emitter follower a current or voltage series negative feedback amplifier?

[koyel123]

Hi,

In a textbook the emitter follower is depicted as a voltage series negative feedback amplifier and a current series negative feedback amplifier as well. Difference between the two circuits shown is only that the one, which is a current series feedback amplifier uses another load resistor coupled to the emitter resistor using one capacitor that is the load resistor and emitter resistor are in parallel only for ac and for dc load resistor is absent. In the one, which is shown as voltage series negative feedback amplifier doesn't have any load coupled to emitter resistor and output is taken across emitter resistor only. Does the coupling capacitor changes emitter follower from a voltage series to current siries negative feedback? Please clarify.

Thanks and regards,
Koyel

 

[LvW]

For my opinion, the terms "voltage series" or "current series" are somewhat confusing because it is not inicated what is (a) the controlling quantity and what is the controlled quantity.
Hence, I prefer expressions like "voltage controlled voltage feedback" (which, for example, applies to non-inverting opamp circuits).

In the actual case of an emitter follower it is the voltage developped at the emitter node which causes the feedback. Because this feedback voltage is caused by the emitter current we can say that the common collector configuration (emitter follower) realizes "current-controlled voltage feedback". This is true for both cases you have mentioned - DC feedback and ac feedback.
However, because in both cases the emitter current(s) and the effective resistances are different the feedback factor differs also.

 

[Venkadesh_M]

Yeah by connecting just a capacitor you are making diff in load voltage

if the load is parallel to RE then the output voltage is proportional to Ie

But if there is a cap in coupling the output voltage is proportional to change in output voltage that is main part of current Ie.