FEEDBACK AND OSCILLATION

On the top of page 137, positive feedback in a relay system was seen to lead to a latched condition, where the relay continues to feed operating signals back to itself and therefore continue in one position, until the current is somehow interrupted by some other action. It should be noted, however, that when a relay armature operates, it hits the normally open contact, which is a hard barrier to any further motion. It might bounce back a little bit, but it can not overswing. Therefore, positive feedback leads to stable latching and no further motion. 

Transistors are so fast that their natural delay times are in the billionths of a second, so we ordinarily use capacitive delays to produce oscillations with somewhat longer time constants. (For example, to produce a musical note that we can hear, the delay should be roughly a thousandth of second.) We can use either delayed negative feedback (bottom of next page) or fast positive feedback (top of page 182). With transistor circuitry, it is more often directly positive. 

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