Question

Figure shows a communication system. What is the output power when input signal is 1.01 mW?

(Gain in dB = 10 log$10$ $\frac{P_{0}}{P_{i}}$)

• A. 90 mW
• B. 101 mW
• C. 112 mW
• D. 120 mW

Answer 1

Answer: (B)

101 mW

Answer 2

:

Here, transmission path $= 5$ km

Loss suffered in transmission path

$= - 2dBkm^{- 1} \times 5km$

$= - 10dB$

Total amplifier gain

$= 10dB + 20dB = 30dB$

Overall gain of signal

$= 30dB - 10dB = 20dB$

As, gain in dB $= 10\log_{10}{}\left( \frac{p_{o}}{p_{i}} \right)$

$\therefore 20 = 10\log_{10}{}\left( \frac{p_{o}}{p_{i}} \right)$

Or $10\log_{10}{}\left( \frac{p_{o}}{p_{i}} \right) = \frac{20}{10} = 2 = \log_{10}100$

Or $\frac{p_{o}}{p_{i}} = 100$

Or $p_{o} = p_{i} \times 100 = 1.10mW \times 100 = 101mW$