Purpose of this amplifier circuit with op. amplifiers - Page 1
Forum section: electronics / beginners

author: bonzer
author_rank: Regular Contributor
timestamp: February 02, 2019, 10:00:00 pm
content: Hello everyone! Please help me to understand the main purpose behind such a configuration (see attached image). What I see here is the formula of an amplifier with the gain from image gain.jpg attached. The request is to build a passband filter, so the gain I found is almost certainly right (but I'm not 100% sure). What surprises me the most is what goal made the creator to put them in that way? It feels like it could be a way to increase the open loop gain by putting a sequence of two so this way the gains get multiplied (in bode diagram it's the sum). I tried to build a block diagram to show what I mean.NOTE: this circuit might be unstable and alsouncomplete(so without compensation components or other additional stuff needed for big gain and so on).

author: Yansi
author_rank: Super Contributor
timestamp: February 02, 2019, 10:04:13 pm
content: Seems like some textbook schematic where there was too much (you know what) smoke present when writing the book?Otherwise, it resembles a kind of PID regulator transfer function, but not really. Hard to tell what meaning it could have, when the context is not known.

author: bonzer
author_rank: Regular Contributor
timestamp: February 02, 2019, 10:16:44 pm
content: Actually it is posted by a university professor so it still could only be a result of his imagination. Whatever, the circuit should be considered as a passband filter in my opinion (even though it says that it's an amplifier, but it's because the central frequency gain should be something like 34dB).There's still some work to be done but if in your opinion (all you readers, not only Yansi) it's just a mind-scratching exercise then is my gain result 100% true (assuming op. amps are ideal) ? And what do you think about my block diagram?

author: Yansi
author_rank: Super Contributor
timestamp: February 02, 2019, 10:20:56 pm
content: I am used to different pass-band filters. What advantages does this configuration have, above the common Sallen-key and MFB topologies?I am just widely guessing, never seen this practically used anywhere.

author: edavid
author_rank: Super Contributor
timestamp: February 02, 2019, 10:27:21 pm
content: It appears to be a Wien Bridge filter, so do some reading about that.Without knowing component values, it's hard to say.BTW, in English we say "bandpass filter", not "passband filter".

author: rstofer
author_rank: Super Contributor
timestamp: February 02, 2019, 11:05:40 pm
content: Why not simulate it in LTspice and see how it works?

author: MasterT
author_rank: Frequent Contributor
timestamp: February 02, 2019, 11:52:19 pm
content: The circuits resembles "composite amplifier". Google brings this tutorial, how it's any better to single OPA:https://www.audioxpress.com/article/a-50w-channel-composite-amplifier

author: bonzer
author_rank: Regular Contributor
timestamp: February 03, 2019, 09:38:48 am
content: Damn thanks a lot MasterT! Did you know it or you found the key words to search for through google? It's always hard for me when surfing on the internet to hit the right words as english isn't my mother language and also all the courses I do are in italian.Besides I kinda predicted that the open loop gain was gonna be increased but didn't expect it to be that way they show it. NevermindAbout LTSpice: I'm still at the beginning with simulation software. I have never used LTSpice but only PSPICE few times. Also I wanted to understand what it theorically does so I can choose some values for the components and proove my expectations later.

author: MasterT
author_rank: Frequent Contributor
timestamp: February 03, 2019, 02:27:25 pm
content: I know a circuits, but my native language is also not English, and I tried two phrases "combine amplifier", and "composite amp.". Google turns blank on the first one, probably just not so popular in google's search pattern / images database.

author: Audioguru
author_rank: Super Contributor
timestamp: February 03, 2019, 04:13:58 pm
content: The circuit uses two opamps because the lousy old 741 opamp is 52 years old and has a horrible high audio frequency response. A single low noise audio opamp would be used today.The single RC highpass and single RC lowpass are almost useless since their slopes are very gradual.

author: Yansi
author_rank: Super Contributor
timestamp: February 04, 2019, 11:10:18 am
content: So stacking two nasty opamps one after another, makes for a better amp? What a nonsense. The combined response will likely be even worse, not better (apart from the open loop gain, which will likely rise and make it with less phase margin - i.e. less stable).Or did I miss something?

