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Explain with a neat sketch the two categories of front-end amplifiers used in optical fiber communication systems.
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The basic amplifier configurations that are used in optical fiber communication are front end amplifiers

The two categories of front end amplifiers are as follows:

i. Low impedance preamplifier

ii. High impedance preamplifier

Low impedance preamplifier:

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  1. The bandwidth is determined by passive impedance which is across the detector terminal and is taken as $R_L$.

  2. $R_b$ is the detector bias resistance that is parallel with $R_a$ and $R_L$ may be modified to consider the parallel resistance $R_b$ || $R_a$.

    Total load resistance is $R_{TL} = \frac{R_a R_b}{R_a+ R_b}$

  3. Low impedance preamplifier can operate over a wide bandwidth but they have poor receiver sensitivity.

High impedance preamplifier:

  1. The second configuration has a high impedance together with a large detector bias resistor in order to reduce thermal noise.

  2. This structure gives a degraded frequency response as the bandwidth relationship is not maintained for wideband operation.

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  1. The detector output is integrated over a large time constant and it can be restored by differentiation.

High impedance structure gives better sensitivity than low impedance structure, but equalization techniques are needed plus it has a problem with limited ratio of the maximum to minimum input signal range.

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