What is Important in a Design, Voltage, Current or Power?

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Before beginning a design, it is very important to think about the question, "What is important in this design, voltage, current, or power?" This is because it will determine how you design the electronics. It is such a basic question, that it is not often discussed, because it was determined long ago, and now is taken for granted. The answer to this question determines the best tools to use for analysis and design, and the answer is typically different for different types of electronic engineers. For example, typically. engineers dealing in the area of control systems are more likely to care about voltages, and currents. Typically these voltages or currents represent a controlled quantity (maybe position, velocity, etc), and they want to do some kind of signal processing operation to the voltages or currents in their controller. Typically it is not power that matters to them. On the other hand, RF engineers are often much more concerned with power than voltage, because they are trying to transfer power over transmission lines to and from antennas, and it is the power that is transmitted that determines how well the communications system they are working on works. The signal to noise power ratio is their primary concern. Sometimes a circuit has some parts where power matters most and other parts where voltage or current matter most.

The answer to this question is made further important, because the differences in what you really care about only lead to subtle differences in you design, and so they can be overlooked, but these subtle differences are often important. To illustrate this let's design a simple low pass filter with a cutoff frequency of 300 MHz. Suppose this filter is driven with a 50 ohm signal generator and is connected to a 50 ohm load resistor. 300MHz Lumped LPF.png Suppose you decide that voltage is important. Assuming you can't get inside the signal generator to measure the source Thevenin voltage, the transfer function that is important to you is . This is easily found to be:

Failed to parse (syntax error): {\displaystyle V_o(\omega) over V_i (\omega) = R \over {R + j\omega L} = 1 \over {1 + j (\omega over \omega_0)}} where .