_Signal_FAQ02_dB - The voltage ratio is 8/3 The power ratio...

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What is dB? Basically, dB is a power ratio. Let's take an amplifier. Let's say the input power is 2W and the output power is 5W. The amplifier is said to have a power gain of 5/2 in linear scale or log(5/2) Bel, or 10 log(5/2) dB (deciBel). Here 2W is taken to be the reference power. Although dB is a power ratio, it is so common that people used it to measure absolute power as well. As an example, if the reference power is taken to be 1mW, we have the unit dBm to measure power. So, if the transmit power of your handphone is 0.5dBm, how much power is it giving? In addition to absolute power, dB is so convenient that we use it to measure voltage ratio also, assuming that the input and output resistances are the same and so power is proportional to voltage squared. To illustrate, if an amplifier has input voltage = 3V and output voltage = 8V:
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Unformatted text preview: The voltage ratio is 8/3. The power ratio is 8*8/(3*3) or in dB, 10 log(64/9) = 20log(8/3) dB. Thus, if you go to the shop and buy an amplifier (sometimes called a booster) to amplify your TV signal so that you have better reception and that amplifier has 10dB gain, what is the power gain and the voltage gain? Note that TV and many video and RF signal systems (say, your radio) have 75 ohm impedance at both in and out. This is so that it is matched to the impedance of "air" or "freespace". Otherwise, there will be reflections and you won't receive well. EM wave is light! Mismatch in impedance is the same as light going from say, water to air. However, do note that hifi amplifier do not have the same input and output impedance!...
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This note was uploaded on 08/02/2009 for the course ECE EE2009 taught by Professor Prof.c.c.ko during the Fall '07 term at National University of Singapore.

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