Astable Square Wave Generator

If a transistor can be arranged to have delayed negative feedback, similar to the relay buzzer example on page 138, it can oscillate continuously. This is called astable, meaning not stable. That is, it will not remain in one state, but instead it flips back and forth between on and off.  

With a transistor, if the feedback was immediate, there would be no oscillation. Instead, the negative feedback would simply decrease the degree to which the transistor would turn on, so it would only go part-way on and then remain with that partial status. In fact, this does happen with certain other types of amplifier circuits (to be studied later) but not with multivibrators. However, if the feedback is delayed enough for the transistor to turn on completely before 

It is possible to have immediate positive feedback, and also have the delayed negative feedback, and in fact those two actions are what we will get in the first circuit to be assembled in this chapter. The positive action makes the circuit switch (go completely on or off) very fast, once it starts. Because the switching is enhanced by the positive feedback, the output can be a square wave, where the voltage goes from zero up to 9 volts quite fast (once it starts to change), hold there because of delay, and then go down to zero very fast. 

In the circuit of Fig. 17.1, current goes through the low-resistance bulb, through the capacitor, and into the base of the left-hand transistor, turning it on. 

Since it is turned on completely, the + voltage on the left-hand capacitor is practically zero, so there is no current going into the base of the right-hand transistor, and it is turned off 

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