EE 330 Experiment 7 Fall Diodes and Diode Applications

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1 EE 330 Experiment 7 Fall 2007 Diodes and Diode Applications Objectives: The objective of this experiment is to develop familiarity with diodes and diode applications. The relationship between the actual diode and the ideal diode will also be investigated. Background Information: There are several types of diodes that are widely used in the industry today. When diodes are initially introduced, they are generally in the context of a nonlinear device that ideally passes current in one direction and blocks current in the opposite direction. The I-V characteristics of such a device are shown in Fig. 1. Id Fig. 1 I-V Characteristics of an Ideal Diode Diodes that are fabricated with an intended use that takes advantage of these nonlinear characteristics are often termed signal diodes or rectifier diodes. Other types of diode exist that have some properties that are similar to those shown in Fig. 1 but other properties that are significantly different. Five of the more popular alternative diodes are zener diodes, light emitting diodes, photo diodes, Schottky Diodes, and Varactor Diodes. The property of passing a current in one direction and blocking it in another direction are seldom of interest in zener diodes, light emitting diodes, photo diodes, and Varactor Diodes. The use of Schottky diodes is similar to that of signal diodes. Zener diodes ideally have an I-V characteristic shown in Fig. 2 and are intentionally operated under reverse bias in the breakdown region. The major application of zener diodes is in building voltage references. Light emitting diodes (LEDs) are used primarily as indicator lights or in place of small light bulbs. In contrast to incandescent or other forms of light bulbs, LEDs invariably have a very long useful operating life, have very rapid turn-on and turn-off times, are very efficient at converting electrical Page 1 of 7

2 energy to light, and are very rugged. The symbol for the LED is shown in Fig. 3. The LED device is almost always under forward bias and for typical currents, the diode voltage is around 1V. Photo Diodes are used as light detectors. They also can respond very rapidly and are often the detectors that are used at the end of a fiber cable in which Boolean data is transmitted. They are almost always used under reverse bias and the signal information is carried in the reverse saturation current that flows. The symbol for a PhotoDiode is also shown in Fig. 3. The symbol for the Schottky diode is not standard in the industry. Some vendors do not use symbols that distinguish between a signal diode and a Schottky diode. Others use one or more variants of the diode symbol shown in Fig. 4. Fig. 2 Zener Diodes d d Fig. 3 Light-sensitive Diodes Page 2 of 7

3 Fig. 4 Various Symbols for the Schottky Diode With the exception of the Schottky diodes, essentially all diodes are constructed with a p-n junction. The properties of the diode are strongly dependent upon the materials used to create the junction, the grading of the materials in the junction, the physical size of the junction, and the enclosure in which the junction is encapsulated. Schottky diodes are formed with a rectifying metal-semiconductor junction. Schottky diodes generally operate faster and with a smaller offset voltage (to be discussed later) than junction signal diodes. In this experiment we will be focusing on junction and rectifier diodes and on LEDs. In what follows we will not distinguish between junction and rectifier diodes. The characteristics of a typical junction diode are shown in Fig. 4. It can be seen that this diode conducts little current for < 0.5 V and the current gets very large for >.65 V. The I-V characteristics of the diode can be modeled by the diode equation Diode Characteristics Id (amps) (volts) Fig. 4 I-V Characteristics of Typical Junction Diode nvt I = 1 D I S e (1) where I S is a constant characteristic of the diode, Vt=kT/q where k is Boltzman s constant, T is temperature in Kelvin, n is a constant that depends upon how the diode is made, q is the charge of an electron. k/q= 8.63E-5V/ K and at room temperature Vt is Page 3 of 7

4 approximately 25mV. The constant I S is often in the range of 10fA to 100fA. The parameter n is typically 1 for integrated diodes and varies between 1 and 2 for many discrete diodes. In the lecture portion of the course we assumed that n=1 but since we will be using discrete diodes in this experiment it is necessary to introduce the slightly more complicated model of (1) for this experiment. The diode equation is quite unwieldly and in many applications, particularly applications where the voltages in the circuit around the diode are in the 10V range or larger, the ideal diode model of Fig. 1 is adequate. If lower voltages are used, a piecewise linear model of the diode is often used. This is shown with the thick blue curve in Fig. 5. Diode Characteristics Id (amps) (volts) Fig. 5 Piecewise Linear Model of Diode This model can be expressed mathematically as I = 0 V < 0.6V V = 0.6V I > 0 d (2) Part 1 Measure I S and n for the 1N4148 diode and compare with the values predicted in the datasheet. A plot of the I-V characteristics for the 1N4148 diode can be found in the attached manufacturer s data sheet and one of the key figures from the datasheet is repeated below. Extract these parameters at current levels between 0.5mA and 5mA. In these measurements, it is not a good idea to put a dc voltage source across the diode because small changes in this voltage can cause large, possibly destructive changes in the diode current. Plot the I=V characteristics predicted by the diode model for currents between.1ma and 10mA and compare with those in the data sheet. From the model you extracted, predict the current you expect to flow at a diode voltage of 0.6V and compare with what is measured. Page 4 of 7

5 Hint: The following circuit may prove useful for helping measure the relationship between I D and V D for a diode. Hint: From the diode equation observe that if the current is measured at two voltage levels, then you can write the following two equations 1 nvt I = 1 D1 I S e 2 nvt I = 1 D2 I S e I-V Characteristics for the 1N4148 Diode Page 5 of 7

6 Part 2 Design and test a simple diode rectifier circuit. This circuit should provide the input signal across a 1K load resistor when the input voltage is positive and should provide an output voltage of 0V when the input is negative. Test the circuit with a 100Hz sinusoidal signal with 0-peak value of 1.5V and a 0-peak value of 10V. Compare theoretical and experimental results. Part 3 The cut-in voltage of the diode limits the performance of a simple diode rectifier circuit. This should have been very noticeable when the small input was applied in Part 2. A precision rectifier circuit can be used to reduce this effect. Design and test a precision rectifier circuit when operate with the same test conditions as used in Part 2 and compare theoretical and experimental results. A precision rectifier circuit is shown below. Variants of this concept are widely available. Note the output voltage of the precision rectifier circuit is not at the output of the op amp and that a buffer op amp is needed at the output if a significant load is to be drawn from the circuit. V DD V IN V SS D 1 R 1 V OUT Page 6 of 7

7 Part 4 Light emitting diodes are often used for indicator lights and often come in different colors. A series resistor is generally required to limit the current of a LED. Indicator diode circuits are shown below with the current limiting resistor R LIM. The reverse breakdown voltage of an LED is often not large so a signal diode is often placed in series with an LED to increase the effective reverse breakdown voltage. The second circuit shown below has a series signal diode to increase the effective breakdown voltage. RED and GREEN LEDs are available from the lab instructor. Assume the maximum forward current that can flow in the LEDs without damage is 15mA and that the reverse breakdown voltage can be as low as 2V. a) Design and test an LED indicator circuit that will turn on a red LED when the input voltage is between 4V and 5V and turn the LED off when the input voltage is between 0V and 0.5V. b) Design and test a circuit that will turn on a RED LED when the input voltage is nominally 5V and that will turn on a green LED when the input voltage is nominally 0V. D S R LIM RLIM Page 7 of 7

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