Multivibrator

The multivibrator emerged from WWI's urgent need for reliable wireless communications. In 1918-1919, French physicists Henri Abraham and Eugene Bloch developed the astable multivibrator—a relaxation oscillator using two cross-coupled vacuum tube triodes producing square waves. They called it "multivibrateur" because its output contained rich harmonic content, invaluable for calibrating wavemeters.

Simultaneously, British physicists William Eccles and Frank Jordan invented the bistable multivibrator (patented June 21, 1918). Their trigger circuit connected two triodes in cross-coupled configuration where each tube's grid connected to the other's plate. This created a circuit maintaining one of two stable states after a trigger pulse—a binary switch without moving parts.

The multivibrator required vacuum tube triodes, perfected by Lee De Forest in 1906. The triode's three electrodes enabled amplification and switching—a control electrode could modulate electron flow with minimal power. Cross-coupling—feeding each tube's output back to the other's input—created feedback loops that either oscillated continuously (astable) or latched into stable states (bistable).

Three configurations emerged: Astable (free-running, no stable states—continuous oscillation for clock sources); Bistable (flip-flop, two stable states—the fundamental memory element storing one bit); Monostable (one-shot, one stable state—used for pulse generation and timing).

When Claude Shannon formalized Boolean logic for electronic circuits in 1937-1938, the bistable multivibrator became physical binary memory. ENIAC (1945) relied heavily on flip-flops from 6SN7 dual triode tubes. Before mercury delay lines and magnetic core memory, flip-flops were the only way to store binary data electronically. Every modern computer, smartphone, and digital device descends from this 1918 invention—SRAM cells and processor registers implement bistable multivibrators as cross-coupled transistor pairs.