Hi, How can i determine the cut-off frequency from this op amp graph ? - Page 1
Forum section: electronics / beginners

author: nyame
author_rank: Regular Contributor
timestamp: May 16, 2018, 05:36:05 pm
content: 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 ?

author: rstofer
author_rank: Super Contributor
timestamp: May 16, 2018, 11:31:48 pm
content: 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_frequencyYou could substitute 60 dB wherever I wrote 80 dB above.BTW, I'm assuming the vertical axis is dB

author: Wimberleytech
author_rank: Super Contributor
timestamp: May 16, 2018, 11:43:10 pm
content: Quote from: rstofer on May 16, 2018, 11:31:48 pmGreat answer, but I think the vertical axis is V/V (gain of 10 and gain of 100) and not dB.

author: rstofer
author_rank: Super Contributor
timestamp: May 17, 2018, 12:58:02 am
content: Quote from: Wimberleytech on May 16, 2018, 11:43:10 pmI 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.htmAgain, these definitions are covered in the Wikihttps://en.wikipedia.org/wiki/Cutoff_frequency

author: nyame
author_rank: Regular Contributor
timestamp: May 17, 2018, 12:03:54 pm
content: thanks sir,please one more think, how do i connect the 100k and the 10k curves ? please !

author: nyame
author_rank: Regular Contributor
timestamp: May 17, 2018, 12:05:27 pm
content: thanks sir,please how do i connect the 100k and the 10k curves ?

author: Wimberleytech
author_rank: Super Contributor
timestamp: May 17, 2018, 01:04:22 pm
content: Quote from: nyame on May 17, 2018, 12:05:27 pmI think you are plotting the freq response of an opamp in 1) gain of 10, and 2) gain of 100. Probably some measurement errors here and there. Your plots should line up as shown here.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 17, 2018, 02:18:22 pm
content: Quote from: nyame on May 16, 2018, 05:36:05 pmi believe those graphs are "open loop" gain. to decide cut-off frequency, first you decide how much gain you want in your circuit and draw a horizontal line from y-axis to the right. say if you want the gain of 3, we are looking at somewhere cut-off around 300Khz... if you want gain of 5, that is just over 100KHz. fwiw..Quote from: nyame on May 17, 2018, 12:05:27 pmyou cant and you shouldnt. usually open loop gain will overlap nicely (as Wimberleytech demonstrated). your graph showing different path maybe due to different setup and loading. so to know your opamp exact behaviour, you need to know your real life circuit, feedback and output loading... different loading different plot.thats why we rely on datasheet plot first as ballpark, and then do real life testing to see if our circuit matches the datasheet plot, it can be off but it should not be far off, many more can be said (on circuit setup), but thats the general idea, ymmv.

author: Wimberleytech
author_rank: Super Contributor
timestamp: May 17, 2018, 02:29:37 pm
content: Quote from: Wimberleytech on May 17, 2018, 01:04:22 pmHere is a garden variety 741 open loop frequency plot with the gain of 10 and gain of 100 cases overlayed

author: Lee Leduc
author_rank: Regular Contributor
timestamp: May 17, 2018, 02:47:15 pm
content: This tutorial may help explain what you are seeing.Notes On Gain-Error In Op-Amp Amplifiershttps://www.elexp.com/Images/Notes_on_Gain_Error_in_Op-Amp_Amplifiers.pdf

author: nyame
author_rank: Regular Contributor
timestamp: May 17, 2018, 05:26:00 pm
content: thank you sir,that area is already connect, i though i needed to connect the upper end also, please do i also need to connect all the gains and frequency respond ?or better still how should the complete graph set-up look like ?

author: nyame
author_rank: Regular Contributor
timestamp: May 17, 2018, 05:31:10 pm
content: my wish is to graph this results, and identify the cutoff frequency

author: Wimberleytech
author_rank: Super Contributor
timestamp: May 17, 2018, 05:43:53 pm
content: Quote from: nyame on May 17, 2018, 05:31:10 pmrstofer has answered your question in his first reply.

author: nyame
author_rank: Regular Contributor
timestamp: May 17, 2018, 05:59:35 pm
content: thank you sir,does it required that i plot the gain bandwidth product for the above graph, or better still i need you to guide me, i need to produce a graph out of this result ?please what should the complete graph look like ?

author: nyame
author_rank: Regular Contributor
timestamp: May 17, 2018, 06:03:32 pm
content: Hi Sir,thanks for the reply it helps.please how should a complete graph look like, appart from plotting the results and the cutt-off frequency, is the any additional thing i need to do to make the graph complete ?

author: rstofer
author_rank: Super Contributor
timestamp: May 17, 2018, 07:18:57 pm
content: Wimberleytech gave you a combined graph above. What you have is a pair of graphs, one for a gain of 100 and one for a gain of 10. It should be pretty easy to graph a gain of 20, or 30, or whatever. You know the outer edge of the gain-bandwidth product. That's the sloping line. Pick a gain, get a bandwidth.I pointed out that the sharp corners don't occur in the real world and, at the cutoff frequency, the output would be 3 dB down or 70.7% of the original value.The Wiki I linked shows you that the corner is rounded and, in fact the curve passes through the point that is 3 dB down or 70.7% of the original value.Look at the graph in the Wiki an notice how it is curved.We get the sharp corners when we approximate the response manually then sketch in the curve to fit. These are really just approximations, the only real response curve is obtained with a network analyzer like the tool in Digilent Waveforms for the Analog Discovery 2. Here a signal generator sweeps the frequency spectrum and the analog input is used to compute the graph.

author: rstofer
author_rank: Super Contributor
timestamp: May 17, 2018, 07:40:28 pm
content: Here's another documented related specifically to gain-bandwidth product.Note the definition for cutoff frequencyhttps://en.wikipedia.org/wiki/Gain%E2%80%93bandwidth_product