Why does this NPN transistor operate differently if it comes before the switched element?

 This is my answer to SE EE question Why does this NPN transistor operate differently if it comes before the switched element?

My answer

I want to further develop the philosophy inside some of the answers here making a few comparisons between the two circuits.

In fact, both circuits are voltage followers but they follow different voltages - the first circuit "follows" the supply voltage V1 and the second circuit follows the input voltage V2. And because the supply voltage is higher than the input voltage, the current through the LED in the latter case is lower.

If we continue with such simple comparisons, we can say that the transistor in the first circuit acts as a *closed switch* with zero resistance. No voltage is lost on it, so all the supply voltage is applied to the LED network. In the second circuit, the transistor acts as a *variable resistor* ("rheostat") on which, roughly speaking, the difference between the supply and input voltage is lost.

Both circuits are used here as switching (digital) circuits but, in fact, both they are analog circuits made act as digital circuits. For this purpose, the first is "overdriven" while the second follows the input voltage by the help of negative feedback.

Let us now see how, in principle (when the transistor is not "overdriven"), moving the load (LED circuit) from the collector (first circuit) to the emitter (second circuit) drastically changes the transistor behavior:

• When the LED network is in the collector, the transistor keeps up a constant current through the LED (acts as a current "source"). For this purpose, it changes its collector-emitter "resistance" so that to keep the total resistance constant. This is its genuine behavior.
• When the LED network is in the emitter, the transistor's behavior is changed by the mechanism of the negative feedback and now it keeps up a constant voltage (acts as a voltage source). For this purpose, it changes its collector-emitter "resistance" so that to keep a constant ratio between its "resistance" and the total resistance. This is its modified behavior.

So, by means of the negative feedback phenomenon, a current source (common-emitter stage) is converted to a voltage source (common- collector stage).

True, these are a few unusual considerations that resemble rather 99% Lindt dark chocolate than the sweet Milka chocolate:-) but they can make us think...