TABLE OF CONTENTS Then at the balance let s be mute,
We never can adjust it;
What s done we partly may compute,
But know not what s resisted(John Burns)
Definition. A preamplifier is an amplifier that is used at the front-end of a transmission system to raise the output of a low level source so that the signal might be further processed by the main part of the system without appreciable degradation in the S/N ratio.
Note
A preamplifier may also include provision for equalization and/or mixing.
A preamplifier is essential when the signal to be processed is so weak that its amplitude becomes comparable to that of spurious signals or intrinsic noise. Typically, the receiving antenna of a radio communication system or a sensor in a noisy industrial environment are low-level sources that both require preamplifiers.
Theory. We have already seen in Sect. 4.5.6 that for several cascaded two-ports, the overall noise factor can be computed with the expression (4.78):
This equation states that if the gain of the first stage is greater than unity (G 1 > 1), then all fractions are divided by G 1 and consequently their noise contribution to F is considerably reduced. The greater the gain of the first stage, the less significant the contribution of the following stages.
In contrast, when G 1 < 1, the contribution of the following stages is artificially augmented and the overall noise factor is seriously degraded. This...
