| Electronics > Beginners |
| Hi, How can i determine the cut-off frequency from this op amp graph ? |
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| nyame:
Hi, please how can i determine the cut-off frequency of this op amp graph, and please how can i connect the two curves together ? |
| rstofer:
You already have the cutoff frequencies where the flat portion of the curve begins to turn down. About 6 kHz and 80 kHz. This looks a lot like a gain-bandwidth problem. If you want 100 dB of gain, the bandwidth is a little over 6 kHz. If you want 10 dB of gain, the bandwidth is about 80 kHz. Usually, the cutoff frequency is considered to be the frequency at which the output is 3 dB down. That's pretty hard to see on a graph of 100 dB but it's pretty easy on a 10 dB graph. Since the graphs aren't exactly real in the sense of the fall off, it's a little difficult to say whether the lower graph cuts off at 60 kHz or 80 kHz. Normally, that sharp turn down is at the frequency where the output is already 3 dB down and there is a curve where that sharp turn it located. Look at the Bode' plot here: https://en.wikipedia.org/wiki/Cutoff_frequency You could substitute 60 dB wherever I wrote 80 dB above. BTW, I'm assuming the vertical axis is dB |
| Wimberleytech:
--- Quote from: rstofer on May 16, 2018, 11:31:48 pm --- BTW, I'm assuming the vertical axis is dB --- End quote --- Great answer, but I think the vertical axis is V/V (gain of 10 and gain of 100) and not dB. |
| rstofer:
--- Quote from: Wimberleytech on May 16, 2018, 11:43:10 pm --- --- Quote from: rstofer on May 16, 2018, 11:31:48 pm --- BTW, I'm assuming the vertical axis is dB --- End quote --- Great answer, but I think the vertical axis is V/V (gain of 10 and gain of 100) and not dB. --- End quote --- I just came back in to rewrite my reply. I agree, the graph is probably in V/V. That makes a lot more sense given the logarithmic scale. My bad... The nominal cut-off frequency is still the point where the signal is 3 dB down but that needs to be converted to a voltage ratio and 3dB is 0.707 times the unattenuated voltage. So, for the upper graph, the cut-off frequency would be the point where the value is about 70. For the bottom graph, the cut-off frequency is where the value is about 7. http://www.sengpielaudio.com/calculator-gainloss.htm Again, these definitions are covered in the Wiki https://en.wikipedia.org/wiki/Cutoff_frequency |
| nyame:
thanks sir, please one more think, how do i connect the 100k and the 10k curves ? please ! |
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