THE CURRENT-SWITCHING PRINCIPLE - THE VERSATILE LONG-TAILED PAIR

The circuit technique known as current-switching is versatile and fund amental to the operation of digital systems working at high digit swit ching rates. For these reasons, it merits close study by engineering s tudents, particularly those intending to become proficient in circuit design. This paper presents a tutorial-review of the long-tailed pair current-switch using bipolar junction transistors. A brief historical survey sets the scheme in context. Then, a dc characterization, employ ing a simplified equivalent circuit, is followed by a first-order stud y of the transient response based on the use of a charge-control model of the transistor. Finally, a miscellany of applications is listed to illustrate the adaptability of the scheme to the design of logic and waveform generation and shaping circuits.