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FOR MORE COURSES VISIT WWW.CIE-WC.EDU OBJECTIVE To measure the improvement in bias stability when the base-bias resistance is decreased. FOR MORE COURSES VISIT WWW.CIE-WC.EDU INTRODUCTION Experiment 2 illustrated the base must be held at a constant value in order for the emitter to work properly. In experiment 2, the base was biased through a 100kW resistor and a 1kW potentiometer. FOR MORE COURSES VISIT WWW.CIE-WC.EDU This was a simple arrangement, but did not provide a very stable voltage to the base of the transistor. This experiment used an alternate biasing method, with a lower resistance as seen from the base. • The 100kW base current-limiting resistor has been reduced to 10kW. FOR MORE COURSES VISIT WWW.CIE-WC.EDU • A 1.2kW resister was also placed in series with the potentiometer and the -10V supply. • The action of the emitter feedback will be improved because the base is held to a steadier voltage. • The following slide illustrates the Experiment 3 schematic diagram. FOR MORE COURSES VISIT WWW.CIE-WC.EDU EXP. 3 SCHEMATIC DIAGRAM FOR MORE COURSES VISIT WWW.CIE-WC.EDU REQUIRED PARTS 1 220W, 2W resistor (red-red-brown) 1 470W, ½ W resistor (yellow-violetbrown) 1 1200W, ½ W resistor (brown-red-red) 1 10kW, ½ W resistor (brown-blackorange) 1 2N2431 Germanium transistor (PNP) FOR MORE COURSES VISIT WWW.CIE-WC.EDU PROCEDURE 1. Construct the circuit for Experiment 3. 2. Turn the trainer on. a) Adjust the positive power supply to 15V b) Adjust the negative power supply to -10V. 3. Bend the leads of the transistor so the case is away from the 220W resistor. FOR MORE COURSES VISIT WWW.CIE-WC.EDU EXP. 3 SCHEMATIC DIAGRAM FOR MORE COURSES VISIT WWW.CIE-WC.EDU PICTORIAL DIAGRAM OF EXP. 3 CKT. FOR MORE COURSES VISIT WWW.CIE-WC.EDU 4. Adjust the 1kW potentiometer so you have 5V across R1. 5. Bend the leads of the transistor so that the case now touches the resistor. a) Allow the transistor to heat for 5 minutes 6. Measure the voltage across R1, and calculate the current through R1. a) Record calculation in lab book. FOR MORE COURSES VISIT WWW.CIE-WC.EDU CIE RESULTS 6. 5.9mA FOR MORE COURSES VISIT WWW.CIE-WC.EDU FINAL DISCUSSION We saw a reduction in base-bias current drift. The improved stability was: • Experiment 3; 18% • Experiment 2; 28% • Experiment 1; 96% FOR MORE COURSES VISIT WWW.CIE-WC.EDU For best stability with an emitter-feedback resistor, then, the resistance of the basebias network should be as low as possible. 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