Exploring Electronics: Understanding Circuit Design and Its Limitations

[dijkarte]

Okay maybe this is something I should try as an assignment. :D

How do I do looping in circuits? For example apply the same circuit to signal n-times or t-time?

 

[carlgrace]

I agree, this is what I meant by synthesis. However the example you gave about a power-supply design is more a composition (building from existing components, topologies and tweaking) process than synthesis (starting from scratch). Let me give an example to clarify more what I'm talking about. An engineer is required to design an IC that takes in an analog signal and produces a signal which is the inverse of the original signal's function.

i.e input f(x) = x^2 the result is g(x) = sqrt(x)

Starting from a scratch, using transistors, diodes, and passive elements, how would he/she approach such a problem?

Please note, this circuit probable invented 1000 years ago, I don't know nothing, so my example is just for illustration purpose only. :D

I think I understand. Well, in that case I would say 99% or more of what working analog design engineers do is composition. You only design something from scratch when you have no other choice. Traditionally, progress has been made incrementally with a breakthrough new topology or something only rarely.

To answer your example, it is impossible to do that in any general way using analog techniques. A more tractable question would be "How do you make a circuit that takes a square root". Honestly what an engineer would do first is search the literature and see if there are published circuits to do a square root. In fact there are. There are also very well known circuits to invert a signal. So I would create a first draft circuit combining these two blocks. Then I would see where it needs to be improved (for example, speed, power, etc) and then use my experience and knowledge to address those needs.

Now, since you asked about synthesis assuming I really wanted to design the example circuit from scratch here is what I would do.

First, I would start with a MOSFET since that is the key device in modern analog IC design. Then, I would ask myself, "what is the core operating equation of the MOSFET"? Since it's a square-law device, to first order the drain current is proportional to the square of the difference between the voltage at the gate and the voltage at the source when the device is operating in saturation. Then it would hit me, "well, by simple algebra, then, the voltage between the gate and the source of the MOSFET is proportional to the square root of the drain current!" So, now, I would have the core of a solution. Then, I would figure out what support circuits I need to ensure the MOSFET is biased in saturation under the expected operating conditions. If the input signal is a voltage, I would have to design a simple circuit to convert the voltage to current (I'd use a PMOS device in a common-drain configuration). Also, I would have to consider how to buffer the output signal such that it could drive whatever load was specified.

So, you see, even in the case where I designed the core of the circuit from scratch, there is still a lot of "composition". I'm sure you've heard the phrase "necessity is the mother of invention", one reason for this is why invent something when it already exists!

Incidentally, based on your definition, digital design would be considered "composition", since digital circuits are built using a small number of standard cells such as inverters, AND or OR gates and the like.

 

[carlgrace]

Okay maybe this is something I should try as an assignment. :D

How do I do looping in circuits? For example apply the same circuit to signal n-times or t-time?

you would probably need to take a sampled-data approach. Look up "sample-and-hold".

 

[carlgrace]

I agree, this is what I meant by synthesis. However the example you gave about a power-supply design is more a composition (building from existing components, topologies and tweaking) process than synthesis (starting from scratch). Let me give an example to clarify more what I'm talking about. An engineer is required to design an IC that takes in an analog signal and produces a signal which is the inverse of the original signal's function.

i.e input f(x) = x^2 the result is g(x) = sqrt(x)

Starting from a scratch, using transistors, diodes, and passive elements, how would he/she approach such a problem?

Please note, this circuit probable invented 1000 years ago, I don't know nothing, so my example is just for illustration purpose only. :D

I thought a bit about this issue and what you would really do in practice is design an IC that digitizes the signal then computes the inverse using known mathematical techniques. There would be a million specifications to be thought through before this circuit could be designed.

 

[dijkarte]

I'm now thinking about applying a circuit to a signal multiple times. For example, I want to differentiate a signal f(t) using a differentiating amplifier. But I'm interested in finding nth derivative of the signal, and I don't want to hard wire a series of amp's. I just want to use the same amp.

