This might sound like a crazy bad question. But I'm wondering even though current and voltage inevitability co-exist, why do we use the term for one concept(current or voltage)? If there is a signal out there somewhere, it is both current and voltage at the same time. But we name only one of its property.
Is that about knowing one of them well? I mean lets say we have an amplifier and if we only set the amplifier with a "known voltage gain" we call it voltage amplifier and if we set it with a "known current gain" we call it current amplifier? Or is that because the nature of the input signal?
Could you give an input signal example and explain why it is called a voltage or current signal?
EDIT: My confusion didn't settle. Lets say we have a single stage amplifier. And it has an input and output. So when you look at such circuit and its input and output, what makes you to conclude it is amplifying a current input signal or a voltage input signal? What is the method to name the type of the input signal? Imagine it is increasing both. I still don't understand how to distinguish.
EDIT2: imagine a typical common emitter stable biased single npn bjt transistor amplifier. such as: http://www.electronics-tutorials.ws/amplifier/amplifier9.gif?81223b . the base voltage in this case is "bias voltage + small signal voltage – emitter voltage". and the input current current is very low. now look at the output. output voltage increased. okay. but wait.. output current also increased and became "beta*Ibase". So now the input current increased and the input voltage also increased.. is this a current or voltage amplifier? and if it is X amplifier does that mean the input signal is X signal. (X is current or voltage)
For a signal, generally one of voltage or current is what is being controlled, and the other is a bi-product that is dependent on the load.
Consider a normal digital logic signal running between two CMOS chips on the same board. That's a voltage signal. Only the voltage is specified. Not only is the current not specified, but it can vary hugely and isn't known by the designers of the transmitting chip since it depends on the load the receiver presents.
If the only receiver in the above example is a CMOS chip, then very little current will flow in steady state. The load is almost purely capacitive, so current will flow in short blips when the logic level is changed. If instead this signal drives a LED and resistor so as to light the LED when high, then the current will be very different from the previous case. It will again be very different if the LED is wired between power and this signal (lit on logic low) instead of ground and this signal (lit on logic high).
Sometimes the signal value is encoded in the current, in which case the voltage ends up what it ends up. A common example is the industrial 4-20 mA sensor standard. The data is encoded by allowing from 4 to 20 mA to flow, but the voltage can vary over a range. In fact, this is called the compliance range, with a larger range allowing for more flexible use of the device. In this case, you can't look at the voltage to get the value being transmitted.