Circuit Idea/Combining and Varying Electrical Quantities

The Idea
A large part of the electronic devices we can "invent" by combining and varying with the parameters of the studied object. For this purpose, in a series of experiments, we choose successively an independent and a dependent variable, keeping the other quantities constant. We then begin to vary the independent variable and observe the behavior of the dependent variable. By making such separate "sections" of the studied object, we "reverse" and combine the cause-and-effect relationships between individual quantities, get new points of view and penetrate deeply into the essence of things

Examples
If we assume that in a basic electrical circuit voltage causes current (the usual cause-and-effect relationship), and start varying the quantities voltage and resistance, we get a voltage-to-current, resistance-to-current, and voltage-to-current dividers already mentioned.


 * Conversely, if we assume that the current causes voltage (reversing the cause-and-effect relationship) and vary the quantities current and resistance, we obtain the remaining converters (current-to-voltage, resistance-to-voltage, and current x resistance multiplier).


 * Using the same techniques, we can turn a digital-to-analog converter into a programmable amplifier and multiplier, and an analog-to-digital converter into a ratio meter and digital sample & hold circuit.