As seen in Figure 1, the circuit is a 1 k ohm resistor in series with a diode and DC voltage source of 1 V. The diode is oriented such that when the input voltage is positive, the diode is in forward bias. A 1N4007 silicon diode was used in the physical measurement of the circuit. The Vout is measured across the diode and the 1 V DC voltage source. The Vout as a function of time is shown in Figure 2.
The Vout curve is in phase with the AC input sinusoid, but is cut off, or limited, at approximately 1.7 V. This behavior is easily explained using the constant voltage drop model of the diode. On the most basic level, a p-n junction diode acts as a shor t circuit when the voltage placed across it is greater than its characteristic turn on voltage. When this input voltage is less than the turn on voltage, the diode represents an open circuit, according to the model. Thus, no voltage is dissipated across the diode. For this circuit then, the output voltage would be the same as the input voltage. Adding the 1 V DC source in series with the diode increases the Vout turn on voltage required to achieve a limiting state. The input voltage then must be grea ter than 1.7 volts for this to occur. This explains the rectifying behavior observed in Figure 2 above.
The 1.7 V limiting value is the most important characteristic of this circuit. The transfer characteristic of Figure 3 best shows this voltage limiting effect. For input voltages less than approximately 1.7 ·V, the relationship is linear with a slope of 1. The linearity is due to the fact that the diode approximates an open circuit for voltages less than the turn on voltage, according to the constant voltage drop model. The ratio of output to input voltage is 1:1 then, indicating that they are equal. When the input voltage is greater than 1.7 V, the output voltage is limited to 1.7 V, because the input voltage is high enough to overcome the turn on voltage of the diode - DC source combination. The diode- DC source combination then is effectively a s hort circuit, and no additional voltage is dropped across it. No matter how much higher the input voltage becomes, the output voltage stays at 1.7 V. Thus, the circuit acts as a voltage limiter for forward bias voltages greater than the circuit's turn o n voltage. By choosing different values for the DC source voltage, the output voltage can be limited at different levels. This circuit becomes important in designing many signal processing systems that require specific limited voltages.
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