Understanding the Behavior of Diodes Under Forward and Reverse Bias
Introduction: Diodes, a fundamental component in electronic circuits, function based on their biasing conditions. When a diode is forward biased, it allows current to flow, while in reverse bias, the current is blocked or limited to a very small value. This article aims to explain the behavior of diodes as voltage varies, specifically within the range from 0 to 12 volts, and how to interpret the biasing state of diodes under these conditions.
Diode Operational Characteristics
Most silicon diodes turn-on at around 0.6 to 0.7 volts, at which point they begin to conduct significant current. Below 0.6 volts, the diode is not forward or reverse biased, but it does conduct a negligible amount due to thermal effects. This minimal conduction is considered insignificant until the 0.6-0.7 volt threshold is reached.
Unbiased State: 0V
At 0 volts, the diode is not biased at all. The diode can conduct a tiny current with any positive voltage, but this is negligible until the forward voltage threshold is met. When the anode is positive and the voltage exceeds around 0.5 volts, the diode starts to conduct. Once this transition occurs, the diode is considered forward biased.
Forward Bias: VI > 0.6V - 0.7V
As the current is increased above the forward voltage threshold, the forward voltage required to maintain this current increases slightly. If the polarity is reversed, the diode conducts tiny leakage currents, which vary with temperature. The diode is reverse biased under these conditions. If the reverse voltage is increased to the diode's reverse breakdown voltage, it becomes a zener diode, where the voltage does not increase significantly. However, if the reverse current is increased further, the diode exhibits a drastic increase in current.
/Application in Practice: Vi from 0V to 12V
Considering the situation where VI ranges from 0V to 12V, let's analyze the behavior of a typical silicon diode (D1) and a zener diode (D2) in the given circuit.
D1 - Silicon Diode
For a typical silicon diode, the forward voltage (VF) is around 0.7 volts. Assuming VI goes from 0V to 12V with no load on the Vo terminal, D1 will start to conduct when VI exceeds 0.6V - 0.7V. This means D1 will be forward biased when VI reaches 0.7V or more.
D2 - Zener Diode
Assuming the breakdown voltage of D2 is greater than 14 volts, D2 will remain in the reverse biased state throughout the voltage range because the forward voltage threshold (0.7V) is always below 14V, and therefore, D2 will not conduct.
Analysis for Vi
When Vi goes negative, D1 will not conduct (it will be in reverse bias). D2 will conduct when Vi exceeds -2.7 volts (the zener voltage of a typical silicon diode), indicating a transition from reverse bias to forward bias for D2.
Incorporating Load Resistance
If the Vo terminal has a load that draws current, the voltage drop across a 1 kΩ resistor due to the current must be accounted for. This means the actual voltage at the Vo terminal will be less than the applied voltage, and the biasing state of the diodes will be affected accordingly.
Conclusion
The behavior of diodes under forward and reverse bias can be understood through their operational characteristics and the applied voltage. The forward voltage threshold (typically between 0.6V and 0.7V for silicon diodes) is crucial in determining when the diode conducts significant current. Understanding these concepts helps in designing and analyzing electronic circuits effectively.