author: vealmike
author_rank: Regular Contributor
timestamp: February 04, 2019, 11:44:35 am
content: Quote from: Yansi on February 04, 2019, 11:10:18 amGain bandwidth product is a constant. Theoretically cascading two amps would increase this.

author: ogden
author_rank: Super Contributor
timestamp: February 04, 2019, 12:08:07 pm
content: Quote from: Yansi on February 04, 2019, 11:10:18 amYes indeed.https://www.eevblog.com/2014/01/25/eevblog-572-cascading-opamps-for-increased-bandwidth/

author: Zero999
author_rank: Super Contributor
timestamp: February 04, 2019, 12:18:55 pm
content: Quote from: ogden on February 04, 2019, 12:08:07 pmDoing it with two op-amps within the same feedback loop isn't a good idea though, as it increases the risk of oscillation. The usual way to do it is to have two separate amplifiers in series, each with the square root of the required gain. For example, if you require a total gain of 100, use two amplifiers, each with a gain of √100 = 10, in series, giving a total gain of 100.

author: Yansi
author_rank: Super Contributor
timestamp: February 04, 2019, 04:30:35 pm
content: Stacking multiple stages with low gains each makes sense. Stacking them directly like will improve close to nothing, just raise problems with stability.

author: vealmike
author_rank: Regular Contributor
timestamp: February 05, 2019, 09:35:46 am
content: Quote from: Yansi on February 04, 2019, 04:30:35 pmAu contraire, it will work beautifully on the simulator. If you wanted to teach students about the pitfalls of assuming simulated circuits will always work, this would be a fine example to pick.

author: Zero999
author_rank: Super Contributor
timestamp: February 05, 2019, 09:52:43 am
content: Quote from: vealmike on February 05, 2019, 09:35:46 amThat depends on how accurate the models are.

author: vealmike
author_rank: Regular Contributor
timestamp: February 05, 2019, 10:33:58 am
content: Fair point.If they accurately model the parasitics (including supplies), noise and shonky soldering in the breadboard / veroboard lab model, the simulator will give the correct result.But generally people just use the same schematic for both sim and lab build.

author: nick_d
author_rank: Regular Contributor
timestamp: February 05, 2019, 10:50:47 am
content: The link to the 50W/channel composite amp is golden. If I was going to build a class B amp I would definitely build it like this. Actually, it's sort of logical because it makes sense to drive the power stage with an op-amp IC rather than building your own driver stage. The fact that the power stage is designed to work as effectively an op-amp, is just an added bonus that makes this circuit a lot cooler.In regards to the stability issue, yes certainly if you put two very quick devices in series and an overall feedback loop around them, it's a recipe for oscillation, since the devices will respond to the input in open-loop fashion before the feedback arrives. There are two main ways around. One way is to slow down each of the individual devices (give each one its own feedback loop, inside the overall feedback loop). Another way is to generate the feedback signal as some function of both op-amp outputs, so that there's a "quick" feedback coming from the first op-amp and a slower feedback coming from the second.I've done a bit of this before while trying to generate stable feedback loops and integrators for sigma-delta converters. Honestly the maths does your head in a bit, but in principle the response of the circuit can be calculated by complex analysis. Given some frequency (omega), you can calculate the gain and phase shift of the amplifier in the same way you would calculate the DC gain, except using impedances instead of resistances, and obviously giving any capacitors or inductors their correct impedances, which will be imaginary and will depend on omega. You just have to make sure the gain is <1 when the phase shift is in the 90..270 degree region. The correct feedback components will ensure this.I tend to use a trial and error approach, with either Spice or Matlab or Python with Matplotlib... so that I can easily change components and then graph the response of the circuit as a frequency-vs-magnitude and frequency-vs-phase plot. There are other plots such as the Bode plot and the pole-zero diagram, which I'm honestly not as familiar with, but can be used to read off stability information if you know how. (I try to avoid using too many different kinds of plot though).cheers, Nick

