It is a great idea to see the connection between situations of our everyday life and abstract electronic circuits... If we can do that, it would allow us to understand and explain electronic circuits in a natural human way...
For example, we know very well from our experience that people obey the ubiquitous negative feedback principle when realize their goals - when encounter any sort of obstacle they make further efforts to overcome it... and this makes them stronger... And if we are observant enough, we will notice that the operational amplifier in circuits with negative feedback does the same... and we can see how by following the evolution of some popular op-amp circuit, e.g. the non-inverting amplifer... We can do it in three steps as follows:
1. Voltage follower. It may sound strange but the main task of the operational amplifier in a circuit with negative feedback is just to maintain (almost) zero voltage between its inputs (remember the H&H golden rules)... or more precisely, to keep the voltage at the inverting input (almost) equal to the input voltage applied to the non-inverting input. The op-amp will easily do it if we simply connect its output to the inverting input thus obtaining the popular voltage follower (here we assume zero output resistance ROUT and moderate load resistance RL).
2. Disturbed voltage follower. If, for some reasons, the op-amp output resistance increases... and the load resistance decreases... the voltage follower will be naturally disturbed by the attenuation RL/(RL+ROUT). The op-amp will compensate this obstacle by increasing (RL+ROUT)/RL times its internal output voltage (before the internal output resistance)... thus restoring again the "so desired" zero voltage at the differential input. Thus, as the negative feedback is taken after the undesired disturbance, the external output voltage will continue staying equal to the input voltage. You can observe this mechanism in the funny movie Strange things (see the attachments below) if you choose a resistor from the library on the left, and then click on the button Hide the dog on the right.
3. Intentionally disturbed voltage follower. If we are observant enough... or we recall the compensatory human behavior in daily life, we will guess to use as an output the increased voltage before (instead the equal voltage after) the disturbing voltage divider... thus obtaining the desired non-inverting amplifier. For this purpose, we have to disturb intentionally the voltage follower by an artificial voltage divider consisting of two steady resistors R1and R2 thus making the op-amp amplify (R1+R2)/R1 times the input voltage. You can observe this clever trick in the funny movie Strange things if you click on the button Show the dog.
So, we can make the conclusion that the op-amp non-inverting amplifier is an intentionally disturbed voltage follower... and formulate a general rule for creating amplifying negative feedback circuits:
Insert a disturbing element in the negative feedback loop and take the output signal before the disturbing element.
Related questions are:
https://www.researchgate.net/post/What_is_the_role_of_the_T_resistor_network_connected_in_the_negative_feedback_loop_Can_we_think_of_it_as_of_a_double_disturbance
https://www.researchgate.net/post/Can_we_reverse_a_voltage_divider_by_applying_the_input_voltage_to_its_output_and_taking_the_output_voltage_from_its_input
https://www.researchgate.net/post/What_is_the_simplest_negative_feedback_system_What_is_its_structure_How_does_it_operate_How_do_we_implement_simplest_negative_feedback_circuits
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
https://www.researchgate.net/post/What_are_actually_the_input_and_output_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/Why_the_gain_of_the_non-inverting_amplifier_is_1_R2_R1_while_the_gain_of_the_inverting_amplifier_is_only_R2_R1_Is_this_a_desired_difference
https://www.researchgate.net/post/Does_the_op-amp_in_non-inverting_circuits_with_negative_feedback_behave_as_a_voltage_source_with_negative_internal_impedance
http://www.circuit-fantasia.com/collections/tools/strange-things.html
Cyril,
Thank you for this fascinating idea. I have always thought of the common mode rejection ratio as a human characteristic. When we wish to focus on a specific thought or specific sensory input, such as hearing, we seem to have the ability to reject common mode inputs. Peter
Dear Peter,
Thank you for the appreciation of the evolutionary approach in the op-amp circuitry. Really, the common mode rejection is another extremely interesting phenomenon that can be explained in such an intuitive manner.
The ubiquitous long-tailed pair (two emitter-coupled transistors forming a differential stage) can be presented as two negative feedback systems (voltage followers) with separate inputs (inverting and non-inverting) and joined outputs... but these outputs are not used... Instead, the reaction of one of the systems (its current converted to voltage) is used as an output. We can see this estravagant idea in our daily life, e.g. in the games of arm wrestling and tag of war...
At differential mode (the two input voltages change in opposite directions), the two voltage (emitter) followers oppose each other (fight:) and try to change the common "output" voltage (of the common emitter point) in different directions. There is a conflict in the common output... they react to the else's "intervention"... and we use this reaction as an output.
At common mode (the two input voltages change in the same directions), the two voltage followers cooperate with each other working together on the common high-resistive emitter load (the "long tail"). They all together increase or decrease the voltage of the "output" (the common emitter point)... There is no conflict in the common output... there is no reaction... there is no output change... and the common mode input signals are rejected... In our life this is the situation when people cooperate and mutually help...
Regards,
Cyril
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