This particular laboratory is an investigation of input and output resistances. The objective is to determine what they are, where they come from, and how they affect electrical devices such as radios, CD players, function generators, and so forth. There are many electrical devices that can be modeled as

- Measure the output resistance of the HP function generator using the Fluke Hydra.
- Using a method such as Thevenin's Theorem, determine the output resistance of the function generator by making some circuit measurements.

There are various ways to determine the output resistance of the F.G., but I used a very simple and logical method to determine the output resistance. Since we can model the F.G. as a voltage source with some internal resistance in serious, I can measure the open circuit voltage, and then apply various test loads and measure the load voltage until I get a voltage equal to half of the open circuit voltage. When I find the load resistor that drops half of the open circuit voltage I know I've matched the output resistance of the F.G. because I've created a 0.5 voltage divider. Experimentally I found the load resistor that dropped half of the input voltage to be **47 ohms**, which agrees to within three ohms of the specified value.

- Connect the line-out of the portable CD player or Walkman directly to a speaker. Do the same for the earphone jacks. Discuss what you hear, explaining why.
- Measure the resistance of the speaker.
- Measure the output resistance of the CD player. Compare the resistances of the line-out and the earphone jacks for the CD player.

The resistance of the speaker was measured to be 7.5 ohms. To measure the output resistance of the line-out I measured the output voltage at a given instance of a song for no load. Then I added trial loads, and measured the output voltage at the same instant until I found a resistor that dropped a voltage equal to half of the un-loaded voltage. This resistor value was the output resistance of the line-out jack, because I had created a 0.5 voltage divider just as in Part A. The line-out resistance was **49 kohms**. A similar procedure was followed to determine the output resistance of the earphone jack, and I found the resistance to be about **9 ohms**. These values make sense; for the line-out resistance, almost all the voltage is dropped inside the CD player because of the voltage divider equation. For the earphone jack, the output resistance was 9 ohms and the speaker resistance was 7.5 ohms, so there will be some voltage dropped across the speaker, but not even 50%; this is why the quality of the sound is so poor.

- Measure the input and output resistance of the HP as an amplifier. Comment on which resistance is higher.
- AC couple(use a capacitor in between) the output of the CD player to the input of the HP amplifier. Comment on whether or not this will work as an amplifier. How does it sound? Can you vary the gain?

From the experiment we see that in applications, we want input resistances of devices to be higher than the output resistances of the device supplying the signals. When the ratio of input over output resistance is high, more voltage is st the input which is what we want, especially for something like a speaker where we want all the voltage we can get.

Please send questions or comments to Ryan Sherry.

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