author: Zero999
author_rank: Super Contributor
timestamp: February 05, 2019, 12:34:57 pm
content: Quote from: vealmike on February 05, 2019, 10:33:58 amThe problem with using two op-amps in the same loop is the delays add, so there's an increased risk of the gain being above unity, at 180ophase shift, resulting in oscillation.You're mistaken that this won't show in a simulator. With the appropriate models, it's fairly straightforward to use SPICE to predict if it will oscillate or not. Whether the parasitics make any difference or not depends on the impedance and bandwidth of the circuit. If low value feedback resistors, say under 100k or so, are used and the op-amp is slow, such as the old 741, the parasitics will make little difference and the simulator should closely match reality, unless the construction is extremely bad.Attached is a simulation showing four different inverting op-amp configurations with a total gain of 100. The simulation clearly shows the one with the two op-amps in the same loop, #2, will oscillate. Using two separate amplifiers in series, #3, is the most stable configuration, but will have a higher offset error, as the second amplifier multiplies the firsts offset. Local feedback with two amplifiers in the same loop, #4, gives a much better offset a and a wide bandwidth, but selecting the value of the compensation capacitor is tricky: too low = oscillation, too high = less bandwidth. I've tweaked it in the simulation, but in real life err on the side of caution and keep it on the higher side: only design for as much bandwidth as you really need.

author: ogden
author_rank: Super Contributor
timestamp: February 05, 2019, 03:45:06 pm
content: Quote from: Zero999 on February 05, 2019, 12:34:57 pmTo make your blanket statement work, you "conveniently forgot" to include capacitor "C2" of original circuit

author: Zero999
author_rank: Super Contributor
timestamp: February 05, 2019, 04:18:04 pm
content: Quote from: ogden on February 05, 2019, 03:45:06 pmYes, that capacitor was a bodge and isn't needed if the local feedback resistor network is configured correctly. Now the circuit is stable and there's no need to mess around with compensation capacitors.

author: Yansi
author_rank: Super Contributor
timestamp: February 05, 2019, 04:38:57 pm
content: Quote from: nick_d on February 05, 2019, 10:50:47 amNot sure which link, but most composite amplifiers suck. There are more issues brought to the design, rather than solved, when a complex feedback loops with additional gain stages after the opamp.There is normally no point in building composite audio amps like this. There is nothing that difficult on designing a nice stable discrete amplifier, if enough care is given to it and circuit built wisely. Opamps does not really make that any easier.There are numerous fully discrete designs availabel for free on the web, some of them very well executed, across wide variety of topologies (even CFA).

author: nick_d
author_rank: Regular Contributor
timestamp: February 06, 2019, 02:50:59 am
content: QuoteI'm not an expert on this so I don't doubt what you say, however I would be keen to know what these issues are and why. I understand there are things like crossover distortion, offsets, noise and so on. Also, that various kinds of distortion products are easier or harder to get rid of. I also understand that feedback doesn't always remove distortion fully, especially in conditions where the open loop gain is low.On the other hand, the arguments advanced previously seem convincing: the feedback compensates for wonkiness in the second stage, such that only the first stage has to be a high quality amplifier. The criticism is that it requires a complicated feedback network and will have overall slower response, but this seems surmountable with correct design (and audio doesn't require a super high bandwidth in any case).cheers, Nick

author: Zero999
author_rank: Super Contributor
timestamp: February 06, 2019, 08:58:57 am
content: Quote from: nick_d on February 06, 2019, 02:50:59 amAdding multiple op-amps in the same feedback loop complicates frequency compensation, making oscillation more likely.It's normally better to just use a more expensive op-amp, with wide bandwidth and low distortion.The only legitimate reason I can think of for having multiple amplifiers in the same feedback loop, is an op-amp, followed by a discrete power stage. Most op-amps are only rated to 40V maximum and the output current is typically limited to under 20mA, so using one to drive a power stage makes perfect sense.

author: ogden
author_rank: Super Contributor
timestamp: February 06, 2019, 09:30:32 am
content: Quote from: Zero999 on February 05, 2019, 04:18:04 pmIt is not messing around but Miller CompensationOP circuit may be demonstrator of exactly that - how to compensate unstable opamp.https://www.edn.com/electronics-blogs/analog-bytes/4436589/Miller-Compensation-and-the-RHPZ-

author: Zero999
author_rank: Super Contributor
timestamp: February 06, 2019, 02:17:17 pm
content: Quote from: ogden on February 06, 2019, 09:30:32 amBut it's tricky to select the correct capacitor, but it can avoided in this case just by configuring the feedback network properly. It works, because each op-amp is designed to be unity gain stable