Amplification - Characteristics of inputs

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As already mentioned, the input signal entering an amplifier must never exceed the values indicated by the constructor. On the other hand, when the input signal has a very low amplitude, as is the case with a microphone signal, one should make sure that the signal loss is as small as possible by the time it reaches the preamplifier. To illustrate the situation, let's refer to the following circuit:

Amplification - Amplification of a microphone: equivalent circuit

Amplification of a microphone: equivalent circuit

The circuit shows a microphone represented as a voltage generator. Its internal resistance is connected to an amplifier whose input impedance is shown in the picture. I is the current flowing through the circuit and VA is the voltage measured between the microphone and the entrance of the amplifier.

This is the equation describing the situation:

Equation 14.3. Analysis of a circuit's input stage 

Analysis of a circuit's input stage

Analysis of a circuit's input stage

Analysis of a circuit's input stage

Analysis of a circuit's input stage

Analysis of a circuit's input stage

Summing up: E is the tiny voltage generated by the microphone, and VA is the voltage that reaches the amplifier's input. If we now assume that Zin is much greater than ri (in symbols Zin >> ri), in other words, that the input impedance of the amplifier is far greater than the microphone's internal impedance, what results is that in the sum (ri + Zin) we can ignore ri in relation to Zin, and this means that VA nearly equals E.

It is clear to see that in this way (making Zin >> ri) we can transfer almost all the voltage generated by the microphone to the amplifier's input. Otherwise we would have VA << E, in other words, we'd have deteriorated the microphone signal. This rule is generally true, and usually we'd consider impedance adaptation to be correct when:

Amplification -

This brief explanation, though perhaps a bit heavy-going for those of you who aren't as yet well acquainted with electric circuits, is fundamental for the comprehension of the transferring of signals from one stage to another (in this case, from a microphone to an amplifier).








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