 

[carlgrace]

I'm now thinking about applying a circuit to a signal multiple times. For example, I want to differentiate a signal f(t) using a differentiating amplifier. But I'm interested in finding nth derivative of the signal, and I don't want to hard wire a series of amp's. I just want to use the same amp.

You're going to need a sample-and-hold (S/H) circuit. Here's the algorithm.

1. Sample the input
2. loops = order
3. do
4. Apply S/H output to the differentiator
5. Reset S/H
6. Sample the output of the differentiator with S/H
7. loops = loops - 1
8. until loops == 0
9. enddo

you're also going to need a clock to drive the S/H.

This is a slightly advanced design.

 

[dijkarte]

Thanks :)

I see how it's complicated now. Where can I learn all these tricks, which electronic book?

 

[carlgrace]

Thanks :)

I see how it's complicated now. Where can I learn all these tricks, which electronic book?

It will take a while to digest them all. The links I posted for the Analog Devices information and the Analog Chips book will go a long way.

 

[dijkarte]

I read some article by an expert advising against Spice:

http://electronicdesign.com/article/analog-and-mixed-signal/what-s-all-this-spicey-stuff-anyhow-part-iii-5632

I'm not sure if it's Spice in particular or any circuit simulation software?

The way to go is do it in the lab and test a real circuit, but this is for a learner like me a very expensive approach since those test equipment are ridiculously over priced. A decent oscilloscope costs thousands.

I tried the OrCad PSpice and it's not bad except for the GUI which sucks. Usability from a software interface design perspective is below average.

 

[carlgrace]

I read some article by an expert advising against Spice:

http://electronicdesign.com/article/analog-and-mixed-signal/what-s-all-this-spicey-stuff-anyhow-part-iii-5632

I'm not sure if it's Spice in particular or any circuit simulation software?

The way to go is do it in the lab and test a real circuit, but this is for a learner like me a very expensive approach since those test equipment are ridiculously over priced. A decent oscilloscope costs thousands.

I tried the OrCad PSpice and it's not bad except for the GUI which sucks. Usability from a software interface design perspective is below average.

This is a well known article by Bob Pease. I knew him, he passed away last year. The article, however, is obsolete. SPICE has been developed continuously since then (he wrote it around 1990 I think) and all the issues he described have been largely dealt with. Even at the time Bob wrote this he held a highly minority viewpoint. SPICE is the gold standard for circuit simulation. It is impossible to test an integrated circuit idea in the lab before fabricating it, so SPICE has become a truly indispensable tool for design. Since you're interested in learning electronics, think of SPICE as an amazing, inexpensive learning laboratory.

I agree PSpice kinda sucks. Much better (and free!) is LTSpice from Linear Technology. Get it here: http://www.linear.com/designtools/software/

 

[dijkarte]

So there are different versions of GUI-based Spice all sit on top of the same base non-gui Spice code? Then using different versions does not affect the simulation result.

 

[carlgrace]

No, SPICE is not "one program" anymore (hasn't been since the 60s.). It is more a way to approach solving linear differential equations. It turns out this is a good way to solve circuits too, since they can be described well as a large set of linear differential equations. The code underlying LTspice will be completely different from the code underneath Pspice. Assuming they are both correct implementations, you're right the simulation outputs should agree (particularly for the simple simulations you are going to run). The difference is how easy to use it is and how robust it is to non-ideal situations.

You have probably read about convergence issues in SPICE. The problem stems from difficulties in inverting ill-conditioned sparse matrices. Different versions of SPICE deal with it in different ways, with different levels of success. Commercial versions of SPICE (or SPICE-like simulators) such as HSPICE, Spectre, and Eldo can handle huge circuits without too many problems with convergence. My point is that SPICE is more a philosophical approach to circuit simulation than it is a relevant code base.

 

[dijkarte]

IC. Spice is more like an open specification for a circuit simulation model.

 

[carlgrace]

Yep. As far as I know the source code is in the public domain. Modern simulators use much for sophisticated techniques because they have to deal with circuits with millions of devices.

