The long-tailed (differential) pair is another example of a an elegant simplicity. There is a lot of philosophy in this configuration that deserves to be discovered even almost 70 years after its invention.
1. MAIN IDEA. It is amazingly simple - to (self)destroy the undesired input quantites by subtracting, and to enhance the useful input quantities by summing.
2. IMPLEMENTATION. This clever idea is implemented in an extremely original way (as long as we can realize this fact) in a circuit configuration called with the figurative name "long-tailed pair". If you try to explain it to human beings (students), you can do it in a figurative and colorful manner to stamp your explanation on their memory as follows:
♦ First, you can present the long-tailed pair as a circuit of three elements - two variable pull-up "resistiors" (the transistors T1 and T2, temporarily neglecting the collector resistors) connected to the positive supply rail and one steady pull-down "resistor" (the resistor Re, an constant-current element or a current source) connected to the negative rail (the ground)... or present this combination as a kind of a "forked voltage divider" with two separate upper legs and one common lower leg... or, even better, present it nonelectrically as a mechanical system consisting of three springs - two pull-up and one pull-down...
♦ Then, you can do it more professionally if present this arrangement as a combination of three "sources" in parallel (affecting the same common point) - two voltage sources (the emitter followers T1 and T2) and one current source (the emitter transistor TE). Actually, they are not sources but sooner non-linear elements (two constant-voltage and one constant-current).
♦ Finally, you can generalize (and thus immortalize:) this arrangement if you present it as three interacting (mutually helping or opposing) negative feedback systems with joined outputs (see the attached picture below).
3. OPERATION. It is extremely interesting to see how these three devices (non-linear resistors, sources or systems) interact during the circuit operation.
Biasing mode. First, we have to "bias" this system of three stretched elements (if you prefer, stretch them in a form of a star). For this purpose, the pull-down element (the lower spring) begins slacking the common joint upward until it reaches the desired "quiescent point" position (close to the zero level, in the case of a "split supply").
In other words, the emitter current source varies trying to set the desired biasing current. The two voltage sources cooperate and help the current source; the sum of their currents is equal to the input biasing current.
Common mode ("blowing" a constant current into the pair). At common mode (two positive input voltages), the two pull-up elements become stronger and begin simultaneously pulling the common joint upward while the pull-down element weakly resists them pulling it downward. As a result, they lift the common point to some height and the three elements (springs) remain stretched (an equilibrium).
In other words, the two voltage sources cooperate and "move" side by side the common emitter point (change the common emitter voltage). Тhe emitter current source helps them keeping up a constant biasing current. As the voltage sources do not "strain" at all (they "see" an open circuit), the currents through them do not change. Thus the pair is "lifted" to the level of the common input voltage and it is kept up at this "height" by the mechanism of the series negative feedback...
Differential mode (steering the current between the two legs of the pair). At differential mode (one increasing and the other decreasing positive input voltage), the pull-down element does not do anything. The one pull-up element becomes even stronger and begins pulling the common joint upward while the other pull-up element becomes slack to the same extent and looses the common point. As a result, it stays immovable... as though it is fixed to the real ground... it is a virtual ground...
In other words, the two transistors mutually keep up their emitter voltages constant... they mutually "virtually ground" their emitters... and act as "transistor stages with virtually grounded emitters"... They are like two voltage sources that oppose each other and mutually neutralize the voltage changes at their outputs (an emitter virtual ground). At the same time, the emitter current source "watches" passively and does not do anything (there is no reason to do anything). As the voltage sources extremely "strain", the currents through them vigorously change (are steered); they represent the reactions of the two "fighting" negative feedback systems.
Related questions:
https://www.researchgate.net/post/What_is_current_steering_and_how_is_it_implemented_Is_there_a_dual_voltage_steering_If_so_what_is_it_and_how_is_it_implemented2
https://www.researchgate.net/post/What_is_the_truth_about_the_exotic_current_feedback_amplifier_Is_it_something_new_or_just_a_well_known_old_Is_it_really_a_current_feedback_device
https://www.researchgate.net/post/Can_we_connect_a_floating_voltage_source_between_the_two_inputs_of_a_differential_amplifier_If_so_at_which_conditions
http://www.circuit-fantasia.com/my_work/conferences/cs_2005/paper2.htm
Dear Cyril,
I agree with Remi that you introduced a visualized explanation of the differential amplifier circuit.You want always to enter inside the circuits and feel the operation of every element in it and you want to see how do all elements work together in a harmony to carry out the required function.
You see the circuit as an orchestra and you want to follow every member of it and photograph him.
This may work well at simple circuits with few elements and devices. However when the circuit gets complicated and you want to analyze it you have to split it into functional building blocks. This what one does when he synthesized a complex circuit. It is synthesis by analysis.
Sometimes when you desire to make the things more clear and imaginable you may make it more mysterious.
What you did is the intuitive analysis of the differential pair circuit. Yes,there are many approaches for explaining the operation of such circuit. For example, there is one which is based on knowing the the operation of the basic transistor amplifier circuits the CE, the CB and the Emitter Follower.
This circuit is concepted to work as an EMITTER FOLLOWER for the common mode input voltage leading to equal currents in the collector pairs giving zero output differential voltage between the the two collector.
In case of a differential input voltage, the circuit works as a common emitter as the emitter will be at virtual ground. This is a consequence of the long emitter tail and the Emitter Follower operation of the pairs. Since two equal and opposite singn voltage will be dropped on the tail canceling each other rendering the the common emitter at virtual ground. Consequently, under differential mode, the differential pair works common emitter amplifiers.
Every teacher has its vision and style for qualitatively explaining the circuits operation. At the end, we have to determine quantitatively the performance parameters of the circuit. At this case, we must all agree on the answer.
Thank you Cyril. You remind me with the great Greece philosophers.
Wish you more success.
An excellent answer, Abdelhalim! I like it... and I agree with your observations. I especially want to thank you for the picturesque words with which you have portrayed my approach in explaining circuits; it is a great recognition for me. Thank you for the contribution.
Best regards,
Cyril
We, that we would like thank you for your tremendous efforts to make the circuit transparent and clear for the students. I believe very firmly in every word which i said about you. Really you deserve more than that. I feel that you spend great efforts to simplify the electronic circuits.
Thank you my colleague and friend.
Best regards.
Abdelhalim
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