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Inside op-amps |
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Graham Slee 
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Retired Joined: 11 Jan 2008 Location: South Yorkshire Status: Offline Points: 16110 |
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 Topic: Inside op-ampsPosted: 25 Jul 2022 at 6:56pm |
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Inside op-amps Seems the occasional member is swayed to the ARSE website which spends most of its time rubbishing real designers, probably because they haven't a clue themselves. How do you get inside an op-amp without cracking it open?  This image shows one method. No it isn't mine but it is in an Analogue Devices book. So, if you want to change the characteristics of an op-amp to suit a particular purpose, how do you do it? For example, how would you make an AD711 emulate an AD817? You do it by getting an education!!! |
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Graham Slee
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Posted: 25 Jul 2022 at 7:09pm |
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First, go to and try to find the internal schematic. Good luck. |
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Graham Slee
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Posted: 25 Jul 2022 at 7:23pm |
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I can't get the 711 open-loop Fc at 10kHz, but I can get the open-loop Fc at 1kHz, and make it the same open-loop gain as the 817.
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Graham Slee
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Posted: 26 Jul 2022 at 5:48am |
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So why would I want to emulate an AD817 using an AD711? Because the AD817 has a shot noise problem, and also in 2006, was not going to be made RoHS compatible. It was far later that ADI realised they were no match for the EU! The AD817 had a great open-loop bandwidth - that means flat to a mid frequency with no NFB. Great to be able to base the curves of RIAA on. But, its low frequency gain falls a bit short, so if you understand transistors, and a.c., and negative feedback, you'll know that the computed RIAA NFB feedback is not going to work at low frequency. So, what you do is iterate it out and get the curve to fit. Now, what if you decide you've had enough of throwing away half the AD817's you bought because of their shot noise? You redesign the product to use something else. But how do you keep that incredible bass? Well, you change the new op-amp to have the same open-loop gain, and that is in the 50Hz to 500Hz region. So, how do you change the op-amps open-loop gain? By understanding its schematic! But there is no schematic! You have to know the "family characteristics" of the op-amp. How can you tell "family characteristics"? I think you have to work with them - perhaps design broadcast audio equipment using them - kind of get to know them in the real world. In which case you can see the schematic from the specification, and then you read the description, which confirms it. So go read the AD711 data sheet, and discover its "family characteristics". Then after you've done that, then calculate and construct a graph of its open-loop bandwidth.
Edited by Graham Slee - 29 Jul 2022 at 10:13pm |
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Graham Slee
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Posted: 26 Jul 2022 at 6:13am |
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Exercise:
View the Functional Block Diagram of an NE5534 op-amp Q1, Which output is class-A? Q2, Which input is that of the VAS (voltage amplifier stage)? Q3, How would you change the open-loop gain?
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Graham Slee
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Posted: 26 Jul 2022 at 12:56pm |
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The Gm of an op-amp differential input stage is that low that the voltage amplifier stage does the voltage amplification and that's why it's known as the voltage amplifier. Looking at the NE5534 schematic the voltage amplifier exists between comp/bal and comp, pins 8 and 5. Therefore, pin 5 is the class-A output (which outputs sufficient current to drive a capacitor, Ccomp). That answers Q1. The input to the voltage amplifier stage (VAS) is pin 8. That answers Q2. And the compensation capacitor changes the open-loop gain at its Fc. That answers Q3. And a further answer to Q3, is that any impedance between pins 5 to 8 will change the open loop gain!
Edited by Graham Slee - 29 Jul 2022 at 10:14pm |
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Graham Slee
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Posted: 26 Jul 2022 at 1:19pm |
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Now, perhaps at Btec level they teach op-amps, but it is only the basics, but I've had clever arses trying to foist their son's Btec notes on me before, and that's because the Devil is in them, and they are so stupid that they see my advanced stuff as gobsh*te, but it's them who should have that title. Obviously, if your education stopped at the nth spiral, you'll not grasp any higher! And based on what I just said, then I'm probably wasting my time here. But, as Ccomp alters the high frequency open-loop gain, then there must be an imaginary resistor in parallel with it (go on, scoff). That imaginary resistor defines the open-loop gain of the operational amplifier. It has a gain bandwidth product, doesn't it? The GBW is the gain where the amp falls to unity, and at the other end, at low (really low) frequency the op-amp has its maximum gain. So, you can plot a slope between that low frequency maximum gain and unity gain, versus frequency, can't you? So what stops it having gain forever? What stops it is Ccomp. Otherwise you have an unstable amplifier (an accidental oscillator). You also have Z-in at the VAS input (pin 8 from last post), and the amplifier's gain is n-times that Z-in, or it wouldn't fall off in a controlled manor. So what is the Z-feedback? You know, the imaginary resistor, which I have just proved to exist! So what about all those computed NFB values for the RIAA filter? Are they not in parallel with Z-feedback open-loop? Edited by Graham Slee - 29 Jul 2022 at 10:15pm |
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