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Principle behind Emitter follower circuit

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Vijay Vinay

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Hi all,
I am designing an emitter follower circuit for my design. I googled about this concept, but I am not getting the exact explanation for that theory. Can anyone please tell me how it works.

Thanks
 

What don't you understand, specifically? The emitter current is equal to the base current times beta+1. The emitter voltage is equal to the base voltage -Vbe.
 
barry said:
The emitter voltage is equal to the base voltage -Vbe.
Click to expand...

I think, we can say that the emitter voltage variation dVE (across the emitter resistor RE) follows the base voltage variation dVB because of the negative voltage feedback effect.
This effect linearizes the exponential function I~exp(VBE/Vth) which - without feedback - describes the relation between the currents (base, emitter and collector current) and the voltage across the B-E junction.
 
Hi barry,
Thanks for your reply. However, what exactly I don't understand is I have an emitter follower circuit and I don't know which current to consider for allthe transistors. Also I need to know how to calculate the respective currents for all the transistors.

emitter_follower.jpeg
 

Vijay, that's not just an emitter-follower. It's a complex combination circuit. Your opening post came nowhere near describing what you were talking about.

I suggest you start by first trying to understand a basic emitter-follower using a single transistor.
 
Pjdd said:
Vijay, that's not just an emitter-follower. It's a complex combination circuit. Your opening post came nowhere near describing what you were talking about.

I suggest you start by first trying to understand a basic emitter-follower using a single transistor.
Click to expand...

Hi Pjdd,
Thank you for your information. Could you suggest some good links for understanding the concept of emitter follower using transistors.

Thanks
 

It looks to me like a current sink loaded, emitter follower
buffer pair for a differential signal chain.

Current is set in this case by the current sink loads,
in turn by (Vbias-Vbe(Ie)-Vee) divided by the 100ohm
tail resistor (at this low Re impedance, the Vbe needs
to be figured, and it's possible that the active devices
may be mis-sized to the tail current setpoint and load).

Output voltage vop is set as follower vinp-Vbe(Ie, temp).
 

Looking at the schematic I see WE narrow and LE long
on the BJTs. In my experience with other kits, this is
backwards and can lead to performance issues at higher
currents (base debiasing etc.). Check your PDK docs.

Since Vbias is a degree-of-freedom be sure you have
set it appropriately.
 

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