A problem of terminology (since 40 years).
The core of a common feedback stage of most amplifying circuits
is a single transistor (or a push-pull). Its collecteur current is mainly
relying on the voltage difference between its base and its emitter
and is used by the rest of the circuit.
The voltage received by the emitter of this transisor
can be buffered by a second transistor in emitter follower.
The stage is then called a Long Tail Pair, but
the real voltage determining the current of feedback
stays the Vbe of the first transistor.
Contrary to the first circuit above, with a LTP, the impedance
of the feedback circuit determinging (resistors, usually named
Rf, Rg) the closed loop gain of the whole circuit has only a very
small effect on its open loop gain.
Since long, I disapprove the terminogy currently used on the difference
between these two kinds of feedbacks. Looking for clearer terms, I found
direct feedback and assisted feedback well appropriate.
I opened a thread about that in a french forum, Melaudia :
https://forums.melaudia.net/showthread.php?tid=12776&pid=214318#pid214318
It starts with this drawing which I find quite expressive :
The core of a common feedback stage of most amplifying circuits
is a single transistor (or a push-pull). Its collecteur current is mainly
relying on the voltage difference between its base and its emitter
and is used by the rest of the circuit.
The voltage received by the emitter of this transisor
can be buffered by a second transistor in emitter follower.
The stage is then called a Long Tail Pair, but
the real voltage determining the current of feedback
stays the Vbe of the first transistor.
Contrary to the first circuit above, with a LTP, the impedance
of the feedback circuit determinging (resistors, usually named
Rf, Rg) the closed loop gain of the whole circuit has only a very
small effect on its open loop gain.
Since long, I disapprove the terminogy currently used on the difference
between these two kinds of feedbacks. Looking for clearer terms, I found
direct feedback and assisted feedback well appropriate.
I opened a thread about that in a french forum, Melaudia :
https://forums.melaudia.net/showthread.php?tid=12776&pid=214318#pid214318
It starts with this drawing which I find quite expressive :