 

[nsaspook]

I read some article by an expert advising against Spice:

http://electronicdesign.com/article/analog-and-mixed-signal/what-s-all-this-spicey-stuff-anyhow-part-iii-5632

I'm not sure if it's Spice in particular or any circuit simulation software?

The way to go is do it in the lab and test a real circuit, but this is for a learner like me a very expensive approach since those test equipment are ridiculously over priced. A decent oscilloscope costs thousands.

IMO circuit simulation software at the beginner level is great for letting you think you can handle a Superbike while physically you can only manage to pedal a tricycle. Software assisted design is great after you know how to do the same thing with a pencil, paper, vector board and wire jumpers but "electronics" is not engineering. Learning "electronics" is a physical act that requires real circuits. You seem to want to skip actual electronics and that's great if you never plan to actually build anything.

Buy some used or cheap equipment and a soldering iron.

 

[dijkarte]

IMO circuit simulation software at the beginner level is great for letting you think you can handle a Superbike while physically you can only manage to pedal a tricycle.

Then this is bad not great for beginners.

Software assisted design is great after you know how to do the same thing with a pencil, paper, vector board and wire jumpers but "electronics" is not engineering.

I think you are misunderstanding the purpose of circuit simulation software in learning.
Doing it on paper and and understand the fundamental concepts come first and are very essentials.

I see here 3 main uses at least for someone novice like me:

Verifying answers that I do on paper.

Try more complex circuits that doing it on paper will take very long trying to solve the math.

Develop a common sense of how circuits work by easily modifying a circuit and see the effect of this modification.

In short, to speed up the learning rather than skipping the essentials.

Like learning programming, it cannot be always on paper, must see things in action.

Buy some used or cheap equipment and a soldering iron.

I'm not sure if this is affordable to many. I would rather postpone this till I get the basics and then move to simple projects to implement.

 

[nsaspook]

I see here 3 main uses at least for someone novice like me:

Verifying answers that I do on paper.

Try more complex circuits that doing it on paper will take very long trying to solve the math.

Develop a common sense of how circuits work by easily modifying a circuit and see the effect of this modification.

I'm not sure if this is affordable to many. I would rather postpone this till I get the basics and then move to simple projects to implement.

I think tactile and mechanical memories are very important while learning the basics of a skill that requires physical activity in a setting other than a school lab. Circuit simulation just like a calculator in math should be used when you are no longer learning basic circuits or the math at hand but need to speed up a mechanical process or optimization. The sad truth is guys with great math and simulation skills are a nickle a dozen these days and people with excellent troubleshooting and repair skills are treasures.

 

[dijkarte]

Circuit simulation just like a calculator in math should be used when you are no longer learning basic circuits or the math at hand but need to speed up a mechanical process or optimization.

Your comparison here is not accurate. You know that in many fields that require physical training such as in aerospace, military, space missions, pilots require some decent training that can be made safe and efficient with simulation systems. These systems are a mix of mechanical and computer/software equipment through which they can experience things that could be dangerous to try in real. All simulation systems whether they are a mix of mechanical and software or pure software are all fit in one category to serve the same purpose.

However you example of the calculator more fits in the tools category, something different.
An CAD design program for architectural drawing is a better example of a software that is used for drafting vs. hand drawing. But there's no simulation. Take for instance someone designing a car engine, how would it be without using a simulation system? or even students will have to learn how to use these systems to study their designs in an efficient manner, much less cost, less time. And all of this can be done either after or while learning the basics.

Learning is different than what it was 1000s years ago. If you can get a sense of how things work while learning and see it in action in a virtual world better than postpone until you lose interest in the subject and probably drop out of the course.

Something similar to programming. I'm learning the basics, data structures and how to create linked list, hash tables, binary trees, some sorting techniques...etc. At the same time I can use libraries that provide these tools for me. I do it both ways and I learn both at the same time. I cannot say I'm a better programmer if I create my own data structures and algorithms every time I make a project, than someone who uses well-tested and very likely more efficient tools that help in speeding up the work.

So even calculators save us a lot of time, but they don't not make people less skilled engineers. :)