Electronic – Why is power so often associated with current and not also voltage

currentpowervoltage

I was reading one of my textbooks the other day about linear BJT emitter-follower amplifiers (non-relevant) and came across the following passage:

Although the small-signal voltage gain of the emitter follower is slightly less than 1, the small signal current gain is normally greater than 1. Therefore the emitter follower circuit produces a small signal power gain.

But I've learned that the power can be expressed:

\$P = IR = \frac{V^2}{R} = I^2R\$

Which means that power is directly proportional to both current AND voltage. Wouldn't this mean that a large voltage gain also provides a power gain?

This is not the only place I've seen this discrepancy either. It seems that whenever people talk about power, they are only really concerned with current and not voltage, even though the math seems to suggest that is not the case.

Can anyone elaborate on this?

EDIT: One explanation I can think of is that there can be voltage across an open circuit, so increasing that voltage would theoretically increase power, even though nothing is getting hot… whereas if there's current flow increasing power by increasing current would make the component dissipate more energy…

Best Answer

It seems that whenever people talk about power, they are only really concerned with current and not voltage

Ignorant people perhaps, but every competent electrical engineer knows that both voltage and current must be taken into account.

Although the small-signal voltage gain of the emitter follower is slightly less than 1, the small signal current gain is normally greater than 1.

There is no discrepancy here. The authors are pointing out that if there is current gain then voltage gain is not required to increase power. So the myth they are trying to dispel is that you can't have power gain without voltage gain - exactly the opposite of what you think people are concerned about.

One explanation I can think of is that there can be voltage across an open circuit, so increasing that voltage would theoretically increase power

If the output is open circuit then it will draw no current so there cannot be any power gain. However it may still be useful to consider the voltage gain if that voltage can be maintained with a load.

In some circuits (eg. video amplifier) the source and load impedances are matched, resulting in half of the output voltage and power being lost in the source. In this case you would normally only consider the voltage gain under load (so a video amp with open circuit voltage gain of 2 is actually a unity gain buffer).

In others (eg. audio amplifier) the load impedance is normally much higher than the source impedance, so the voltage gain remains (almost) constant whether driving the load or open circuit. If the load impedance is reduced then it will draw more current and power at the same voltage. This is not important for low level signals so usually only the voltage is considered. The output power of an audio power amplifier is very important, so speaker impedance is always considered - but output voltage and current are rarely mentioned.

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