Starting Network for Astable Multivibrator


A flip-flop that generates two unstable states (high-low) is called astable multivibrator, a fancy name for you to say it’s an oscillator. The very basic of such circuit consist of two transistor where the collector of each transistor is wired in such way they try to short the base of the other transistor through a capacitor. At the beginning of the powering up the circuit, all transistor will be in open state and both transistor’s base will be pulled-up, and the slight different of the transistor’s characteristic would cause one transistor tend to fired first and short the other transistor base through a capacitor, making it turned on until the capacitor is saturated and give a turn for the other transistor to swap the state. Here is a schematic diagram of astable multivibrator circuit with a starting network to ensure the circuit will always be astable:

Starting Network for Astable Multivibrator

The starting network is built around two 1N4148 diodes and a 0.22uF capacitor.  Without this starting network, a simple astable multivibrator/flip-flop circuit  could lead to a lock-up failure   at the power start up, especially when dealing with slow rising voltage power supply.


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