AN INTRODUCTION TO INPUT AND OUTPUT RESISTANCE

By Bob Zorn

Why can't your stereo amplifier drive an infinite number of speakers? The easy answer is lack of power. Everyone is familiar with the concept of power and understanding the specifics is an important part of electrical engineering. You are familiar with the DC voltage supply model of an ideal voltage source in series with an internal resistance with the internal resistance determining the maximum power available from the supply. The internal resistance is the source or output resistance and the power i s transferred to the load or input resistance. The same basic situation applies for any system that uses alternating frequencies, from a bass note through a satellite signal. The major differences are that the resistance of the load changes when the fre quency changes so we substitute the term impedance for resistance. The other is that the details require three calculus classes to fully appreciate. Fortunately, we can use the DC model. The stereo amplifier can supply a limited amount of voltage and c urrent, or power. The speaker connections can be modeled as an ideal source in series with an internal resistance. This internal resistance is the output impedance specified by the manufacturer, usually 4 or 8 ohms. A connection to the speaker terminal s forms a voltage divider between the internal resistance and the speaker or load resistance. When you connect a 4 ohm load to a 4 ohm output, the impedances are "matched" and maximum power is transferred to the load. When the load resistance drops belo w 4 ohms, the voltage drop across the internal resistance becomes greater and the power transferred to the load becomes less. Two eight ohm speakers connected in parallel would be an optimal load but clearly there is a limit on the resistance of the load . The same reasoning applies to any system where power is being transferred between devices. The amplifier has to have expensive low output impedance circuitry to drive the low speaker impedance but its input impedance can be much higher. The connectio ns between source component (CD) and the load component (amplifier) are both high impedance. We can measure the input and output resistance with an ohmmeter but the reading may not be accurate. Ohmmeters measure DC resistance but we are interested in the AC resistance or impedance. A low DC resistance will appear as a low impedance to the sourc e but an input with a high DC resistance can become a low impedance when AC signals are applied. In lab, we found the unknown impedance by monitoring the signal with an oscilloscope and changing a known impedance. When the signal voltage drops to 0.7 of its unloaded value, maximum power is being transferred to the load and the impedances of the two components are equal. Using the voltage = 0.7 for maximum power transfer will be covered in EE209 but for now consider the fact that power is a function of voltage and current.
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