The current-switching principle-the versatile long-tailed pair

The circuit technique known as current-switching is versatile and fundamental to the operation of digital systems working at high digit switching rates. For these reasons, it merits close study by engineering students, 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, employing a simplified equivalent circuit, is followed by a first-order study 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.