"Feedback is a process in which information about the past or the
present influences the same phenomenon in the present or future"
I find the idea of feedback quite fascinating and i could realize how universal it is ( applying not only to EE but to society, biological systems,etc ).
But for now, i'm focused on EE.
I'm interested in trying to capture all kinds of feedback used in electronical circuits ( of whatever kind, whether digital or analog ), so that i have the ability of looking a feedback in any electronical circuit and know which task its implementing there.
So far i got three different kinds of feedback.
1 – Positive feedback that tries to reinforce the value of the input as time passes. It is used in any circuit that implements hystheresis, in multivibrators, oscilators, etc …
2 – Negative feedback that tries to drive the output value to desired/reference values. It is used in any control system.
3 – The kind of feedback used in sequential circuits, which acts as a way of the combinational circuit and the inputs to make the states evolve.
Now i'm curous … My question is :
Are there more kinds of feedback other than those 3 being used in EE circuits or these three kinds would cover all kinds of feedback used in EE circuits ?
Thanks a lot in advance
Best Answer
Positive feedback circuits (e.g. a Schmitt trigger) are in unstable equilibrium, the same as a ping pong ball on top of an upside-down plastic cup. You only have to move the cup one way or the other a little bit and it wants to roll off. Assuming the cup has a large lip and the ball doesn't roll off the end, then you can make it go the other way by moving the cup the other way, but the ball wants to stay at the ends - it's hard to make it stay in the middle.
This applies to multivibrator circuits, that either oscillate from one side to the other (astable, like a clock) or wait for an input (monostable, like a pulse generator or bistable, i.e. a flip-flop). This feedback is all about reinforcing a state for constant inputs and switching it to the other state for changing inputs, which can be used to create memory and state machines (i.e. sequential logic circuits, like you mention). A flip-flop for example can be used to store 1 bit of information (you can check the last position (i.e. state) you left the ball in - left or right).
A negative feedback circuit (e.g. amplifier, active filter) is the opposite - it's a stable equilibrium system. Now the cup is turned round (the right way up) and the ball is in it. You can move the ball to the edges of the cup by moving the cup either left or right, but once you stop it will return to the centre, assuming you don't exceed the limits of the system (which would be dependent on the centre of mass in the ball example). This type is about control/processing.
In an amplifier, for example, the output wants to be zero. An increasing input (in either direction) causes a increasing output (with some gain possibly), which is fed back and taken away from the input (as opposed to adding it in the case of positive feedback), making the output smaller. You have to keep increasing the input or it will go back to zero.
Another example is automatic gain control (AGC), where you might have a radio receiver amplifier with a variable gain. The amplifier output is fed back to a control circuit that adjusts the gain of the amplifier depending on the strength of the received signal, thus maintaining a constant output. Other control examples include applications where there is fine-tuning of movement (like a robot hand that grips objects without crushing them by sensing the exerted pressure, feeding it back and adjusting the gripping force, converging on a constant value).