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UNIVERSITY OF TEHRAN Electrical and Computer Engineering Department Electronic Circuits Lab I Experiment 1 Characteristics of a P_N Junction Diode 1. Objectives In this experiment, characteristics of the P_N junction diode will be introduced. You will measure some of these characteristics you may have previously found in catalogs, making you familiar with the real functionality of a diode. 2. Necessary Equipment Diodes to be used are 1N4148, 1N4001, 1N4002 or 1N4004 as well as Zeners diode with break voltage of 4 to 7 volts. Furthermore, you will need 1KΩ, 10KΩ, 100KΩ, 100Ω, and 560Ω and 10Ω resistors. Other essential items include a DC power supply, an oscilloscope, a function generator, a breadboard, an ohmmeter and wires. 3. Pre-lab 3.1 Consider the circuit below: a) Draw the output characteristic diagram, V O versus V I . 1 b) Draw the approximate diode characteristics diagram I O versus V I and calculate the diagram slope in the work point, as you know we refer to this value as diode’s dynamic resistance. c) Now assume that sinusoid signal with amplitude of 6 Volts and frequency of 1KHz is applied to the circuit, draw V O (t) diagram. Assume the threshold voltage or Vγ for the diode is 0.6 Volts. d) Now replace the diode D with a Zener diode with a reverse break voltage of 7 Volts and a forward turn-on voltage of 4 Volts, draw the zener characteristic diagram, I O versus V Z . Also draw the circuit’s output characteristic diagram. V O versus V I . e) Once again, replace the input with a sinusoid signal ‘similar to the one in section (C)’ and draw V O (t) diagram. 3.2 As you may have already guessed the circuits described above can be used as voltage rectifiers; however actual application for such circuits in their current is limited by to loading issues. Consider the circuit below; each diode has a Vγ of 0.6 Volts. Assume R L =1KΩ. Draw output characteristic diagram V O versus V I for this circuit. a) Consider circuit3. Draw the V O / V I characteristic for this circuit, now add a 1KΩ resistor to the output and sketch the said diagram again. Compare the two diagrams. Is this circuit suitable when loaded with large loads? Why? b) Re-do segment A for a zener diode with Vγ=4 and Vbreak=7 volts. Which of the two circuits are more suitable? (Note that the maximum reverse current for a zener diode is 50mA) 2 4. Procedure 4.1 Static Characteristic Diagram: 4.1.1 Assemble Circuit 1. Input must be sinusoid signal with amplitude of 6Volts, at 1 KHz frequency. Hint: with your knowledge of electronics, you must be able to detect the cathode and anode of a diode; nevertheless the line on the body of a diode shows the Cathode. Circuit 1: The circuit to show the characteristic diagram of a diode. 4.1.2 Connect the GND of the oscilloscope to point 2 and main probes of channel 1 and 2 points 1 and 3 in order to see diode’s characteristic diagram on screen. Set the mode on X-Y and observe the characteristic diagram of the diode. Sketch the conclusion. Compare it with what you have drawn in Pre_lab. Measure the Vγ. 4.1.3 Repeat these steps with 1N4004 diode. Try again with a 4 to 7 volt zener diode. Find the reverse break point voltage in the related zener diagram. Using the diagram, calculate the Vγ for all the diodes and VZ for the zener diode. (Vγ is the forward turn-on voltage and V Z is the reverse break voltage) Write down how you have found these two parameters. 4.1.4 Back to Circuit 1 with 1N4148 diode, 1K resistance and sinusoid 6-volt input; connect the GND of the oscilloscope to point 3 and check points 1, 2 as input and output. Set the input frequency at 100Hz. Explain the difference between input and output positive amplitudes. 4.1.5 In the sinusoid mode for D1N4148, increase the frequency continuously and follow the output. Find the reason why you see difference when you increase the frequency. 3 Hint: there are junction capacitors in the diode, which collapse when frequency is increased. 4.1.6 Change the sinusoid input to a square shaped. Explain what you see. 4.1.7 Sweep the frequency while watching the output. Measure the turn-on and turn-off delay times. 