|
A multivibrator is an electronic circuit used to implement a variety of simple two-state systems such as oscillators, timers and flip-flops. It is characterized by two amplifying devices (transistors, electron tubes or other devices) cross-coupled by resistors or capacitors. The name "multivibrator" was initially applied to the free-running oscillator version of the circuit because its output waveform was rich in harmonics.〔''Multivibrator'' in ''IEEE Std. 100 Dictionary of Standards Terms 7th ed.'',IEEE Press, 2000 ISBN 0-7381-2601-2 page 718〕 There are three types of multivibrator circuits depending on the circuit operation: *Astable, in which the circuit is not stable in either state —it continually switches from one state to the other. It functions as a relaxation oscillator. *Monostable, in which one of the states is stable, but the other state is unstable (transient). A trigger pulse causes the circuit to enter the unstable state. After entering the unstable state, the circuit will return to the stable state after a set time. Such a circuit is useful for creating a timing period of fixed duration in response to some external event. This circuit is also known as a one shot. *Bistable, in which the circuit is stable in either state. It can be flipped from one state to the other by an external trigger pulse. This circuit is also known as a flip-flop. It can be used to store one bit of information. Multivibrators find applications in a variety of systems where square waves or timed intervals are required. For example, before the advent of low-cost integrated circuits, chains of multivibrators found use as frequency dividers. A free-running multivibrator with a frequency of one-half to one-tenth of the reference frequency would accurately lock to the reference frequency. This technique was used in early electronic organs, to keep notes of different octaves accurately in tune. Other applications included early television systems, where the various line and frame frequencies were kept synchronized by pulses included in the video signal. == History == The classic multivibrator circuit (also called a ''plate-coupled multivibrator'') is first described by Henri Abraham and Eugene Bloch in ''Publication 27'' of the French ''Ministère de la Guerre'', and in ''Annales de Physique 12, 252 (1919)''. It is a predecessor of Eccles-Jordan trigger〔William Henry Eccles and Frank Wilfred Jordan, "(Improvements in ionic relays )" British patent number: GB 148582 (filed: 21 June 1918; published: 5 August 1920).〕 derived from this circuit a year later. Historically, the terminology of multivibrators has been somewhat variable: * 1942 – multivibrator implies astable: "The multivibrator circuit (Fig. 7-6) is somewhat similar to the flip-flop circuit, but the coupling from the anode of one valve to the grid of the other is by a condenser only, so that the coupling is not maintained in the steady state." * 1942 – multivibrator as a particular flip-flop circuit: "Such circuits were known as 'trigger' or 'flip-flop' circuits and were of very great importance. The earliest and best known of these circuits was the multivibrator." * 1943 – flip-flop as one-shot pulse generator: "It should be noted that an essential difference between the two-valve flip-flop and the multivibrator is that the flip-flop has one of the valves biased to cutoff." * 1949 – monostable as flip-flop: "Monostable multivibrators have also been called 'flip-flops'." * 1949 – monostable as flip-flop: "... a flip-flop is a monostable multivibrator and the ordinary multivibrator is an astable multivibrator." 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Multivibrator」の詳細全文を読む スポンサード リンク
|