ELEC 242 Lab

Experiment 3.3

Common Collector Transistor Circuit

Equipment

Components

The source resistance ($R_S$ ) of the function generator is 50 ohms. We used this fact to our advantage in Lab 1 to convert it into a pseudo current source, but sometimes it works against us. If we want to drive a low resistance load (e.g. a motor) with a voltage, there will be a voltage drop across the internal resistance of the function generator. This means that the voltage actually delivered to the load can be significantly less than that generated by the source.


Step 1:

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

Step 2:

 Turn the function generator AMPLITUDE fully counterclockwise. Pull out the DC OFFSET control and adjust it until the output is 4 V.

Step 3:

 Adjust the AMPLITUDE control so that the signal has a p-p amplitude of 6 V.

Step 4:

 Connect a 47 ohm resistor to the function generator output.
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Verify that the output is reduced by about 1/2 by the loading.

Step 5:

 Wire the following circuit.

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Note that this inserts an emitter follower buffer between the function generator output and the load.

Step 6:

 With the scope, measure $v_{OUT}$ . Sketch the waveform, noting the p-p and average value.

Step 7:

 Compute the voltage gain, $\displaystyle A_v = \frac{\Delta v_{OUT}}{\Delta v_{IN}}$ and the offset $V_{ofs}={\rm avg}(v_{OUT}) - {\rm avg}(v_{IN})$ .

Question 3:

 Compute the average power delivered to the load ($P_L$ ) and dissipated by the transistor ($P_D$ ). The power rating for the load resistor is 1/4 W and for the transistor is 625 mW. Have we exeeded the rating of either component?

Step 8:

 Replace the 47 ohm resistor with a 470 ohm resistor. Measure $v_{OUT}$ and compute the voltage gain and offset. Are they significantly different from the previous values?