In the context of a diode's I-V (current-voltage) curve, "series resistance" refers to the effect of an external resistor connected in series with the diode. This resistor, also known as the "load resistor," is a real-world element that influences the diode's behavior in a circuit.
When we plot the I-V curve of a diode, it shows the relationship between the current flowing through the diode and the voltage applied across it. In an ideal diode, the curve is characterized by zero current flow until a certain threshold voltage is reached. Once the threshold voltage is exceeded, the current increases rapidly with a small change in voltage, indicating that the diode becomes highly conductive.
However, real-world diodes have inherent resistance, known as "dynamic resistance," that causes the I-V curve to deviate from the ideal behavior. Additionally, when a diode is used in a circuit, it often operates with an external series resistor.
The presence of a series resistor affects the diode's I-V curve in the following ways:
Voltage drop across the series resistor: When current flows through the diode, it also flows through the series resistor. According to Ohm's law (V = I * R), this current creates a voltage drop across the resistor. As a result, the voltage applied to the diode is reduced by the amount of voltage drop across the series resistor.
Flattening of the curve: The series resistor limits the maximum current that can flow through the diode. As a result, the I-V curve becomes flatter, showing a less steep increase in current with increasing voltage compared to an ideal diode.
Threshold voltage shift: The series resistor can cause a shift in the effective threshold voltage of the diode. The actual threshold voltage required to start significant conduction may differ from the ideal value due to the voltage drop across the series resistor.
Reduced maximum current: The series resistor restricts the amount of current that can flow through the diode at a given voltage. Consequently, the maximum current level attained in the I-V curve will be lower than what an ideal diode without series resistance would exhibit.
Overall, the presence of series resistance affects the diode's performance and should be considered when designing and analyzing electronic circuits that involve diodes. The series resistor is often utilized to limit current and protect the diode from excessive currents, particularly in applications like voltage regulation and current control circuits.