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The output of a single-pole lowpass filter (in an opamp or in an audio amp) causes the output to drop 6dB (half the voltage) per octave, not 3dB per octave.
Its minimum rolloff is -3dB at 75kHz so its output will be reduced a little at 15kHz.
There is no LM357. Maybe you mean the LF357? Its GBP is 20MHz, not 10MHz.As we all know the bandwidth is a freq where the volatge gain of an amplifier reaches to 0.707 of its max (i.e -3dB of reduction in voltage gain). so the bandwidth is where the volatge gain drops by 3dB
the GBWP for LM357 is 10MHz.
the formula for caculating the bandwidth says"
Gain x bandwidth = GBWP
Know suppse that the voltage gain of an op-amp is 5.6 (i.e 15dB), so the bandwidth would be:
10MHz/5.6 = 1.8MHz of bandwidth.
so the question is:
Can we tell that at 1.8MHz (which is the bandwidth of LF357 by the gain of 5.6)the voltage gain drops by 3dB and reaches to 12db or to 3.96?
There is no LM357. Maybe you mean the LF357? Its GBP is 20MHz, not 10MHz.
The gain at 1.8MHz is 20 on the graph on its datasheet but it shows a GBP of 28MHz.
The GBP of an opamp doesn't matter because its slew rate reduces the max output level at high frequencies.