# Why these can't be an amplifier? [closed]

I am asking if we have a voltage signal must be amplified why we can't mount it in a resistance less than one and take the output as a current by using Ohm's law I = V/R and R is less than one ?

• Because in I = V/R the left side of the equation is current, whereas you are saying that you wish to amplify voltage. Commented Apr 26, 2015 at 20:56
• But can i take current as indication of voltage? Commented Apr 26, 2015 at 22:02
• Amplifier increases power of the signal, by definition. Do you wish to take voltage $V$, apply it across resistance $R$, dissipate power $P = V^2/R$, and call that an amplifier? What are you ultimately trying to accomplish? Commented Apr 26, 2015 at 22:15
• Current is current, and voltage is voltage. Confusing the two doesn't make you look clever. Commented Apr 26, 2015 at 22:27

You are right that on constant resistance, you can determine the voltage by measuring the current. But in no way, current an voltage are equivalent.

Voltage tells you how much energy is in a single electron, which can also be interpreted as a kind of pressure on the electrons. On the other hand, current tells you how many electrons are passing through a wire in a specific time.

That's why we talk about voltage sources (that output a constant voltage, independent of the load resistance), and as they have a maximum current they can produce, they need a minimum resistance. Check your audio amp - it likely is specified for speakers with at least 8 or 4 Ohms. Or check your wall wart power supplies, they are specified with a maximum current (typically 1 to 2 amps for smartphone chargers), which means they can not keep up the specified voltage, if the resistance gets too low (below 5 Ohms on a 5 Volts, 1 Amp supply).

On the other hand, we talk about current sources, those are outputting (we often call it sourcing) a specific current, independent of load. These kind of sources have a maximum voltage (called the *compliance voltage) they can emit. If the resistance is too high, these sources fail to emit the current they are specified to emit. Some LED drivers are built as current sources, because the LED itself needs a specific current for effective operation, while its resistance (and thus the voltage) are dependent on the temperature and age of the LED.

You are talking about amplification in your question. Actually, you need to specify what you want to amplify, and how you want your amplifier to behave. For example, you can have (typical audio amps are built this way) a voltage source, whose output voltage is controlled by the input voltage. (The modelling theory, for example used by Spice) calls this a voltage controlled voltage source). Is this case, you can write down a number, called the voltage gain, which is the factor between output and input voltage. The output current depends on the output voltage and the output resistance. On the other hand, you might already have a voltage of the value you want, but the voltage source is only able to source a couple of milliamps. You need hundreds of milliamps for your application (e.g. a DC motor), though. You can build an amplifier that outputs the same voltage as it has on the input, but only takes a little amount of current on the input, but is able to emit large currents on its output. This amplifier has a voltage gain of 1, but the current gain (ratio between output and input current) can be several orders of magnitude.

What's definitely wrong is the idea to compare the number of volts to the number of amperes, and concluding that something got amplified, because there are more amperes than volts (that's what you get on a resistance below 1 Ohm). If something gets amplified, you have more of the same quantity than you had before. So you might have more voltage than without the amplifier, or more current than without the amplifier, and in most but very exotic cases, you have more power (product of current and voltage) than without the amplifier.

You are wanting to increase voltage, so you need to look at the equation as:

V = I x R


What happens if you keep the current constant, and make R small - notice that the voltage will get smaller?

Now if you could some how keep the current constant and increase the resistance, notice that the voltage goes UP. That all sounds great, why not add a bigger resistance, who needs those silly amplifiers...

Well, not so fast. There is another equation you need to think about, and its the power one.

P = V x I


If you keep the current the same, but increase the voltage, then the power in the signal will have to increase - this is really what amplification is, adding power to a signal.

Unfortunately, resistors are passive devices, so called because they cannot add power, only dissipate it. If you think about it, if a resistor could add power, it would have to have some power supply, otherwise you would be getting free energy.

• I do not want to increase voltage , i want to take the current as an indication to the voltage , it is just a Resistance factor between them Commented Apr 26, 2015 at 22:03
• @MohamedOsama how are you planning on measuring the current? How much current can the source of your signal provide? Commented Apr 26, 2015 at 22:11