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The brightness of an LED depends upon _____; when source voltage is much _____ than the diode threshold voltage, the brightness of the LED remains approximately constant.
forward current, higher
reverse bias, lower
junction capacitance, equal
ambient temperature, smaller
forward current, higher
The brightness of an LED is directly proportional to the forward current flowing through the device, as the photon emission process is dependent on the recombination rate of charge carriers. When the source voltage significantly exceeds the threshold voltage (knee voltage), the current becomes limited by the external series resistor, causing the LED to operate in a stable region where brightness remains constant for a fixed current.
The brightness of an LED is directly proportional to the forward current flowing through the device, as the photon emission process is dependent on the recombination rate of charge carriers. When the source voltage significantly exceeds the threshold voltage (knee voltage), the current becomes limited by the external series resistor, causing the LED to operate in a stable region where brightness remains constant for a fixed current.
LтИЭIfтАЛ тАФ Relationship between light intensity and forward current
IfтАЛ=RseriesтАЛVsourceтАЛтИТVthтАЛтАЛ тАФ Determination of forward current in a standard LED circuit
As the forward voltage VfтАЛ across the LED exceeds the threshold voltage VthтАЛ, charge carriers are injected into the junction region. The recombination of electrons and holes releases energy in the form of photons. The intensity of light L is proportional to the forward current IfтАЛ, expressed as LтИЭIfтАЛ. Once VsourceтАЛтЙлVthтАЛ, the circuit current IfтАЛ=RseriesтАЛVsourceтАЛтИТVthтАЛтАЛ determines the light output.
Brightness is directly proportional to the number of electrons recombining per unit time.
LEDs act as current-controlled devices rather than voltage-controlled devices.
A current-limiting resistor is essential to prevent thermal runaway and permanent damage.
Saturation of light intensity occurs due to non-radiative recombination at very high currents.
High luminous efficiency
Long operational lifespan
Fast switching capability
Requires constant current driver
Sensitive to temperature fluctuations
Indicator lamps
Digital displays
Optical communication links
Threshold voltages typically range from 1.8V for red LEDs to 3.5V for blue/white LEDs.
Option B is incorrect because reverse bias prevents current flow and inhibits light emission.
Option C is incorrect as junction capacitance affects switching speed, not steady-state brightness.
Option D is incorrect because ambient temperature typically degrades brightness due to increased non-radiative recombination.
A is correct тАФ The brightness of an LED is proportional to forward current, and it remains constant when the source voltage is significantly higher than the threshold voltage, provided a current-limiting resistor is present.
Always remember that LEDs are current-driven devices; treat the LED voltage drop as nearly constant (VfтАЛ) in circuit analysis problems.