Laboratory No. 01: Small & Large Signal Diode Circuits Electrical Enginnering Departement By: Dr. Awad Al-Zaben Instructor: Eng. Tamer Shahta Electronics Laboratory EE 3191 February 23, 2014
I. OBJECTIVES 1. To understand small signal diode circuits. 2. To understand large signal diode circuits. II. EQUIPMENTS REQUIRED 1. DC-voltage source. 2. Digital Multimeter (DMM). 3. Coupling wires. 4. Probes. 5. Object board. 6. Resistors (270Ω, 11kΩ). 7. Variable resistor (Potentiometer). 8. Ordinary diode. 9. Function generator. 10. Oscilloscope III. INTRODUCTION A pn diode is composed of two sections of semiconductor with different types of doping. An n-type semiconductor is one that has material added to it that contributes extra free electrons (material that has 5 valence electrons ). These atoms are donors. A p-type semiconductor has material added to it that contributes extra holes ( material that has 3 valence electrons). These atoms are acceptors, since they accept a free electron to form a bond with a neighboring silicon atom. 2
When a p-type semiconductor is placed next to a n-type semiconductor, then we have a concentration gradient of carriers (i.e. more charge in some areas than others) This results in positively charged holes diffusing from the p-type material into the n-type material, where they recombine with an electron (i.e. an electron fills a hole). This leaves behind a negatively charged depletion region in the p-type material - i.e. depleted of holes, and so the net negative charges on the acceptor atoms leads to a net negative charge in the depletion region. The opposite happens to electrons in n-type material. Since we have a region with net negative charge, and one with positive charge, we end up with an electric field across the depletion region. The electric field in the depletion regions results in a voltage across the pn junction, known as the built-in voltage, given by ( ) NA N D = V T ln V D where N A is the acceptor concentration in the p-type material and N D is the donor concentration in the n-type material, and is typically around 0.7V olts. n 2 i (1) Anode Cathode Anode Cathode Anode Cathode Depletion Regions FIG. 1: Two sections of semiconductor with different types of doping IV. DIODE CHARACTERISTICS The current through the diode can be calculated using the following equation: ( I D = I S e V D /nv T 1 ) (2) where I S is the saturation current, on the order of 10 15 A at room temperature but doubles every 10 o C. V T is the thermal voltage, given by V T = kt q (3) where K is Boltzmanns constant = 1.38 10 23 J/K, T is the absolute temperature in kelvins, q is 3
the magnitude of the charge on an electron = 1.60 10 19 C, and n is a device-dependent constant between 1 and 2. At room temperature, V T 26 mv. The diode characteristics described by equation (1) is plotted in figure (1.2). I D 0.025 0.02 0.015 0.01 0.005 0.4 0.2 0 0.2 V D 0.4 0.6 0.8 I S FIG. 2: Diode characteristics Under DC conditions, the diode has only one point of operation, this point (V DQ, I DQ ) is called the quiescent point or the Q-point (see Figure(3)). Since we have V D across the diode 0.0035 I D 0.003 0.0025 0.002 I DQ 0.0015 0.001 0.0005 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 V DQ V D FIG. 3: Q-point location on the characteristics under dc conditions and I D that flows through the diode, then we have resistance. In this case, the resistance is called the DC resistance or the static resistance and is given by: R = V DQ I DQ. 4
Under AC conditions, the sinusoidal input will move the instantaneous operating point up and down a region of the characteristics as shown in Figure(4). In this case, the resistance is called the Ac resistance or the dynamic resistance and is given by: r d = V d I d. 0.0035 I d 0.003 0.0025 0.002 Id 0.0015 0.001 0.0005 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 V d V d FIG. 4: Operation point under ac conditions V. EXPERIMENTAL WORK A. Forword Biased Diode Behaviour Obtain a diode from your instructor. Use the DMM to determine the Anode terminal and the Cathode terminal. What is the value of the diode resistance? 1. Construct the circuit shown in Figure(5). 2. Vary the input voltage from 0 to 0.75 in steps as shown in table(1). Record both the current through the diode and the voltage across the diode. (Note that the current can be obtained be measuring the voltage across the resistor (10K) and then dividing by (10K). 3. In your report, plot the characteristics curve of the diode, and determine the dynamic resistance in the vertical rise region. 5
10K V S I D V D FIG. 5: Circuit to determine the forward biased diode characteristics B. DC Load line Use the same circuit in Figure(5). Use fixed supply (V s = 2 Volts). Determine the Q- point. You need to plot the dc load line of this circuit on the characteristics curve you obtained in the previous part. Determine the static resistance at the Q-point. C. Reverse Biased Behaviour 1. Construct the circuit shown in Figure(6). 2. Vary the input voltage from 0 to -15 in steps as shown in Table(2). Record both the current through the diode and the voltage across the diode. 3. In your report, plot the characteristics curve of the diode. V S 1M I D V D FIG. 6: Circuit to determine the reverse biased diode characteristics 6
D. Light Emitting Diode (LED) Characteristics 1. Construct the circuit shown in Figure(7). 2. Confirm that there is no current flowing in the circuit. 3. Swap the power supply polarity. The LED should light. Measure the voltage across the resistor and the LED. 4. Substitute 330Ω, 3KΩ, 30KΩ, and 300KΩ resistors for the 100Ω. How does the brightness of the LED change? How does the voltage across the diode change? Explain these results 5V 100Ω I D V D FIG. 7: Circuit to investigate the LED characteristics E. Zener Diode Characteristics 1. Confirm that the Zener diode is working as a normal diode in the forward biased region. (you have to build the circuit) 2. Construct the circuit shown in Figure(8). 3. Confirm that the circuit is working as a regulator even if the input voltage V s is changing. (i.e. the output voltage is constant even if the input voltage is varaible). 4. Find the minimum input voltage such that the circuit is working correctly. 7
5. Confirm that the circuit is working as a regulator even if the load resistance R is changing. (i.e. the output voltage is constant even if the load is varaible). 6. Find the minimum load resistance such that the circuit is working correctly. 1KΩ Vo Vs I D R L FIG. 8: Circuit to investigate the Zener Diode characteristics VI. REPORT Your report should include in addition to the theory, the following: 1. Your results of the diode characteristics including the graphs of the characteristics. 2. Reverse biased characteristics. 3. Load line analysis. 4. LED characteristics and analysis of the experiment results. 5. Zener experiment results along with your discussion. 6. Discussion 8
V s V D I D 0.0 0.1 0.2 0.3 0.4 0.5 0.55 0.60 0.62 0.64 V s 0.0-5.0-10.0-12.0-15.0 V D I D 0.66 0.68 TABLE II: Diode Reverse Characteristics 0.70 0.72 0.75 TABLE I: Diode Forword Characteristics 9