Circuit Idea/Using Cause and Effect Relationships
Revealing causality edit
Classical electronics courses do not reveal cause and effect relations in electronic circuits. For example, who cares if there is a causality and what causes what (what quantity is an input and what an output) in Ohm's law? Authors just suppose that voltage and current change simultaneously; they do not mind how the famous rule is written (I = V/R, V = I.R or R = V/I).
Also, the classical approach considers active elements as proportional, inertialess devices where the input and output quantities change simultaneously. For example, it is believed that the collector current of a transistor changes simultaneously with its base current, the output voltage of an operational amplifier changes simultaneously with the input its voltage etc. Viewed in this way, the operation of the devices can hardly be understood because the cause-and-effect relationship between the input and output quantities is not visible.
Only, we human beings consider every change in this world as a result of some cause (in electronics that means the output quantity is a result of the input quantity). We cannot imagine that the input and output quantities can change simultaneously. We know that always the input is first and the output is second; so, the output always follows (delays) the input. So, in order to understand the operation of electronic devices, cause and effect relations must be seen.
Introducing causality edit
In the case when apparently there is no causality in electronic circuits, we can introduce it. Let's for concreteness consider the example above of Ohm's law. There, we first assume that voltage causes current (I = V/R) in a voltage supplied Ohm's circuit; thus we "invent" the simplest voltage-to-current converter.
Changing causality edit
But we know that this cause and effect relation is an arbitrary choice; so, we can change (reverse) it. This means we can assume with the same success that current causes voltage (V = I.R) in a current supplied Ohm's circuit; thus we "invent" the reverse current-to-voltage converter.
Evolving this powerful idea we will (re)invent a lot of useful and original circuits by using any accessible circuit points (including circuit outputs, supply terminals, etc.) and component parameters as an input. For example, varying with resistance as an input quantity we will obtain a resistance-to-current converter (in the case of a voltage supplied Ohm's circuit) and a resistance-to-voltage converter (in the case of a current supplied Ohm's circuit). Then, applying an input voltage to the output of an emitter follower we will "invent" the odd common-base transistor amplifying stage. Later, changing causality, we will transmute a digital-to-analog converter into a digital controlled amplifier.