Experiment 14 - Feedback

OR gate with feedback.

Consider again an OR gate. In the diagram shown the result Q is connected to the input of the same gate. This is called feedback.

Initially, this logical circuit behaves quite normal...

• If A = 0, then Q = 0 if, and only if the value of Q had already been 0.
• Turns A = 1, then Q = 1.

Circuit of an OR gate with feedback. (Enlarge)

So far so good. But now something new happens. You can no longer influence the outcome of Q. No matter what value you put to the input A, the result remains Q = 1...

An OR gate with feedback stores if ever was A = 1.

With the circuit shown you build this "storage unit".

Once you understand the principle of feedback you can move to circuits with more practical use...