Introduction To Amplifiers

By: Ryan Sherry

Background


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 two port networks, where a two port network is nothing more than a device with two input terminals and two output terminals. Due to internal circuitry, which we cannot necessarily see, there will be certain impedances or resistances across both the input and output terminals. These resistances are what we are talking about when we discuss input and output resistance. These terminals behaviors play an importany role in the branch of electrical engineering concerned with modeling systems based on transfer characteristics, but that will be the topic for later labs.

Procedure


Part A: Function Generator

The output resistance of the HP F.G. is specified to be 50 ohms. I measured the resistance to be 50.43 ohms with the Fluke, by simply attaching the Fluke leads to the two output leads of the F.G. Note that the F.G. should be turned on when the output resistance is measured.

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.

Part B: Source and Load

When I connected the line-out of the CD player directly to a speaker, I didn't hear anything at all. When I connected the earphone jack to the speaker, I heard sound, but it was very low quality. The explanation for not hearing any sound from the line-out, is that the output resistance of the line-out jack was very much larger than the resistance of the speaker, and no voltage could drop across the speaker. The earphone jack had an output resistance that was much lower and closer to the resistance of the speaker, therefore there was some voltage dropped across the speaker.

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.

Part C: HP 6825A Programmable Power Supply/Amplifier

Amplifiers are other electrical devices that have input and output resistances associated with them. The input resistance was measured as 10.9 kohms and the output resistance was 3 ohms. Notice the output resistance was much smaller than the input resistance. We should be able to use the HP as an amplifier for the CD player. And indeed, when we amplify the input and connect a speaker to the output of the amplifier, we hear sound that is of respectable quality. The gain can be varied by adjusting the voltage on the amplifier, which now functions basically just like the volume control on the earphone jack.

Conclusion


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.


Back to the Lab 9 Page