Nifty Diode Circuit 1: The Voltage Limiting Circuit


To: Professor Lord

From: Jose Carbajal and Brian Cagno


Objective

The objective of this lab was to explore the functions of nifty diode Circuit 1.

Procedure

The first part of the lab consisted of building the circuit shown in Figure 1, and experimenting with various features of the circuit in order to get some insight on how the circuit works.

Analysis

After analyzing the circuit at a 4V (peak) sinusoid wave and, at a frequency of 100Hz, we discovered that the circuit had a clipping characteristic as shown in Figure 2 , which shows the output as a function of time. Because th e output voltage is less than the input voltage above 1.7V, a better name for the circuit would be a "Voltage Limiting Circuit". Clipping occurs because the diode is not a perfect device, meaning that it has a resistance built into it. Since current is going through this device, the resistance causes a voltage drop, which for a typical voltage Si diode is .7V. Therefore, the total voltage drop from A to B on the circuit, Figure 1, is 1.7V. When Vin is equal to 1.7V, the diode is in forward bias and acts like a short. This short holds Vout at 1.7V at any input voltage above this value. When the voltage drops below 1.7V, the diode is in reverse bias and it can be modeled as an open circuit. Thus, Vout is equal to Vin at any voltage below 1.7V. Using the XY mode on the oscilloscope, the graph shown in Figure 3 was produced, which enables the transfer characteristics to be viewed. As Vin increases, Vout also increases until the input voltage reaches 1.7V, which is when Vout levels off as can be seen in Figure 4, a graph of Vin verses Vout.

Next, we analyzed the frequency characteristic for the "Voltage Limiting Circuit". At a frequency of 2MHz, the output which was once level at 1.7V began to rise to about 2V, then slowly slanted down to about 1.6V where it once again leveled off. Refer to Figure 5, which shows a graph of Vout verses Vin with Vin set at 8Mhz to exaggerate the shifting properties. At a frequency of 450kHz, the output shifts to the right, and the most negative region of the output signal shifts up wards.

Switching the polarity of the diode greatly changed the graph of Vout vs. Vin to Figure 6. When the polarity of the diode was switched, we observed that the output voltage did not drop below .3V. The voltage across the diode ( .7V) needed to be subtracted from VBIAS (1V). When Vin was less than .3V, the diode was in forward bias, which held the output voltage at .3V. When Vin was greater than .3V, the diode was in reverse bias making Vout equal to Vin (short circuit).

We also experimented with the type of input going into the circuit. When a triangle wave was put in, the same characteristics that were present in the sine wave were present in this signal as well. It would level off at the same voltages, and also shif t at the same, previously mentioned frequencies when a sine wave was used as the input to the circuit. Similar results were also observed when a square was used as an input, which is shown in Figure 7. This graph shows Vout ver ses Vin when the input is a square wave.

Varying other values of the circuit also effect the functioning of the current. When VBIAS varied, a shift where the diode changed from reverse bias to forward bias was observed. This changed where the output voltage levels off. Besides varying VBIAS, we also varied the amplitude of the input sine wave. This variation had no effect on the shape of the output signal. Changing Rload did have an effect, however. The output voltage leveled off at a smaller voltage, 1.5V for a 51kilo ohms resistor, whic h is seen in Figure 8.

Conclusion

Nifty Diode Circuit 1 has some properties that maybe useful in certain applications. It is easy to alter the circuit to obtain the desired characteristics such as varying VBIAS in order to change where the peak voltage levels off. It is also important t o realize there are limitations to the frequency input as shown in the analysis of the frequency characteristics for the "Voltage Limiting Circuit". It is always good to know the characteristics of the circuit so it can be manipulated and applied to a sp ecific application.


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