Three things they should have taught in Engineering 101, Part 3: Learn an intuitive approach

Darren Ashby -March 21, 2013

Note: The following is adapted from the book "Electrical Engineering 101, Third Edition" by Darren Ashby (Newnes).

[Part one explains how, by understanding unit math, you can solve nearly any problem. Part 2 covers some basic comparisons that use physical equivalents of basic electronic components to create an intuitive understanding of a circuit.]


Intuitive Signal Analysis
I'm not sure if intuitive signal analysis is actually taught in school; this is my name for it. It is something I learned on my own in college and the workplace. I didn't call it an actual discipline until I had been working for a while and had explained my methods to fellow engineers to help them solve their own dilemmas.

I do think, however, that a lot of so-called bright people out there use this skill without really knowing it or putting a name to it. They seem to be able to point to something you have been working on for hours and say, "Your problem is there. "They just seem to intuitively know what should happen. I believe that this is a skill that can and should be taught.

There are three underlying principles needed to apply intuitive signal analysis. (Let's just call it ISA. After all, if I have any hope of this catching on in the engineering world, it has to have an acronym!)

  1. You must drill the basics. For example, what happens to the impedance of a capacitor as frequency increases? It goes down. You should know that type of information off the top of your head. If you do, you can identify a high-pass or low-pass filter immediately. How about the impedance of an inductor—what does it do as frequency increases? What does negative feedback do to an op-amp; how does its output change? You do not necessarily need to know every equation by heart, but you do need to know the direction of the change. As far as the magnitude of the change is concerned, if you have a general idea of the strength of the signal, that is usually enough to zero in on the part of the circuit that is not doing what you want it to.
  2. You need experience, and lots of it. You need to get a feel for how different components work. You need to spend a lot of time in the lab, and you need to understand the basics of each component. You need to know what a given signal will do as it passes through a given component. Remember the physical equivalents of the basic components? These are the building blocks of your ability to visualize the operation of a circuit. You must imagine what is happening inside the circuit as the input changes. If you can visualize that, you can predict what the outputs will do.
  3. Break the problem down. "How do you eat an elephant?"the knowledge seeker asked the wise old man. "One bite at a time,"the old man replied. Pick a point to start and walk through it. Take the circuit and break it down into smaller chunks that can be handled easily. Step by step, draw arrows that show the changes of signals in the circuit, as shown in Figure 1.6. "Does current go up here?" "Voltage at such and such point should be going down." These are the types of questions and answers you should be mumbling to yourself.6 Again, one thing you do not need to know is what the output will be precisely. You do not need to memorize every equation in this book to intuitively know your circuit, but you do need to know what effect changing a value of a component will have. For example, given a low-pass RC filter and an AC signal input, if you increase the value of the capacitor, what should happen to the amplitude of the output? Will it get smaller or larger?

    You should know immediately with something this basic that the answer is "smaller." You should also know that how much smaller depends on the frequency of the signal and the time constant of the filter. What happens as you increase current into the base of a transistor? Current through the collector increases. What happens to voltage across a resistor as current decreases? These are simple effects of components, but you would be surprised at how many engineers don't know the answers to these types of questions off the top of their heads.

FIGURE 1.6: Use arrows to visualize what is happening to voltage and current.

6 Based on extensive research of talking to two or three people, I have concluded that all intelligent people talk to themselves. Whether or not they are considered socially acceptable depends on the audibility of this voice to others around them.

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