Introduction to Amplifiers

Input Resistance, Output Resistance:

What are they and who cares?

To: EE 222 Class and Others in the EE Community

From: Stephen Pietrucha

Lab Partners: Doug Hoffman, Dave Strickland

Date: 12-6-95

This laboratory experiment explored the concept of input and out resistances. Also explored were amplifiers, and how they work. Essentially all devices which have an input or output have a resistance across the terminal. We found that this resistance should be measured while the device was on since that is the state in which it will be operating.

Our first step was to take a Hewlett Packard Function Generator and determine its output resistance. This was done while it was on, and was found by the Fluke to be 50 ohms, which agreed with the rating on the HP's face plate. An alternate method depends upon placing a resistor of known value across the output terminals. Next find the open circuit voltage, and the output voltage. From here we can find the Thevinin Equivalent Circuit, and determine the value of the internal voltage which was found to be 56 ohms . This method is more accurate since we only care about the internal resistance when a load has been applied.

Next we connected the earphone jack of a Realistic Stereo-Mate portable cassette player to a standard speaker which was rated 8 ohms. Using the fluke a measurement of 7.6 ohms was obtained. The tape player was playing Frank Sinatra's rendition of "Oh Come All Yea Faithful", and the same portion of the song was played while making the following measurements in order to keep the input signal relatively constant. Here using the Oscilloscope we were able to calculated values for the open circuit voltage and the output voltage. These were used in order to obtain a value for the stereo's output resistance, this was calculated to be 8.32 ohms. The sound coming out of the speaker at this point sounded dull and was not very loud.

Using the Hewlett Packard 6825A amplifier we measured the input resistance, using the Fluke, to be 7.6 kohms, while the output was found to be 2 ohms. What an amplifier does is accept the high resistance at the input and lower its value at the output. This allows us to use the 2 ohm output of the amplifier as the input to the 8 ohm speaker. When this is done we can hear Frank the way he should be. This concept of matching resistances is an important one. When you are buying a stereo receiver, you should get one where the output resistance is as low as possible. The reason for this is that if the load resistance is greater then the internal resistance, then the voltage will be dropped over the speakers. If you attach speakers whose resistance is less then the internal resistance then the majority of the voltage is dropped inside the receiver and the output signal will sound pretty lousy. You want to get the maximum voltage drop to be across the speakers. Using the amplifier we can control the gain by turning the adjustment on the amplifier which allows more voltage to drop.

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