, since now the input impedance becomes beta times re' + Re if Re>>re' then its roughly beta time Re and so the excessive temperature dependance of the input impedance is removed and also the input impedance is greatly increased making the circuit suitable for voltage transfer.

 

[cetc1525]

emitter follower has the wide band &low output resistance.

 

[muoinhohn]

Hi al
The resistor RE that used for the stability of amplifier in dc bias is exists in CE and also CC amplifier. So that if we use the linear model to analyze, we going to get the same input impedance for both.
In power amplifier using CC, the signal is large. So I can not analyze the circuit in using the small signal model. I'd like to have the good explanation for the case: large signal with CC amplifier.

Thank for all your responses
Muoinho

 

[strabush]

muoinhohn
You forgot something, the power amplifier using an Emitter follower has the emitter bias stabilization resistor AND the load (loudspeaker) in series at the Emitter, so the input impedance of the Emitter Follower is even much higher.

 

[qutang]

commen emitter has hign gain.but emitter follower can driver big load

 

[electronrancher]

you should think about the mos case (no base current).

for example. a cmos voltage follower has 3v on the gate, so the source wants to rest at about 3v-Vth, maybe 2.2v or so. it's drain is connected to VDD, so it can just POUR current into the load to satisfy Vout=2.2v, therefore it is considered VERY low output impedance.

only problem is you need at least 1Vth to drive it so you can't output all the way to VDD, only up to VDD-Vth. (body effect is next troublemaker if you only have single-well process)


same goes for emitter follower. you'll need a good strong base drive, but other than that the principle is very similar. no body effect and your max Vout is VDD-Vbe, assuming you can drive the base to VDD. (Usually you can only drive to VDD-VCEsat in bipolar so that's why I say that)