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Engineering LibreTexts

16.1: Theory Overview

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  • LEDs behave similarly to switching diodes in that they conduct current easily in forward bias and appear as an approximate open circuit when reverse biased. Unlike standard silicon switching diodes, however, the forward bias potential is not approximately 0.7 volts. Instead, this potential will vary depending on the design of the LED but typically will be in the neighborhood of 2 volts for everyday devices. The brightness of the LED is directly controlled by its current: the higher the current, the brighter the LED. Consequently, it is important to drive LEDs with constant current sources to ensure consistent brightness.

    Many circuits cannot drive LEDs directly so an intervening circuit is used (a driver) to boost the current up to the value the LED requires for a given brightness. The driving signal is attached to the base while the LED is situated in the collector, thus the transistor’s current gain, beta, is exploited. Unfortunately, beta is not a particularly stable and consistent parameter so methods are required to alleviate this shortcoming. A saturating switch works by operating at the extreme ends of the DC load line; that is, either cutoff or saturation. If no signal is applied to the base, both base and collector currents will be zero, and thus, the LED is off. When a base signal is applied, it is designed to be large enough to force the transistor into saturation even with very modest betas. Therefore, the LED will always see the saturation current, regardless of the normal beta value. In contrast, the non-saturating circuit works by placing a resistor in the emitter. This establishes a constant emitter current (and thus, constant collector and LED current) in spite of beta changes. That is, if beta changes, the effect is seen in the base current, not the collector current. The non-saturating circuit has the advantage of using one less resistor, however, the saturating switch has the greater advantage of using the same collector and base voltages (the nonsaturating circuit requires a collector source potential at least a few volts greater than the base voltage).

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