This discussion is old as the very bipolar transistor... but it is still interesting...

In my opinion, a BJT can be controlled either by voltage or current depending on the situation. If we want a maximum gain (the case of amplifying stages), we apply a small (to be a linear device) AC voltage (e.g., ±100 mV) with an additional appropriate DC biasing voltage (e.g., 0.65 V) directly to the base-emitter junction; then we can consider the BJT as a voltage-controlled device. The problem is that the base-emitter junction, like a diode n-p junction, is not so suitable for a voltage control since it behaves as a voltage source (more precisely, as a constant-voltage nonlinear "resistor"). Figuratively speaking, there is a conflict between two constant-voltage devices connected to each other (an incorrect arrangement). The dual incorrect arrangement is if we try to inject a current into the BJT collector (the "dynamic load" odd idea).

If we want a maximum input voltage range, we drive the BJT base-emitter junction through a resistor Rb (the picture in the attachment below); then we can consider the BJT as a current-controlled device. We may look at this arrangement from two viewpoints. First, we can think of the combination "Vin + Rb" as a simple current source that is shorted by the transistor input (the base-emitter junction acts as a current load). Second, we can think of the combination "Rb + base-emitter junction" as a transistor with a large voltage range (a voltage load).; as though the transistor has enlarged many times its input voltage range and it has transmuted into a kind of FET-:) The increased linearity will be an additional advantage...

This is an old V-to-I technique widely used in the measurement in the past - imagine an old-fashioned analog voltmeter built by an ammeter (a movement) and a resistor connected in series. Like in the case of the BJT above, we can control a real ammeter by a small voltage (a few hundred mV). The only difference will be the linearity...

But yet, what does "voltage controlled" and "current controlled" mean? Is this a property of the input source or of the very BJT?

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