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Analog Angle Blog

Simple circuits, regulator analysis teaches solid lessons

With so much analog-circuit design being done by automated tools and circuit-performance analysis done with tools such as Spice, it may seem anachronistic to actually go through a basic circuit in detail, in order to understand what each component does, and what the tradeoffs associated with component selection may be. It's all too easy, though, for circuit-designers (especially aspiring ones) to be so far removed from the down-in-the-dirt circuit design that they lack the needed understanding of the basics. Yet it’s the grasp of these basics which translate into solid designs with fewer unpleasant surprises.

The dominant role which ICs have doesn’t help here, either, for two reasons. First, by their nature, ICs conceal their inner workings. Second, there are many valuable design tricks and techniques which are viable only with ICs, but which don’t make sense for discrete circuits. Examples are the use of matched components on the die to cancel out some errors and temperature-related drifts, or the use of a large number of “free” components such as extra transistors and resistors to achieve the desired performance, options which are not possible or attractive in a discrete circuit.

Nonetheless, there are alternatives for those who want to learn about the reality of basic analog-circuit design. A good place to start, not surprisingly, is with the venerable 555 timer—but not as an IC. Check out this all-discrete version of the 555 from Evil Mad Scientist, Figure 1 , which they cleverly call a “dis-integrated circuit. Students and aspiring analog designers can build it, then use it to try many of the thousands of 555-based circuits available on the web or in books, while observing the internal waveforms and effects of changing the external component values and arrangements. Since it is all relatively low speed, a low-cost scope or even DVM can be used. (The same company also makes an all-discrete version of the 741 op amp.)

Figure 1

The hidden, 'black box' nature of an IC is revealed and accessible with this discrete kit version of the classic 555 timer  from Evil Mad Scientist.

The hidden, “black box” nature of an IC is revealed and accessible with this discrete kit version of the classic 555 timer from Evil Mad Scientist.

Another “learning tool” is a basic, all-analog Lightning Detector circuit (not, you don't use it by setting it up at spots it will likely strike ground, so there's no danger). Nothing is critical about this four-transistor circuit: use of alternate parts, component tolerance, layout, packaging, or supply. Yet it allows the user to observe how a circuit reacts to a stimulus—and if natural lighting is not available, you can simulate it by placing one of those low-cost piezoelectric igniters near the antenna or using a sparking circuit such as unsnubbed relay contacts.

Of course, observing a circuit's waveforms and behavior is one thing, but at some point you have to do some mathematical analysis of currents, voltages, and topology to become a better engineer. That's why a recent “Power House” application note from Texas Instruments really caught my attention. In Power Tips: How to design a robust series linear regulator with discrete components, author Manjing Xie walks you step-by step through the design of this relatively simple analog circuit.

She shows you how to start with a basic one-transistor circuit, Figure 2 (yes, we still need our discrete devices) and then add what you need to make the regulator work properly. To do this, you have to anticipate shortcomings and then provide enhancements which overcome them. You also have to do basic real-world analysis, such as the power dissipation of the transistor if the output is shorted (here, it's 2.4 W, which is not good; so you have to add a current-limiting resistor).

Figure 2

A simple one-transistor linear regulator circuit, here from a Texas Instruments blog, can be a useful tool for basic circuit analysis ad topology investigation.

A simple one-transistor linear regulator circuit, here from a Texas Instruments blog, can be a useful tool for basic circuit analysis ad topology investigation.

This basic, discrete-component analog regulator circuit is a teaching tool, a learning tool, and even a potential testing tool: if you have a job candidate who claims some analog expertise, talking through this circuit will show how much he or she actually knows of basics, before getting to the more difficult higher-frequency issues such as parasitics.

There really are a lot of good options for learning basic, realistic analog circuits and analysis. Are there teaching tools, circuits, and projects do you use or recommend?

Related

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What Analog’s ‘Imperfections’ Taught Me

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Do Analog ICs Still Need Perfection?

3 comments on “Simple circuits, regulator analysis teaches solid lessons

  1. Victor Lorenzo
    November 17, 2015

    Nice post Bill,

    Your reference to that simple hobby lightning detector called my attention, especially your humorous comment “not, you don't use it by setting it up at spots it will likely strike ground, so there's no danger “. I currently work on projects where I must say, “yes, the circuit will and must be right there, and must survive for continuing receiving more strikes”. Fortunately I can stay sitting in a very safe place, far from those circuits. 😉

    Another circuit category that serves well for learning purposes is analog guitar effects. I used to play a lot with them when started playing electrical guitar and used some very ilustrative vintage effect circuits (like the Fuzz Face) for practical demonstrations when I worked years ago as an Electronics Department professor at the UCLV University in Cuba.

  2. Andy_I
    November 17, 2015

    That lightning detector circuit you referred to, was featured in the **April** issue of QST Magazine several years ago.  Because of the date of the issue, it was considered an April Fool's joke!

    Of course real lightning detectors do exist and pocket ones like that one can be purchased.  I have no experience to know how effective they really are, when they are this simple.

    I enjoyed a recent article about a highly directional microphone array, with the signal processing for the array done entirely in analog rather than digital.  Yes, there are situations where traditional designs still need to be done, and someone needs to know how to do them.

    SPICE is interesting because, although it shows you what a circuit (maybe) does, it doesn't show you how to make it work when it doesn't.  And then you need to pull out your “slide rule” or make those back-of-the-envelope calculations, until you get it working.  I see a lot of folks who use SPICE but have trouble designing their circuit — which may end up inside an IC.  There seems to be a lot of reliance on a schematic they lifted from a textbook, taking on faith that it must work, when a few simple calculations show that it can't.

    Used properly, SPICE is an effective adjunct to help design something, but it never replaces the need to do basic circuit design.

    Speaking of QST — there are a few EE professors or curricula that now require their students to get an Amateur Radio license as a prerequisite for graduation.  Ham radio gives them the *opportunity* to learn how real circuits work, especially if they have to diagnose and repair an old radio transceiver that quits.  And there is an incentive to understand more about Fields and Waves.  Alas, Heathkit is long gone, so there are fewer opportunities to use ham radio to learn how circuits work from the ground up.

  3. Victor Lorenzo
    November 18, 2015

    During lightning events current waveforms are not exactly stochastic signals, they present several characteristic impulses with a frequency bandwidth reaching from near DC to beyond 2MHz.

    Upward flashes in tall structures use to present lower maximum current peaks and higher DC components (due to the initial constant current condition) than cloud to ground (CG) flashes. But most lightning types (+/-, CG, GC, IC, CC) radiate energy in the very low frequencies band that can be detected and amplified with simple receivers using the appropriated antenna.

    There are many commercial and DIY hobby devices for detecting aproaching electrical storms and most use the same principle, detect the impulse current signal and use it to shape an output pulse. Good ones try to detect the lightning interference in both electric and magnetic fields to avoid false triggers.

     

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