# How is abstraction used in Analogue circuit design?

Digital circuit design is a domain in which we abstract a complex analogue transistor circuit as a logic gate, effectively abstracting away even the electrical nature of the device as we use boolean algebra to describe its behaviour. I am sure that when we design analogue circuits we must be following a similar approach but not abstract things to such a level.

I understand the concept of voltage and current sources and that they can be used with Resistors-Capacitors-Inductors to abstract the behaviour of a given electrical circuit and make it easier to comprehend how it behaves. In all these cases, depending of the problem, one does away with some amount of information which is less significant e.g parasitics can be ignored at low frequencies.

I can see analogue circuit design done using schematics containing BJTs and FETs and Resistors-Capacitors-Inductors. This would be bottom up design. I want to find a resource which does a "top-down" approach to analogue design. In other words, an analogue problem is desribed and a top-down approach is taken to solve it. First a block diagram is created and than one figures out what goes into those blocks.

I understand that his is an "open question" but really, I don't know where else to post it on this internet. Any article, book, website will be appreciated. If it gets taken down, well ....

Note: I know that OP-Amp is an example of abstraction in analogue circuit design but Op-Amp exists as an actual component within the circuit on the PCB.

I think any engineering design is handled top-down when you really look at the design process. This is not limited to digital electronics or even electronics at all.

Design of analog circuits is no different. You start out with what a circuit needs to do, then you come up with a overall strategy to solve the problem, then keep on drilling down until you get to whatever low level details your kind of design requires. For every engineering design I can think of, there are additional levels of abstraction both above and below the design. Again, analog electronics design is not different in that regard than any other engineering discipline. The same general process and existance of multiple levels of abstraction applies whether you are designing a audio amplifier, digital computer interface, the front wheel suspension of a car, or the cooling system of a nuclear power plant.

Let's use the design of a audio amplifier as a example. Higher levels above the design come to you as specifications. There is some limit above which you don't get to make choices (perform engineering design). The need may be to drive a small speaker so that someone within proximity can hear voice, like something built into a automated teller machine at a bank, or a gas pump. If you work for the ATM company, then you don't get to decide you want this to be a 300 W HiFi audio system for a theater.

Often determining the true requirements is part of doing a design. The company has decided to put a speaker in the new ATM so that it can issue voice prompts. They haven't said how many watts must go into what kind of speaker. This is your job to figure out. On the other hand, where the voice signal comes from may have already been determined and you don't get any wiggle room. All you can do is make sure that interface is clearly documented.

Once you have the requirements well decided and documented, you sit back and think of various high level ways to solve the problem. Pretty quickly, you'll narrow this down to a few alternatives, like getting a off the shelf class-D audio chip, a whole module that pretty much does everything between the source signal and the speaker, or something you design from lower level building blocks.

Good engineers don't just take the requirements at face value if they see something that might possibly be a small change externally but that could be a bigger advantage to their design. For example, you might prefer the audio came at you as digital words over SPI or something, but it is specified as line level analog. You go to the engineer that designed the circuit that produces the line level output. Perhaps you find that he actually has digital audio and is running that into a D/A because he figured that's whatever is downstream would want. Feeding digital audio to you not only saves money in his design, but also in yours.

This sort of looking at the bigger picture one or two levels up and trying to do the best for the overall system is all too rare nowadays. This is something the chief engineer of the project should be looking into too, but chief engineers are rare nowadays too. Often you get a project manager with little engineering skill, with nobody really coordinating the engineering effort between disciplines or subsystems.

Anyway, after you look at a few alternatives, you pick one going forwards. Let's say you decided that the does-all module is too expensive for the volume of this product. Your own discrete solution would work, but take more board space than it should, especially since it would be inefficient power-wise and dealing with the heat in the cramped space you have would be a real problem. So you go with the class D audio chip, except that none of the ones for the required power level can work directly with the power supplies you have available.

Now you have decomposed the overall amplifier into some power supply conversion, and the class D chip. Each of those will have details, etc. Eventually you get to a schematic of parts you can buy and put on a circuit board.

That may be the end of your design, but of course there are many levels below that. You're going to lay out the board, but you're not going to design the details of the physical board yourself and the process that will be used to make it. You specify holes and traces, and someone else makes sure that milling, drilling, plating, etc, all happen according to your spec. You buy a class D amplifier chip, but there is obviously significant design inside that block. If this is a super high volume product, then you might design a chip with a class D amplifier being just a part of it. Even then, someone else will design the silicon fabrication process so that you can simply talk in terms of transistors instead of masks and interconnects and doping levels and the like.

I don't see anything different here just because the design happens to be a analog circuit.

I want to find a resource which does a "top-down" approach to analogue design. In other words, an analogue problem is described and a top-down approach is taken to solve it.

That's pretty much how it works - if you start designing the nuts and bolts before comprehending what the "big aim" is then you'll probably run into difficulties and end up throwing away all your imperial (English-English for non-metric) nuts and bolts and using metric. OK it's not a great analogy!

What you may be misunderstanding is that decent analogue engineers will probably do the top-down, higher order stuff in their head and use the actual circuit diagram to define what the problem is - you can do this with circuits and block diagrams - the circuit design defines the problem (and the solution of course) and competent engineers are able to see this. OK some values may be wrong (eventually fixed) but the beauty of a circuit diagram is that this can happen.

Of course, there's nothing wrong with doing a block diagram first and splitting the design into sub-elements but, if all a person sees is a circuit diagram and they are not great at reading schematics then it might just look like a mess.

A circuit diagram should be much more than a means of stringing nets together so that designing the PCB is made easy.

The biggest problem I see with so-called "top-down designs" in analogue circuitry is that to make it palatable you would need to assume some electrical isolation among the blocks. Otherwise it will be complicated to figure out how to configure the blocks so that when you add them together you get the desired result. It also would require the blocks to be adjustable.

You see this in op-amps. The input of an op-amp block isn't changed by the addition of the op-amp. BTW, I disagree with you and think that an op-amp is a very good example of an abstracted analogue block.

Edit: Here I am speaking of formulaic abstraction of the variety you see in digital circuit analysis.

• I don't think you need isolation, you just need to know that the input impedance of each block is much higher than the output impedance of whatever is driving it. Feb 28, 2014 at 16:19
• True, but high impedance is a form of electrical isolation. Feb 28, 2014 at 16:21