4.1.8 Change the diode with a 4 to 7 volt zener, and at 100Hz see the output. Try to find an explanation for what you see at high frequency for the output. 4.2 Forward Drop and Output Loads 4.2.1 In Circuit 1, replace the function generator with a DC voltage source as shown in Circuit 2. (Point3 is the oscilloscope ground.) Note: when measuring the V d voltage, the ground probe must be attached to point 2 to minimize error. Circuit 2: A DC voltage source is used to determine the DC parameters. 4.2.2 Set the input voltage at about 5v. Measure the two near working points (V D , I D ). Remember the two working points must be very close, explain the reason. You can have a more accurate current measurement by measuring the voltage between the two heads of R 1 . 4 According to the diode characteristic diagram, a little change in the V D results in a considerable change of I D . So you should measure the V D as accurate as possible. You can use the Alt mode of oscilloscope with the invert case in order to observe the V D on the CRT screen then adjust the Time/Div selector for an accurate measurement. If your scope hasn’t the inverter section, disconnect the GND terminal of the oscilloscope; use the Alt mode of oscilloscope in order to observe the voltage difference between two heads of diode. 4.2.3 Following the next equation, determine r d : rd = VD 2 VD1 = I D 2 I D1 4.2.4 Now around I D1 , again calculate Rd using this formula: (assume η=1 and VT VT=26mA) r d = (r d : Dynamic Resistance) ID Compare the two conclusions. Which result is more accurate? 5 4.2.5 Use again those two work point measurements to derive I S and η from the next formula: I D = I S * exp (V D / η* V T ) 4.2.6 Build Circuit 3 using 1N4004 and a 100Ω resistor. Circuit 3: by using diodes, we try to achieve to a constant voltage. 4.2.7 You may expect the output voltage to be fixed at 2.1V, so change the input according to the next table and write output. VI 1.5 volt 2 volt 2.5 volt 3 volt 3.5 volt 4 volt 4.5 volt VO Table 1 Sketch the diagram of the V O according to V I . Determine the diode line regulation (Δ V O /Δ V I ) at several work points. (Change in output voltage / change in input voltage [mV/V].) 4.2.10 Now add the load resistors to the circuit. Connect a resistance in parallel with 3 diodes (between V O and GND) and change it according to this table. Then calculate V O and I O . 6 For each load calculate the voltage regulation percentage i.e. S load = 100 [(V noload – V fullload )/ V noload ] R 100K 10K 1K 560 100 10 VO IO S load = Table 2 4.2.11 Now replace the diode in Circuit 2 with a 4 to 7 zener diode, first place the zener diode in forward and find two working points (V Z 1 , I D1 and V Z 2 , I D 2 ), using the equation given in section 4.2.6 measure the η and IS for the zener diode and compare them with that 1N4148 diode then, change the input voltage to set the current at 5mA, again using the method described in section 4.2.4 ,calculate the Dynamic Resistance of the zener diode and compare it to that of 1N4148. 4.2.12 Now place the zener diode in backward position and change the input voltage to set the current at 5mA, measure Vd1 by changing the input voltage. Set the current at 5.1mA and measure V D 2 . Now calculate the reverse rd using V VZ1 = r d = Z2 I Z 2 I Z 1 and compare it to the forward dynamic resistance. How can you justify your findings with the zener’s characteristic diagram? Can we calculate zener’s backward dynamic resistance using the r d =η V T / I D formula? Why? 4.2.13 Now return to Circuit 3 and replace the 3 diodes with a zener diode and add and the load resistors according to the table in section4.2.8 measure the V O and Io accordingly and calculate S load for each step. Also note that maximum reverse current, which the zener can handle, is about 50mA, what is the maximum input voltage you can apply to this circuit? Is this circuit better than the one built with 3 diodes? Why? 7 5. Problem What is the proper range for R L in this circuit? V Z = 7 volts I Z (min)=0.6mA V = 4 volts I Z (max) =50mA 8
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