ELEC 242 Lab

Experiment 3.1

Common Emitter Transistor Circuit

Equipment

Components

The first circuit we'll look at is the basic common emitter (CE) circuit we've studied in class. Since the value of beta varies considerably from one transistor to another, even among devices of the same type, the first thing we'll do is use the circuit to measure the beta of our transistor.

Part 1: Measuring Transistor Beta



Step 1:

 Connect the top bus strip on your breadboard to the red and green binding posts to form power and ground buses (just like Experiment 2.1).
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Step 2:

 Set the Meter Selector switch on the power supply to +20V. Adjust the 0 to 20V voltage control to produce 15 volts.

Step 3:

 Use a red banana patch cord to connect the 0 to +20V terminal of the power supply to the red banana jack on the breadboard. With a green cord, connect the Common terminal of the power supply (blue binding post) to the green banana jack of the breadboard.

Step 4:

 Plug your BNC-banana adapter into the 6V supply terminals. Be sure that the prong with the ground bump is plugged into the negative (black) terminal of the power supply.


Step 5:

 Plug one end of a BNC patch cord into the adapter. Plug the other end into J1-3 on the interface board.

Step 6:

 Find a 2N3904 transistor in your parts kit. The transistor leads are identified below.
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Step 7:

 Wire the following circuit:
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Step 8:

 Turn on the power supply. Turn the 0 TO 6V control fully counterclockwise. Gradually increase $v_{IN}$ until $v_{OUT}$ is 7 V.

Step 9:

 Measure the voltages across $R_B$ and $R_C$ . Use these to determine $i_B$ and $i_C$ .

Step 10:

 Compute $\beta = i_C/i_B$ .

Step 11:

 Repeat these measurements for $v_{OUT}=2\rm V$ and $v_{OUT}=12\rm V$ . Compute the corresponding values of $\beta$ . Is $\beta$ the same at different values of collector current?

Question 1:

 Based on the component values and the average value of beta for your transistor, calculate and sketch the expected voltage transfer characteristic ($v_{OUT}$ vs. $v_{IN}$ ) for your circuit. Be sure to label the voltages for significant points on the curve. Compare this with the results obtained in Part 2. Are the two reasonably close?

Part 2: Common Emitter Transfer Characteristics



Step 1:

 Set $v_{IN}$ to 0 V and measure $v_{OUT}$ .

Step 2:

 Increase $v_{IN}$ in steps of 0.2 V until it reaches 3 V. At each point, measure $v_{OUT}$ and record both values.

Step 3:

 Plot $v_{OUT}$ vs. $v_{IN}$ .

Step 4:

 Increase $v_{IN}$ in steps of 1 V until it reaches 6 V. At each point, measure $v_{OUT}$ and record both values. The limiting value of $v_{OUT}$ is the collector saturation voltage ($V_{CEsat}$ ).

Part 3: Linear amplifier



Step 1:

 Set the function generator to produce a 100 Hz triangle wave.

Step 2:

 Unplug the BNC patch cord from the power supply and plug it into the function generator MAIN output.

Step 3:

 Connect CH 1 of the oscilloscope to $v_{IN}$ . Connect CH 2 to $v_{OUT}$ .

Step 4:

 Set the function generator AMPLITUDE control to minimum. Pull out the DC OFFSET control and adjust it so that $v_{OUT}=7\rm V$ .

Step 5:

 Increase the AMPLITUDE until the peak-to-peak (p-p) value of $v_{OUT}$ is 4 V.

Step 6:

 Measure the p-p value of the input, $v_{IN}$ .

Step 7:

 Compute the voltage gain of the linear region, $\displaystyle A_v = \frac{\Delta v_{OUT}}{\Delta v_{IN}}$ . Is this equal to the slope of the linear region of the transfer curve?

Question 2:

 Show that in the active region, $\displaystyle A_v = -\beta\frac{R_C}{R_B}$ . Is this true for your circuit?