EMT 1255 - Experiment #7 Biasing PDF

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Summary

Lecture - Prof. Jang (chairman and he is a terrible instructor)
Lab - Prof. Bustamante (Recommended) ...

Description

Experiment #7: Bipolar Transistor Biasing

Due Date: 04/11/2018 Objective The objective of this experiment is to be able to construct and analyze three types of

transistor bias circuits specifically, base bias, voltage –divider bias, and collector-feedback bias.

Theory Transistor Biasing is the process of setting a transistors DC operating voltage or current conditions to the correct level so that any AC input signal can be amplified correctly by the transistor. A transistors steady state of operation depends a great deal on its base current, collector voltage, and collector current and therefore, if a transistor is to operate as a linear amplifier, it must be properly biased to have a suitable operating point. Establishing the correct operating point requires the proper selection of bias resistors and load resistors to provide the appropriate input current and collector voltage conditions. The correct biasing point for a bipolar transistor, either NPN or PNP, generally lies somewhere between the two extremes of operation with respect to it being either “fully-ON” or “fully-OFF” along its load line. This central operating point is called the “Quiescent Operating Point”, or Qpoint for short. When a bipolar transistor is biased so that the Q-point is near the middle of its operating range, that is approximately halfway between cut-off and saturation, it is said to be operating as a Class-A amplifier. This mode of operation allows the output current to increase and decrease around the amplifiers Q-point without distortion as the input signal swings through a complete cycle. In other words, the output current flows for the full 360o of the input cycle.

Equipment ● ● ● ● ● ● ●

One 470 ohms resistor One 2.0 K ohms resistor One 6.8 K ohms resistor One 33 K ohms resistor One 360 K ohms resistor One 1.0 M ohms resistor Two npn transistors (2N3904)

Procedure 1. We measured and recorded the values of the 1.0 M ohms and 2.0 K ohms resistors (Refer to table 7-1) 2. We computed the parameter listed in the table 7-2 assuming that Bdc was 200 for the base bias circuit. 3. On the same circuit we then proceeded to measure the values that we had already computed expect for the currents. 4. We then removed the transistor and place a different one on the circuit to see what changes we would encounter. 5. Moving on to the voltage-divider bias we measured the values of a 33 K ohms resistor, a 6.8 K ohms resistor, a 470 ohms resistor, and a 2.0 K ohms resistor and recorded our values on the table 7-3.

6. We computed the parameter listed in the table 7-4 once again for the voltage-divider bias circuit. 7. On the same circuit we then proceeded to measure the values that we had already computed expect for the currents. 8. We then removed the transistor and place a different one on the circuit to see what changes we would encounter. 9. Finally moving on to the collector-feedback circuit we measured the values of a 360 K ohms resistor and a 2.0 K ohms resistor, and recorded our values on the table 7-5. 10. Using Kirchhoff’s voltage law we computed the parameter listed in the table 7-6 assuming that Bdc was 200 for the base bias circuit. 11. On the same circuit we then proceeded to measure the values that we had already computed expect for the currents. 12. We then removed the transistor and place a different one on the circuit to see what changes we would encounter.

Data

Questions 1. The method that shows less variation between transistors is Voltage-divider bias. 2.

3. R1 = 33 K, R2 = 2.5 K; (2.5 K/ (33 K + 2.5 K)) 24 V = 1.69 V; 1.69 V - .7 V= .99 V; . 99V / 47 ohms = 21.1 mA

4. 5. a) Less than .7 V to the the base of the transistor which won’t allow it to function. b) Current will not flow. c) The current through the emitter will be 10 times less than what it is supposed to be.

Conclusion Throughout this experiment we encounter very few difficulties; the lab ran smoothly throughout the class. Some of the difficulties were our power supply which wasn’t working properly, so me and my partner moved to another table, we thought that our data was wrong since the number measured and the numbers computed for our base bias transistor circuit were different, but then we learn that the reason to that was the power gain. We also learn that the current going through the emitter is approximately the same as the one in the collector, therefore it can be used to determine Rc voltage assuming that its current is the same as the one from the emitter....