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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)

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Because that is what we are intrested in; for a voltage signal we measure its voltage and don't care what current was needed to produce this voltage. – PlasmaHH Jan 21 at 14:25
    
A signal is something which contains information of some sort. If you are somehow controlling voltage to represent information, then it's a voltage signal. If you are adjusting current through a loop to signify the information, then that is a current signal. – KyranF Jan 21 at 16:20
    
If you were to add a schematic of the amplifier you speak of, we would likely be able to tell you which it is and why. – horta Jan 21 at 18:15
    
a BJT transistor is a "current amplifier", with it's default/standard behaviour and physical properties – KyranF Jan 21 at 18:47
    
but it has a voltage gain it also amplifies the voltage.. ahh am confused.. – user16307 Jan 21 at 18:47
up vote 15 down vote accepted

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.

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i edited my question. – user16307 Jan 21 at 17:29

Picture two things: an electrical source, and a load that connects to the (two pins of) the source.

The relation between voltage, current, and the load is fixed by Ohms law: current = voltage / resistance.

Consequently, when the load is unknown, the source can choose between 'fixing' (setting, determining) the voltage OR fixing the current, but not both.

Hence when we want the source to convey information, it can do so either by its output voltage, OR by its output current. And that is how we name the source.

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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 current signal or voltage signal. Imagine it is increasing both. I still don't understand how to distinguish. – user16307 Jan 21 at 17:14
    
i edited my question, – user16307 Jan 21 at 17:33
    
An amplifier is designed to have either a voltage (low impedance, high possible current) or current (high impedance, high possible voltage) output. Independently, it can have a voltage (high impedance) or current (low impedance) input. So you look at the input and output stages, and you recognise how they are designed (for low or high impedance). – Wouter van Ooijen Jan 21 at 19:24
    
"voltage (low impedance, high possible current) or current (high impedance, high possible voltage) output." is there some mistyping here? – user16307 Jan 21 at 19:36
    
No, not that I see any. – Wouter van Ooijen Jan 21 at 19:38

Basically, a voltage signal can have any load across it and it will still output approximately the same voltage. Conversely, a current signal regardless of load will give approximately the same current.

An example of a voltage signal would be a bench-top power supply. You set it to a voltage and it tries to output as much current as necessary to maintain that voltage.

An example of a current signal would be something like an inductor with a collapsing magnetic field. It results in a temporary pseudo constant current. If you put a high resistance load across it, you'll get a very high voltage.

No signal is truly a voltage or current signal entirely. We simply name them that way because certain signals are closer to an ideal current or ideal voltage source.


EDIT: You'll know based on the input of the amplifier. If the input is high impedance (a gate of a mosfet), then you're likely amplifying a voltage because a large current isn't meant to pass through a high impedance. If the amplifier is setup with a low impedance input, then you're amplifying a current. An example of this would be something like a current mirror/multiplier. In this case, the input is the drain of mosfets or collector of bjts which is low impedance.

Something to note is that most amplifiers amplify voltage so if you want to measure a current and amplify that, it's sometimes converted to a voltage through a resistor (or other means) and then amplified through an op-amp.

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i made an edit to my question. – user16307 Jan 21 at 17:31
    
imagine a typical common emitter stable biased single npn bjt transistor amplifier. such as:electronics-tutorials.ws/amplifier/amplifier9.gif?81223b . the base voltage in this case is "bias voltage + small signal 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? – user16307 Jan 21 at 18:33
    
@user16307 a common emitter amplifier has its current gain based on beta and the voltage gain based on the ratio of Rc/Re. In this case you have a voltage gain of 10 and a current gain of 100. In this case, you could say it's acting as more of current gain amp than a voltage gain amp. As I mentioned before, there is no such thing as an ideal current or voltage amplifier or an ideal current or voltage signal. Does it matter what you call it? You could increase Rc to 100k and it would then be more of a voltage gain amp than a current gain amp. – horta Jan 21 at 19:06
    
are you saying if beta is greater than voltage gain it is called s current amplifier? – user16307 Jan 21 at 19:08
    
@user16307 If Ai > Av then it's a current amplifier. If Av > Ai then it's a voltage amplifier. Both are nothing more than attempts at descriptive names. If Ai = Av, then you have a perfectly equal voltage and current amplifier. – horta Jan 21 at 19:10

A signal is a means of communication. You decide how you're going to communicate, and then you communicate using that method - whether it's by changing current, changing voltage, changing impedance, two tin cans and a bit of wet string, or whatever. One end talks, and one end listens.

In order to communicate with voltage, you put enough current into the load to get the voltage you need. In order to communicate with current, you put enough voltage into the load to get the current you need. In order to communicate with impedance, your listener applies a voltage and looks at the current, or vice versa.

Why you would choose a particular signalling method is a matter of engineering judgement, looking at the benefits and disadvantages of each.

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