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db

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prasannan82

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Hello,
why should we express gain in db and plot the graph?why dont we take the gain directly on y axis and freq on x axis and plot the graph?
 
The ranges are frequently too big in both gain and frequency, and things frequently make better sense as log plots. We hear logarithmically, for example, so audio amplifier gain is plotted logarithmically. Same with frequency.
 
Imagine deciding a scale for the gain if an amplifier has a bandwidth of 100MHz to 1GHz.

Log plots make this sort of thing possible and if the frequency and gain are both in dB then the logs will cancel each other out so the plot will look the same.
 
prasannan82 said:
Hello,
why should we express gain in db and plot the graph?why dont we take the gain directly on y axis and freq on x axis and plot the graph?


In fact, the graph would be good either way. To graph something in dB simply reflects that sometimes you will be measuring or calculating in dB for other reasons and it becomes convenient to keep the values in dB for graphing.

For example, in analog design, we often prefer to use dB so that when we cascade many stages we can add or subtract the gains (in dB) rather than multiplying and dividing (when not in dB). Adding and subtracting small values is much easier to do in your head than multiplying and dividing big numbers.
 
Also, its much easier to draw a bode plot (the name for a gain vs frequency plot that is logarithmic on both axis) than it would be to draw the same plot without log scales. With log scales you can use relatively straight lines.

https://en.wikipedia.org/wiki/Bode_plot

The way I was taught to draw these was to just ignore the minute curves and draw approximations. Wherever there is a pole in the filter you subtract 20db / decade from the magnitude plot and wherever there is a zero you add 20db / decade.
 
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