Author Topic: Purpose of this amplifier circuit with op. amplifiers  (Read 4915 times)

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Offline bonzerTopic starter

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Purpose of this amplifier circuit with op. amplifiers
« on: February 02, 2019, 10:00:00 pm »
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 also uncomplete (so without compensation components or other additional stuff needed for big gain and so on).
 

Offline Yansi

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Re: Purpose of this amplifier circuit with op. amplifiers
« Reply #1 on: February 02, 2019, 10:04:13 pm »
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.
 

Offline bonzerTopic starter

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Re: Purpose of this amplifier circuit with op. amplifiers
« Reply #2 on: February 02, 2019, 10:16:44 pm »
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?
« Last Edit: February 02, 2019, 10:19:24 pm by bonzer »
 

Offline Yansi

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Re: Purpose of this amplifier circuit with op. amplifiers
« Reply #3 on: February 02, 2019, 10:20:56 pm »
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.  :-//
 

Offline edavid

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Re: Purpose of this amplifier circuit with op. amplifiers
« Reply #4 on: February 02, 2019, 10:27:21 pm »
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".
« Last Edit: February 03, 2019, 12:43:47 am by edavid »
 

Offline rstofer

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Re: Purpose of this amplifier circuit with op. amplifiers
« Reply #5 on: February 02, 2019, 11:05:40 pm »
Why not simulate it in LTspice and see how it works?
 

Offline MasterT

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Re: Purpose of this amplifier circuit with op. amplifiers
« Reply #6 on: February 02, 2019, 11:52:19 pm »
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
 
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Offline bonzerTopic starter

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Re: Purpose of this amplifier circuit with op. amplifiers
« Reply #7 on: February 03, 2019, 09:38:48 am »
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. Nevermind

About 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.
« Last Edit: February 03, 2019, 09:45:22 am by bonzer »
 

Offline MasterT

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Re: Purpose of this amplifier circuit with op. amplifiers
« Reply #8 on: February 03, 2019, 02:27:25 pm »
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.
 

Offline Audioguru

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Re: Purpose of this amplifier circuit with op. amplifiers
« Reply #9 on: February 03, 2019, 04:13:58 pm »
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.
 
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Offline Yansi

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Re: Purpose of this amplifier circuit with op. amplifiers
« Reply #10 on: February 04, 2019, 11:10:18 am »
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?
 

Offline vealmike

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Re: Purpose of this amplifier circuit with op. amplifiers
« Reply #11 on: February 04, 2019, 11:44:35 am »
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?

Gain bandwidth product is a constant. Theoretically cascading two amps would increase this.
 

Offline ogden

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Re: Purpose of this amplifier circuit with op. amplifiers
« Reply #12 on: February 04, 2019, 12:08:07 pm »
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

Or did I miss something?

Yes indeed.

https://www.eevblog.com/2014/01/25/eevblog-572-cascading-opamps-for-increased-bandwidth/
 

Online Zero999

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Re: Purpose of this amplifier circuit with op. amplifiers
« Reply #13 on: February 04, 2019, 12:18:55 pm »
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

Or did I miss something?

Yes indeed.

https://www.eevblog.com/2014/01/25/eevblog-572-cascading-opamps-for-increased-bandwidth/
Doing 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.
 

Offline Yansi

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Re: Purpose of this amplifier circuit with op. amplifiers
« Reply #14 on: February 04, 2019, 04:30:35 pm »
Stacking multiple stages with low gains each makes sense. Stacking them directly like will improve close to nothing, just raise problems with stability.
 

Offline vealmike

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Re: Purpose of this amplifier circuit with op. amplifiers
« Reply #15 on: February 05, 2019, 09:35:46 am »
Stacking multiple stages with low gains each makes sense. Stacking them directly like will improve close to nothing, just raise problems with stability.
Au 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.
 

Online Zero999

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Re: Purpose of this amplifier circuit with op. amplifiers
« Reply #16 on: February 05, 2019, 09:52:43 am »
Stacking multiple stages with low gains each makes sense. Stacking them directly like will improve close to nothing, just raise problems with stability.
Au 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.
That depends on how accurate the models are.
 

Offline vealmike

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Re: Purpose of this amplifier circuit with op. amplifiers
« Reply #17 on: February 05, 2019, 10:33:58 am »
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.
 

Offline nick_d

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Re: Purpose of this amplifier circuit with op. amplifiers
« Reply #18 on: February 05, 2019, 10:50:47 am »
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
 
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Online Zero999

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Re: Purpose of this amplifier circuit with op. amplifiers
« Reply #19 on: February 05, 2019, 12:34:57 pm »
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.

The 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 180o phase 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.

 
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Offline ogden

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Re: Purpose of this amplifier circuit with op. amplifiers
« Reply #20 on: February 05, 2019, 03:45:06 pm »
The simulation clearly shows the one with the two op-amps in the same loop, #2, will oscillate.

To make your blanket statement work, you "conveniently forgot" to include capacitor "C2" of original circuit :)


 

Online Zero999

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Re: Purpose of this amplifier circuit with op. amplifiers
« Reply #21 on: February 05, 2019, 04:18:04 pm »
The simulation clearly shows the one with the two op-amps in the same loop, #2, will oscillate.

To make your blanket statement work, you "conveniently forgot" to include capacitor "C2" of original circuit :)



Yes, 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.
« Last Edit: February 05, 2019, 04:19:38 pm by Zero999 »
 

Offline Yansi

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Re: Purpose of this amplifier circuit with op. amplifiers
« Reply #22 on: February 05, 2019, 04:38:57 pm »
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.

Not 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).
 

Offline nick_d

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Re: Purpose of this amplifier circuit with op. amplifiers
« Reply #23 on: February 06, 2019, 02:50:59 am »
Quote
Not 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.
I'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
 

Online Zero999

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Re: Purpose of this amplifier circuit with op. amplifiers
« Reply #24 on: February 06, 2019, 08:58:57 am »
Quote
Not 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.
I'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
Adding 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.
 


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