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3dB point or pole


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Hi Walid,
A 3 dB point is the frequency that the stage's output is down -3dB from the output at much lower frequencies.
-3dB is half power or 0.707 times the voltage.

A pole is an RC lowpass filter caused by the "Miller capacitor" of a transistor's collector-base capacitance.

Opamps have a dominant pole at a low frequency with a capacitor placed on purpose, to avoid phase-shift from other higher-frequency poles which would cause oscillation.
The -3dB point for an old uA741 opamp is at only about 25Hz. Modern opamps have it at 200Hz or higher.


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What I normally use is db = 20logVout/Vin. Since you are dealing with a capacitor, you might as well use peak to peak voltage. You will notice that you could use the instantaneous voltage, the results are the same, but you should not because there are many instances where the instaneous voltage does not divide.

So, what you are looking for is the frequency at which your output voltage is %70.7 of your input voltage. In this case, you can use peak to peak or instantaneous. Plug in a frequency to your simple RC filter, output referenced to ground, check that your voltage is % 70.7 your input voltage.

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