UNIVERSITY OF NORTH CAROLINA AT CHARLOTTE Department of Electrical and Computer Engineering Experiment No. 2 - Semiconductor Diodes Overview: In this lab session students will investigate I-V characteristics of regular and zener diodes and use diodes in basic circuits. The diode is the simplest and most fundamental nonlinear electronic element. In addition to being an important electronic device, it forms the basis for nearly all other solid-state devices. Figure 1 shows the physical arrangement of the P-N junction diode and its electrical symbol. When p-type and n-type materials are placed in contact with each other, current will flow readily in one direction (forward biased) but not the other (reverse biased) creating the basic diode, a one-way current valve. One of its many applications is rectification as required in DC power supplies. Forward biased diode (a) Reverse biased diode (b) Figure 1. The P-N junction diode showing (a) the diode in forward bias (b) the diode in reverse bias and (c) the diode electrical symbol (c) The positive terminal of the diode is called the anode and the negative terminal the cathode. Figure 2 below shows the typical current-voltage relationship for the diode.
Figure 2. The typical diode characteristic curve showing regions of forward bias, reverse bias, and reverse breakdown This curve can be divided into three regions: 1. The region in which V D > 0, called the forward -bias region, 2. The region in which V D < 0, called the reverse-bias region, 3. The region in which V D < -V Z, called the breakdown region, Figures 3, 4 and 5 below show the typical I-V characteristic for the regular diode, and the zener diode. The regular diode is used to convert AC to DC, perform digital logic, provide circuit protection, etc. Some of the applications for the zener diode are DC voltage regulation, clipping, wave-shaping, and piecewise-linear approximation for function generating circuits. It is very important that the maximum ratings of a diode not be exceeded. Some of these ratings are maximum peak-inverse voltage, maximum forward current (peak and continuous), and maximum wattage at which the device can operate for a particular temperature. There may be other specified maximum ratings depending on the type of diode. 2
Figure 3. A typical I-V characteristic for the regular diode showing an expanded current scale in the reverse bias region Figure 4. A typical I-V characteristic for the zener diode showing an expanded current scale in the reverse bias region Part 5 of the Lab Session illustrates the application of a zener diode (Figure 4) in a simple voltage regulator circuit. A well regulated voltage source is one in which, for a specified range of load current, the voltage does not change significantly as the load 3
current changes. In the circuit of Figure 10, a decrease in load current will tend to increase the output voltage above the reverse breakdown voltage (V Z ); however, a resulting increase in zener current, equal to the decrease in load current, will maintain a constant voltage drop across the series dropping resistor, and, therefore, a constant load voltage. Likewise, an increase in load current will cause an equal decrease in zener current, maintaining a constant output voltage. In the design of such a regulator there are some limitations that must be considered. In the circuit shown, for a particular output voltage and maximum load current, there will be a maximum series dropping resistance given by R S = (12V e o ) / I Lmax. Also, it should be noted that V Z must be equal to e o, and that the power rating of the zener must be greater than the product of I Lmax and e o. Part 6 of the Lab Session illustrates the use of zener diodes to limit or clip the output voltage. In this case the output voltage can never be less than -V Z or greater than +V Z. Similar circuits are often used to drive a circuit that can not tolerate being overdriven. Also, a convenient method for generating a square wave is to drive this circuit with a sinusoid of peak voltage much greater than V Z ; the output then approximates a square wave with a peak to peak value of 2V Z. 4
Pre-Lab Diodes 1. For the circuit shown below, sketch the output waveform assuming that V Don = 0.7V. D1 Vout VAMPL = 10Vp-p FREQ = 60Hz V1 R1 10k 2. Draw a circuit utilizing regular diodes that will perform the full-wave rectification of an AC signal. DO NOT INCLUDE A TRANSFORMER! 5
3. Obtain the manufacturer s specifications for the diodes listed below: Diode 1N4002 (or its equivalent) Forward Bias Voltage Drop V Don V Forward Bias Continuous Current I D A Peak Reverse Voltage PRV V Zener Diode 1N5228 (or its equivalent) Zener Voltage V Z V (INSTRUCTOR S SIGNATURE DATE ) 6
Lab Session Diodes 1. Connect the circuit shown in Figure 5. Set the output of the function generator to 10V p-p and the frequency to 60Hz, using the oscilloscope. Observe and save the waveform across the 10kΩ resistor to your compact flash card. Figure 5. The half-wave rectifier 2. Connect the circuit of Figure 6 and repeat Step 1. Then, set the oscilloscope to AC coupling and adjust the volts/division and seconds/division manually to view the ripple in the output waveform. The ripple should be in the range of 100 to 150mV. Save this waveform to you compact flash card. When you are finished, return the oscilloscope to DC coupling. Figure 6. The half-wave rectifier including a simple capacitor filter 7
3. Connect the circuit of Figure 7. Record the output voltage for the connections given. Figure 7. DRL (diode resistor logic) circuit 4. Connect the circuit of Figure 8. Record the output voltage for the connections given. Figure 8. DRL (diode resistor logic) circuit 8
5. Adjust the decade resistor to 1kΩ before connecting the circuit of Figure 9. Take readings of the load voltage and resistance of the decade resistor as it is decreased from 1kΩ to zero resistance. Figure 9. Zener Diode voltage regulator 6. Connect the circuit of Figure 10 setting the output of the signal generator to 20 V peak to peak at a frequency of 1kHz. Using the oscilloscope, observe and save the waveform of the signal across the zener diodes to compact flash card. Figure 10. Zener Diode clipper circuit 9
Lab Session Diodes (Data Sheet) INSTRUCTOR'S INITIALS DATE: 10
Post Lab Diodes 1. Describe what function the diode is performing in each of the circuits of Figures 5 through 10. 2. What bias region are the diodes in Steps 3 and 4 operating in under each condition (i.e. forward or reverse bias)? 3. Describe what is happening in the waveforms captured in Steps 1, 2, and 6, be sure to include these waveforms in the report. 11