EMT 1255 - Experiment #8 Common-Emitter PDF

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Summary

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

Description

Experiment #8: The Common-Emitter Amplifier

Due Date: 04/13/2018 Objective The objective of this experiment is to be able to compute the ac parameters for a

common- emitter (CE) amplifier, build a CE amplifier circuit and measure de dc parameters, the ac input resistance, and the voltage gain, and to predict and test the effects of certain faults in a CE amplifier.

Theory The most common amplifier configuration for an NPN transistor is that of the Common Emitter Amplifier circuit. The single stage common emitter amplifier circuit shown above uses what is commonly called “Voltage Divider Biasing”. This type of biasing arrangement uses two resistors as a potential divider network across the supply with their center point supplying the required Base bias voltage to the transistor. Voltage divider biasing is commonly used in the design of bipolar transistor amplifier circuits. This method of biasing the transistor greatly reduces the effects of varying Beta, ( β ) by holding the Base bias at a constant steady voltage level allowing for best stability. The quiescent Base voltage (Vb) is determined by the potential divider network formed by the two resistors, R1, R2 and the power supply voltage Vcc as shown with the current flowing through both resistors. Then the total resistance RT will be equal to R1 + R2 giving the current as i = Vcc/RT. The voltage level generated at the junction of resistors R1 and R2 holds the Base voltage (Vb) constant at a value below the supply voltage.

Equipment 1. 2. 3. 4. 5. 6. 7. 8.

One 100 ohms resistor One 330 ohms resistor Two 1.0K ohms resistors One 4.7K ohms resistor Two 10K ohms resistors One npn transistors (2N3904) Two 1.0 uF One 47 uF One 10K ohms potentiometer

Procedure 1. We measured and recorded the values of the resistors listed in table 8-1. 2. We computed the parameter listed in the table 8-2. 3. We constructed the amplifier shown in figure 8-2, and measured the voltages listed on the table 8-2. 4. We computed the ac parameters listed in table 8-3 with a default Vin signal of 300mV pp. We assumed that the B(ac) value 100 to calculate the ac voltage at the collector. 5. We turned on the signal generator and set Vin for 300 mV pp at 1.0 kHz with the generator connected to the circuit. 6. We used the oscilloscope to set the proper voltage and check the frequency.

Data

Questions 1. When the bypass capacitor, C2 is open, the value of RE increase, therefore R’L will be divided by a larger resistance and so the result will be smaller and so will be the gain. 2. Question two does not apply to our lab, since we were not required to do step 6. 3. a) No, is not normal b) The cause of the problem is that the circuit is saturated, because VCE = 0, since VCE = VC – VE 4. a) Will remain the same as before b) 1.24 V

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, it was a bit complicated lab and we were running out of time, therefore we were not

able to complete steps 6 through 11....