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jamescat
Joined: 09 May 2007
Posts: 3
Location: Charlotte NC
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 Posted:
Wed May 09, 2007 11:35 pm Post subject:
decibel and logarithm question |
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I am taking an online electronics course and I am stumped on decibel/logarithm section relating to amplifiers. My problems deal in power or voltage gain, and levels. Here are 2 examples that I cannot solve. (1) A power amplifier has a gain of 26dB, and an input of 1 volt. What is the voltage level? (2) An amplifier has a voltage gain of 60dB, the input is 10 microvolts, what is the output? I have a couple formulas, and I know the answers, but I can't figure out how to get the answers from the formulas. The answer to problem one is 20, and the answer to problem 2 is 10. Here are the formulas:
Gain= signal out/signal in
dB= 10 x log(base 10) (W out/ W in)
dB= 20 x log(base 10) (V out/ V in)
Output voltage= input level x gain ratio
I need a very detailed expression as to how to solve these problems. I have looked in two textbooks for further explanation, but I cannot find the solutions.
Thank you so much if you can help!!!!
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ronsimpson
Joined: 30 Apr 2007
Posts: 11
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neon
Joined: 25 Feb 2006
Posts: 579
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Fri Jun 01, 2007 1:56 pm Post subject:
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i think you do not understand it is a ratio in-out 20db means gain of 100. it can be used for power volts current whatever it is a ratio.
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Electrofix
Joined: 01 Jan 2008
Posts: 5
Location: Worcesterhire, UK
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Mon May 26, 2008 10:23 pm Post subject:
dB calcs. |
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OK - let's go through this step-by-step.
In Q. 1 the gain of the amp. is stated as 26 dB. You are given the input level as a voltage. You are required to find the output voltage. Therefore, you need the relationship that connects input voltage, output voltage and gain (dB) - which is:
dB = 20log(voltage-ratio)
Therefore, 26 = 20log(voltage-ratio)
so 1.3 = log(voltage-ratio)
so 10 to the power of 1.3= (voltage-ratio)
so 19.95 = (voltage-ratio)
Since input = 1 volt, output = 20 volt.
Q2 is essentially the same, only the voltage gain is given.
Same equation applies, which produces Vout of 10mV
_________________
If an electrolytic cap. explodes and there is no one there - does it make a sound? |
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