Electronics > Beginners
Hi, How can i determine the cut-off frequency from this op amp graph ?
(1/4) > >>
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 !
Navigation
Message Index
Next page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod