Lab date: 10/25/95
Subject: Nifty Diode Circuits
From: Mike Gmiterek and Bryan Mathewson
In lab we observed a 1kŻ resistor in series with two 1N4728A diodes, one in forward bias and the other diode is in reverse bias. (See figure 1) When applying a 4V (peak) sinusoidal voltage as the input voltage, it was observed that Vout across the two d iodes displayed an output resembling a sinusoidal wave. The peak of the output was not as sharp as the input. (See figure 1) As we increased the magnitude of the voltage the peak leveled off even more, distancing itself from the original sinusoidal vol tage. According to the characteristics of the output voltage, we concluded the circuit was a voltage limiter. The reason the diodes were put in opposite bias of each other was that it doesn't matter which type of voltage was applied reverse or forward b ias. As soon as enough reverse or forward bias voltage is applied one of the diodes would experience reverse breakdown. When this happens the diode that is in reverse breakdown allows current to flow through Z1 and Z2. The lack of current to Vout causes the clamping effect at reverse breakdown. The transfer characteristics (See figure 3) shows that Vout over Vin looks like the IV characteristics of a resistor in the mid region of figure 3. When the diodes are turned on, however, we see that the ratio becomes constant in both the forward and reverse bias regions. By choosing different zener diodes with different reverse breakdown voltages, we can adjust how much the voltage is limited by our clamping circuit.
FIGURE 1: CLAMPING CIRCUIT DIAGRAM
FIGURE 2: Vin and Vout for Clamping Circuit
FIGURE 3: TRANSFER CHARACTERISTICS
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