Amplification - Characteristics of outputs

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In our case, let's take an amplifier's output which supplies a signal with voltage V and power P.

As mentioned, amplifiers have an output impedance Zu. In this case what we're interested in is to guarantee maximum power transfer from the amplifier to the loudspeaker. In its pertaining section [Impedance of a loudspeaker ] we saw how typical impedance values of loudspeakers or diffusers are: 4 Ohm, 8 Ohm, 12 Ohm.

If the amplifier's output impedance is greater than the loudspeaker's impedance, the latter will demand more power from the amplifier than it can supply, causing an overload which leads to overheating of the equipment which in turn can lead to it getting damaged.

Example 14.1. Voltage and power transfer between two stages 

A numeric example should make the matter clearer. Let's consider an amplifier whose characteristics are:

Output impedance: Zout = 600 Ohm

Output power: 24 dBm [dBm ]

Firstly let's recalculate the output amplitude and power in Watt, starting from the nominal power value expressed in dBm. What we get (if you'd like to have a go yourselves) is that:

Amplification -

Amplification -

So, the amplifier is capable of supplying a maximum power value of 0.251 Watt.

Suppose now we apply an 8 Ohm load to a loudspeaker's output. The power transferred to the latter will be:

Amplification -

As we can see, for the loudspeaker to be piloted demands 19 Watt from the amplifier whereas the latter can only supply a maximum of 0.25 Watt. In this case the amplifier would overheat in trying to supply power that it simply doesn't have.

If we now try and apply a 6000 Ohm load to the amplifier we'd get the following transferred power value:

Amplification -

This time the amplifier can supply all the necessary power. It is important to bear in mind that this way the amplifier shall be underemployed, because it will never be asked to supply all the power it can potentially give. This is why power amplifiers have a low output impedance that are usually equal to the standard impedance values of loudspeakers (let's not forget that these values aren't constant but vary in response to frequency and therefore so do the power transfers).


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