Signals carry information from a transmitter (the signal source) to a receiver (the signal destination). The receiver has to pick up some energy for it to notice that something has arrived. It is a basic principle of information theory that all signals must provide energy to be noticed. The power (amount of energy carried per second) of an electronic signal is equal to the voltage × the current. In general, we can expect the rate at which information can be transferred will depend upon the received power level. The higher the power, the more information per second can be conveyed.

The vital requirement that all real signals must carry power has some important consequences for electronics.

• Anything that receives or responds to electric signals has to have a non-zero, non-infinite input resistance. This determines the amount of current that the receiver demands when you try to apply a given voltage to it.
• Anything which transmits a signal must have a non-zero source resistance. Otherwise, it would be able to provide an infinite output current when providing a signal voltage - i.e. infinite power - which is impossible!

Consider the example illustrated below:
The signal source is trying to send a signal voltage, V_g.
This has to pass through the source's output resistance, R_s.

Viewed by the source, this is in series with the amplifier's input resistance, R_in.
So it has to supply a current
I = V_g / (R_s + R_in).
This produces a voltage at the amplifier input of
V_s = IR_in = R_inV_g / (R_s + R_in).
So the power entering the amplifier's input is
P = V_sI = V_g² × R_in / (R_s + R_in)².


Note that this input power would be zero if the input resistance were zero and if it were infinite. A zero input resistance makes it impossible to obtain a non-zero voltage across the amplifier's input - hence no power enters the amplifier. An infinite input resistance makes it impossible to get a current to enter the amplifier - no power again. To get the maximum possible amount of power to transfer from source to destination - making the signal as obvious as possible to the receiver - we need to choose R_in = R_s.

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