I cannot imagine a simpler negative feedback system than an inverting voltage amplifier whose output is connected to its input. In the most sophisticated case, this circuit can be implemented by an op-amp with an extremely high gain; its main purpose here is to keep its own input voltage equal to zero. Thus the whole system acts as a zero voltage stabilizer. In the simplest case, this circuit can be implemented by a transistor whose collector is connected to its base; here it decreases its present resistance to keep its base input voltage close to zero.
But where is the input of this negative feedback system? Where do we apply the input quantity? I have dedicated another question to this problem:
https://www.researchgate.net/post/What_actually_is_a_negative_feedback_system_What_and_how_does_it_actually_do_How_is_it_implemented_Is_it_really_a_negative_feedback_or_something_else
The present question is closely related to the rest fundamental questions about the ubiquitous negative feedback phenomenon:
https://www.researchgate.net/post/What_is_actually_the_input_of_a_negative_feedback_system_Can_we_consider_the_input_quantity_as_a_disturbance_and_the_output_quantity-as_a_reaction?
https://www.researchgate.net/post/Can_we_see_the_negative_feedback_principle_in_the_operation_of_the_common-base_stage_Can_we_think_of_it_as_of_a_disturbed_common-collector_stage?
Hi Cyril, if we think of electronic systems only (you know, we have negative feedback also in mechanical systems) to me the most simple negative feedback system is a common emitter stage with Re feedback in the emitter path.
Or even simpler: The emitter follower (common collector).
Quote: "In the simplest case, this circuit can be implemented by a transistor whose collector is connected to its base; here it decreases its present resistance to keep its base input voltage close to zero."
Collector is connected to its base? I assume you mean: Through a resistor, OK?
Otherwise, the transistor action is destroyed.
Hi, Lutz! Thank you for supporting the question. I am going on leave for two weeks and will episodically keep track of the RG discussions...
The emitter (source) follower seems to be the simplest negative feedback electronic system but yet it needs an input voltage source to be connected. IMO the simplest negative feedback system (circuit) have not an input source; its input is connected to its output. Maybe a JFET current-limiting diode can serve as a simplest negative feedback system?
In the input part of a BJT current mirror, the collector of the transistor (Q1) is connected directly to its base...
But the question arises:
Is this connection eqivalent to FEEDBACK? I don`t think so because there is no amplifier (Q1 does not act as an amplifying device).
YES, this is exactly a negative feedback! And Q1 is exactly an amplifying device! And this is the simplest example of a system with a voltage-voltage (parallel-parallel) negative feedback!
http://en.wikibooks.org/wiki/Circuit_Idea/How_to_Reverse_Current_Direction
OK - Cyril. I agree.
My first look was a bit too simple. Indeed, the BJT Q1 can be seen as an amplifying device with negative feedback operating at the beginning of the active region.
I just have read your wikibooks contribution - and you have convinced me.
Thank you and regards
Lutz
Maybe these parts of the talk page will be interesting for you too?
http://en.wikibooks.org/wiki/Talk:Circuit_Idea/How_to_Reverse_Current_Direction#Not_a_diode
http://en.wikibooks.org/wiki/Talk:Circuit_Idea/How_to_Reverse_Current_Direction#BJT_as_a_current_mirror_building_component
The simplest negative feedback circuit is a resistor in series with battery. If the battery is non-ideal, the internal resistor "limits" the short-circuit current by generating a voltage opposite to the source voltage. Useless as it may sound, but it is the simplest! If you really insist on including an active device such as a BJT(or a FET), in my opinion, an emitter (or source-) follower circuit is the classic best.
It is a matter of definition - I think.
For my opinion, feedback needs a device that has an input and an output in order to have the chance to feed a signal back to the input.
Thus, for my understanding at least a three-terminal device is needed, which exhibits a certain transfer characteristic between input and output.
To me, your example is a resistive voltage divider - but a feedback system?
Masoud, congratulations on the courage to suggest such an original speculation!
To tell the truth, I have no courage to tell that the humble ohmic resistor introduces a negative feedback. But I used to ask myself if the Zener diode (or the ordinary diode) can be considered as a negative feedback system (like a shunt voltage stabilizer with a parallel-parallel negative feedback)... and if the transistor (the collector-emitter part) - as a current-stabilizer with negative feedback... and generally, if a non-linear resistor - as a system with an intrinsic (inner) negative feedback...
I share the Lutz thought that "the feedback needs a device that has an input and an output". It is interesting to see if it is necessary to have a gain more than one... and if it can be a two-terminal element...
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