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Transcript
FOR MORE COURSES VISIT WWW.CIE-WC.EDU OBJECTIVE 1. To measure the stability of a current feedback circuit. FOR MORE COURSES VISIT WWW.CIE-WC.EDU INTRODUCTION • A substantial improvement in bias current can be obtained using the emitter-feedback resistor shown in the following schematic diagram. FOR MORE COURSES VISIT WWW.CIE-WC.EDU EXP. 2 SCHEMATIC DIAGRAM FOR MORE COURSES VISIT WWW.CIE-WC.EDU • Any change in collector current causes a change in emitter current as long as the base is held at a constant voltage. • As collector current rises with temperature, emitter current rises as well • Thus increasing the voltage drop across the emitter resistor R2. FOR MORE COURSES VISIT WWW.CIE-WC.EDU • This decreases the base-emitter voltage drop, and tends to reduce the base current. • The reduction in base current tends to reduce the collector current as well, thus offsetting the increase in collector current caused by the heat. FOR MORE COURSES VISIT WWW.CIE-WC.EDU • These two opposing factors reach equilibrium, and the original increase in collector current is opposed by the decrease in base current. • This is known as current feedback. FOR MORE COURSES VISIT WWW.CIE-WC.EDU In this experiment we will again set the collector current to 5mA and heat the transistor as in Experiment 1. However, this time we will see less current drift than observed in Experiment 1. FOR MORE COURSES VISIT WWW.CIE-WC.EDU PARTS REQUIRED 1 1 1 1 1 220W, 2 Watt resistor (red, red, brown) 470W, ½ Watt resistor (yellow, violet, brown) 1kW, ½ Watt resistor (brown, black, red) 100kW, ½ Watt resistor (brown, black, yellow) 2N2431 Germanium Transistor FOR MORE COURSES VISIT WWW.CIE-WC.EDU PROCEDURE 1. Construct the previous circuit. 2. Bend the leads of the transistor so that the case is away from the 220W resistor FOR MORE COURSES VISIT WWW.CIE-WC.EDU EXPERIMENT 2 SCHEMATIC FOR MORE COURSES VISIT WWW.CIE-WC.EDU EXP. 2 PICTORIAL DIAGRAM FOR MORE COURSES VISIT WWW.CIE-WC.EDU 2. Identify the leads of the transistor using the diagram below. FOR MORE COURSES VISIT WWW.CIE-WC.EDU 3. Turn the trainer on. a) Adjust the positive power supply to 15V b) And the negative power supply to -10V 4. Adjust the 1kW potentiometer so you have 5V across R1 5. Bend the leads so the case now touches the resistor. FOR MORE COURSES VISIT WWW.CIE-WC.EDU a) Allow the transistor to heat for 5 minutes 5. Measure the voltage across R1, and calculate the current through R1. a) Record the current measurement in the Experiment book FOR MORE COURSES VISIT WWW.CIE-WC.EDU CIE RESULTS CIE measured 5.9mA in step 6. FOR MORE COURSES VISIT WWW.CIE-WC.EDU FINAL DISCUSSION The emitter-feedback resistor improved the stability of the transistor circuit considerably. The percentage of drift was only 28%, compared to 96% change we observed without the feedback. FOR MORE COURSES VISIT WWW.CIE-WC.EDU 5mA – 6.4mA X 100 28% = 5mA FOR MORE COURSES VISIT WWW.CIE-WC.EDU QUESTIONS? FOR MORE COURSES VISIT WWW.CIE-WC.EDU RESOURCES Casebeer, J.L., Cunningham, J.E. (2001). Lesson 1430: Transistors, Part 1. Cleveland: Cleveland Institute of Electronics. FOR MORE COURSES VISIT WWW.CIE-WC.EDU THE END Developed and Produced by the Instructors in the CIE Instruction Department. © Copyright 09/2012 All Rights Reserved / Sept. 2012 FOR MORE COURSES VISIT WWW.CIE-WC.EDU