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This word is rarely discussed in books and magazines in the area we are about to describe. It's a very clever and very important concept to understand.
It applies to a circuit that turns itself ON more and more due to the output signal (from the circuit) being passed to the input. 
There are many reasons why this is done and one of the earliest reasons was for a radio receiver, to make it more sensitive.
Simple feedback - sometimes called negative feedback is not regeneration - however it may be called de-generation.
In this example we study three examples in which the feedback increases the "turn-on" of the circuit to such a point that the circuit cannot turn on any more.
We normally think of "feedback" as "negative feedback," but it can also be positive feedback and this should be called "feed-forward."
But the term "feedback" really means a signal from the "end of the circuit" being passed to the "front of the circuit."
Regeneration can create a "run-away" situation where the circuit breaks into oscillation or some other unwanted effect.
In our cases, it produces a very valuable effect.

The first circuit we will study is a Schmitt Trigger. It has been taken from our BEC course, page 32. The 10k pot is set so that the first transistor is turned on when the thermal probe is cold.

The thermal probe has a Negative Temperature Coefficient and this means the resistance will decrease as the temperature rises. The actual rate of change is not important however if the probe is placed into hot water, the resistance will be about 6k. As the temperature on the probe rises, the voltage on the base of the first transistor reduces and it begins to turn OFF. This will begin to turn ON the second transistor and the relay will start to be energised.
But the interesting point is the feedback line between the two emitters and the 3R9 resistor.

Current will flow in the collector-emitter of the second transistor and also in the 3R9 resistor. This will create a very small voltage across the 3R9 of say about 0.1v.
This voltage also appears on the emitter of the first transistor and this means the voltage between base and emitter of the first transistor is reduced. This causes the first transistor to turn off more and the second transistor turns on more. This continues until the first transistor is fully turned off and the second transistor is fully turned ON.
This action has occurred without any change in resistance of the probe.
In fact the circuit has "snapped" into action and activated the relay fully at the instant the circuit detected the precise resistance of the probe.  
In other words the circuit has created a REGENERATIVE action where the first transistor gets turned off MORE AND MORE until the two transistors have SWITCHED states VERY QUICKLY.
The point at which the circuit changes is due to the value of the emitter resistor (3R9). This resistor creates the hysteresis (the gap between the high and low temperature) and the actual low temperature is determined by the value of 2k2 resistor.

The action of the circuit is such that the relay will open or close very quickly at a defined temperature, even though the temperature does not change.

The action of the two transistors creates a SWITCHING EFFECT where the output changes very rapidly from one state to the other. 
This is very important if you are driving a relay as it prevents it "chattering" and burning-out the contacts. The secret in the snap action lies in the feedback line between the two emitters.

The circuit is designed to turn off a heater or other device when the temperature reaches a certain value.

The second circuit in this discussion is a LED Flasher. The circuit flashes a LED very brightly for a very short period of time, at a rate of about 2 flashes per second. The circuit is actually a very high gain DC amplifier with some added components.
By removing the 330k and 10u electrolytic, we can see the two transistor DC amplifier.
If the 330k is fitted, the LED will illuminate. In fact, the 1k and 10n are not needed to see the LED illuminate, and the 22R current limiting resistor is a very low value and will cause a very high current to flow through the LED, so the circuit should not be operated for any length of time as a DC amplifier.
By adding the 10u electrolytic, the output of the circuit is detected and passed to the input.
Now, let's go over the operation of the circuit again.
Remove the 330k. The circuit will sit in a non-operating mode.
Fit the 330k.
The 330k will turn the circuit on but it will do this slowly because the 10u is also connected to the base of the first transistor and the electrolytic has to charge to allow the voltage on the base to rise.
When the voltage on the base rises to 0.65v, the first transistor begins to turn on and this turns on the second transistor.
Current flows in the collector-emitter circuit of the PNP transistor and this causes a voltage to appear across the 22R resistor. This voltage is passed to the base of the first transistor via the 10u, and now the base of the PNP transistor sees more energy to turn it on. This effect is passed to the second transistor and very quickly both transistors turns each other on more and more until they are both fully turned ON.
This effect occurs without any more input from the 330k resistor. In fact the 330k can be removed as soon as the two transistors start to turn each other on.
Thus the circuit turns itself on by an effect called positive feedback. This effect is also known as REGENERATION.
The 330k must be replaced to start the cycle again and the rest of the description is covered in our Flasher Circuits article.  The main point to note is the effect of REGENERATION. Once the action starts, it does not stop until the circuit is fully turned ON.

The third circuit uses feedback from a transformer to turn the circuit ON to a point where it is fully turned on. It is taken from our LED Torch Circuits article. The cycle starts with the 2k7 resistor feeding current into the base of the transistor. This starts to turn the transistor on and current flows in the 60 turn winding and produces magnetic flux that cuts the turns of the 40 turn winding. The 40 turn winding produces extra voltage that adds to the original voltage and this allows extra current to flow into the base of the transistor to turn it on more.
This continues until the transistor is fully turned on. This action is called positive feedback or more accurately REGENERATION.

The three circuits operate in exactly the same mode. This made is called a SWITCHING MODE.  They change from one state to another VERY QUICKLY.
This action is called a SWITCHING ACTION or DIGITAL ACTION or DIGITAL MODE. There are basically two types of circuits, DIGITAL CIRCUITS and ANALOGUE CIRCUITS (also called audio circuits). An audio circuit operates over a smooth range of low output to high output. A digital circuit goes from one state to the other very quickly.
When this change is produced by the components within the circuit, the action is called REGENERATION because the action cannot be stopped and takes the transitor(s) from the state of not being turned on to the state of being fully turned on.

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