Strange phase behavior on ac-coupled op-amp - Page 1
Forum section: electronics / beginners

author: Fulcrum
author_rank: Regular Contributor
timestamp: May 08, 2016, 12:05:06 pm
content: Hello everyone.I am wondering if anyone can explain the behavior I am seeing the frequency response of this circuit:As you can see, it's simply an OPA847 op amp in an inverting configuration. The capacitor and resistor on the noninverting input are recommended by the datasheet. Running an AC analysis on this circuit results in the following bode plot:Red: VM2, op amp output. Green: VM1, op amp inputOne can see that the op amp phase difference starts out at -180°, as expected. However at around 20KHz, the phase of the input mysteriously increases to a phase difference of -270° at around 3MHz. When the op amp goes past its cutoff frequency, it reaches a phase difference of -360° much faster than it normally would, due to the extra 90° of phase difference that sets in at around 20KHz.I assume the op-amp is the one messing things up, but if anyone has a proper explanation, I'd love to hear it, as well as a way to ac-couple the op amp without getting this behavior.

author: Fank1
author_rank: Regular Contributor
timestamp: May 08, 2016, 12:44:30 pm
content: Run it again only directly ground or remove the 10n cap on the non inverting input.

author: Fulcrum
author_rank: Regular Contributor
timestamp: May 08, 2016, 12:47:17 pm
content: Quote from: Fank1 on May 08, 2016, 12:44:30 pmI just did, and I got the exact same behaviour. Not even worth posting the bode-plot, there's practically no difference.

author: danadak
author_rank: Super Contributor
timestamp: May 08, 2016, 02:32:46 pm
content: There is no current thru R3 (other than bias current) therefore Vr3 = 0,hence C1 effectively out of the circuit. This is true as long as f(aol) stillshows significant G.Regards, Dana.

author: mmagin
author_rank: Frequent Contributor
timestamp: May 08, 2016, 03:48:31 pm
content: Quote from: Fulcrum on May 08, 2016, 12:05:06 pmWow, that Y-axis scale is super confusing.A quick look at the datasheet doesn't show anything obvious that would cause this. I'm having my doubts about the accuracy of the simulation.

author: Fulcrum
author_rank: Regular Contributor
timestamp: May 08, 2016, 04:44:38 pm
content: Quote from: danadak on May 08, 2016, 02:32:46 pmNot sure what that has to do with anything, but the cap is there for a reason. Not sure what, but the AC analysis shows a bit different behavior if I omit it.Quote from: mmagin on May 08, 2016, 03:48:31 pmYeah, I could find nothing in the datasheet either. Using any other op amp shows the same behavior, except of course when using an "ideal" op amp. Google is not giving me any sugar on this topic, so I have resorted to the EEVblog.

author: mmagin
author_rank: Frequent Contributor
timestamp: May 08, 2016, 05:43:53 pm
content: Quote from: Fulcrum on May 08, 2016, 04:44:38 pmNow that I think about it some more, VM1 is a weird place to measure the 'input'. With a theoretical op-amp, that would be at zero all the time.Thinking about it some more, the 60 pF won't even have the same reactance as the 47 ohm resistor until the frequency reaches 56.3 MHz. You might want a much larger input capacitor, depending on what you're doing.

author: Fulcrum
author_rank: Regular Contributor
timestamp: May 08, 2016, 06:02:14 pm
content: Quote from: mmagin on May 08, 2016, 05:43:53 pmWell, the phase difference of an op-amp is measured from its input to its output, yes? And we're not talking about an ideal op amp, neither.Changing the capacitor to a larger value causes the phase response to sort of shift its first breakpoint further back. The frequency response is as expected, with the RC filter cutoff frequency being shifted to a lower frequency. The extra 90 degrees of phase difference is still present and seems to max out at the same point in frequency.5nF:15nF:50nF:

author: whollender
author_rank: Regular Contributor
timestamp: May 08, 2016, 06:03:09 pm
content: An ideal opamp (infinite open loop gain and no frequency roll-off) would drive vm1 to zero. Any real opamp will have finite open loop gain, and a dominant pole at a low frequency.I'm pretty sure the behavior you're seeing is due to the dominant pole rolling off the open loop gain vs frequency.You can test that by modeling the opamp as an ideal opamp, then with finite open loop gain, then with finite open loop gain and a dominant pole.

author: Fulcrum
author_rank: Regular Contributor
timestamp: May 08, 2016, 07:33:25 pm
content: Quote from: whollender on May 08, 2016, 06:03:09 pmAll right. Can anything be done about it? It's gonna reach a 360 degree phase difference awfully fast, no way it'll be stable.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 08, 2016, 07:44:33 pm
content: You're measuring total phase, which has no restrictions.Loopphase and gain must satisfy Nyquist loop stability criterion.Tim

author: danadak
author_rank: Super Contributor
timestamp: May 08, 2016, 11:39:28 pm
content: This is a very challenging OpAmp to work with, its Aol dominant poles are 10 Khzand 100 Mhz, and still plenty of G when loop phase crosses 180 degrees. Even lowCload on output very much issue in loop phase shift.This part has 2 dominant poles, combined with additional poles due to strays, comp,and Zout issues lead to need for very careful examination of loop dynamics.Just layout issues very challenging for a part like this, use their recommended layoutand bypassing to aid in success. Factory probably has a dev board to work with.Also I would do a direct call to TI FAE/factory to see if you can get in touch with designer(s)and establish just how good/accurate the spice model is for RF work. You might find thereis a fair amount of uncertainty especially at HF and into the Ghz area the GBW of this part has.Spice is not a good analytical tool at short wavelengths. S parameter design approach would be moreappropriate. Even consider looking at design with a network analyzer to get the "real" margins.Regards, Dana.

author: Fulcrum
author_rank: Regular Contributor
timestamp: May 09, 2016, 08:52:59 am
content: Quote from: danadak on May 08, 2016, 11:39:28 pmThanks for the in-depth answer. We have a beefy network analyser at our faculty, perhaps I can borrow it. If I have no luck, I guess I'll just have to lower the gain or put a cap in parallell with the Rf.

author: danadak
author_rank: Super Contributor
timestamp: May 09, 2016, 09:37:15 am
content: What are your actual requirements ?1) G2) BW3) C load for OpAmp4) Zin for OpAmp circuit5) Slew Rate6)......Regards, Dana.

author: Fulcrum
author_rank: Regular Contributor
timestamp: May 09, 2016, 10:11:00 am
content: Quote from: danadak on May 09, 2016, 09:37:15 am1) Gain simply as high as possible.2) Bandwidth of about 100MHz3) Capacitive load is about 20pF4) Unknown.5) At least 1000V/µsIf it is any help, the AC coupling capacitor is connected to the output of a common-base transistor amplifier based on the BFR106.

author: whollender
author_rank: Regular Contributor
timestamp: May 09, 2016, 03:22:10 pm
content: Quote from: Fulcrum on May 09, 2016, 10:11:00 amIt sounds like you have some sort of RF application in mind. Looking through the OPA847 datasheet, it looks like it should actually work pretty well for you assuming that you can tame it.As Dana mentioned, the SPICE model for this part may not be particularly accurate, so it would probably be best to either get an eval board from TI to make sure it will work in your application, or mock up the circuit (use an x-acto to cut up some copper clad, dead bug may also work) and play around with different gains, adding additional cap around feedback resistor, etc.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 09, 2016, 04:26:58 pm
content: Er... gain "as high as possible" is limited by thermal noise, 9.2uV from 50 ohms. Or around 100dB. More than that and you're clipping on noise, even with a noiseless amplifier.But that still wouldn't be practical due to the limited dynamic range.Do you actually want an IF amp?And even more importantly: do you actually want a variable tuned IF amp?Tim

author: Fulcrum
author_rank: Regular Contributor
timestamp: May 09, 2016, 06:18:16 pm
content: Quote from: whollender on May 09, 2016, 03:22:10 pmWell, I was able to eventually get to behave! I just had to sacrifice some bandwidth, but that's fine. Cranked up the amplification on the real circuit that I had built and it stopped oscillating.Quote from: T3sl4co1l on May 09, 2016, 04:26:58 pmWell, of course "as high as possible" should have been "as high as possible within reasonable limits".I honestly don't know what an IF amp is, so I can't tell you.But yeah, it's working now, thanks to everyone who took the time to help me! Much appreciated.