As an avid reader of 24×7 magazine and very aware of regular articles such as the “ICC Prep” column, I was very humbled when asked to begin writing this column. With the numerous articles and plethora of valuable information that has been shared over the years by Dave Harrington, PhD, I must admit I have some big shoes to fill. Please bear with me as I begin my journey to help each of you achieve the status of CBET and follow one of the true industry leaders in the biomedical/clinical engineering field.
On behalf of many health care professionals and every BMET/CE who has ever read this column, I would like to extend a most sincere thank you to Dave Harrington, PhD, for sharing your time and talents through the pages of 24×7. Once again, thank you for what you have shared with the readers and the biomedical industry for so many years.
|Figure A: Those taking the CBET exam can be certain the test will contain more than one question about op amps.|
With that said, I would like to begin my first column as sort of a piggyback column to the “ICC Prep” article printed in the August 2008 issue titled, “Electronics—Or the Lack Thereof.”
I could not agree more with this article. With the advancements in electronic technology such as surface mount devices and multilayer printed circuit boards, little room is left for board-level repair. The BMET shops I visit do not have the equipment or expertise to complete electronic repairs at the level of today’s modern health care devices. This, however, does not mean that a well-qualified technician should not understand how these devices work and the electronic circuitry found inside.
In the CBET review seminars I hold, I always go over many of the basic concepts of electronics along with operational amplifiers. As I tell the participants in these review classes, the test is hard enough without missing the “gimme” questions. You know the ones—what is the ground wire resistance limit for patient care equipment, what is the leakage current limit for these devices, etc. In the electronics section of the test, each person sitting for the exam will also receive a few “gimme” questions about electronics.
I consider any questions that can be answered using Ohm’s law a “gimme” question, and most tests will have one or two that can be answered with only this information. My heart always goes out to the exam taker who rushes through and misses one of the “gimme” questions and scores a 104. So do not ignore the obvious such as Ohm’s law.
Make sure you understand exactly what the most common electronic devices, such as a capacitor, do. It is two plates separated by a dielectric; and its functions are to pass AC, block DC, or store a charge. An inductor is nothing but a coil. Its function is to pass DC, and it attempts to block AC. How do these facts play into capacitive reactance or inductive reactance? These are common-type questions, but they are very easy to solve. I would not enter a testing site without this very basic electronic information. There are other such devices, but as the August 2008 article explains, this information is in your electronics 101 text.
It has also been my experience in teaching CBET review classes that many attendees do not remember basics such as how to add resistors in parallel or in series, and capacitors in series or parallel. This is all very basic electronic information, but it is also information that many BMETs do not use on a day-to-day basis and, as the old saying goes, “If you don’t use it, you lose it.” Again, do not go to the testing site without this information clear in your head. This is also the type of information that may help you work through the equipment problem-solving and troubleshooting questions. An example could be, “What does voltage and current do in series and parallel?” Remember, voltage is constant in parallel, and current is constant in series.
I can almost assure anyone who takes the exam that they will receive more than one question about op amps, or operational amplifiers. I was taught electronics from the Malvino texts, and the op amps are introduced with the six ideal properties of an op amp. These ideal properties will never be obtained in the real world, but they do give you an excellent idea of what op amps do and why they are so heavily used in instrumentation circuits. They are as follows:
- Infinite frequency response;
- Infinite open loop gain;
- Infinite input impedance;
- Zero noise contribution;
- Zero output impedance; and
- The inputs tend to follow one another.
With minor changes in circuit configurations, electronic circuit designers can make many useful applications from a single operational amplifier, such as a multiple input circuit—also known as an averaging amp or summing amp. Design engineers can also make a comparator circuit, a differential (or diff amp), an integrator or differentiator circuits, or using three op amps in a special configuration allows what is known as an instrumentation amplifier circuit to be created.
Over the years many test takers have informed me about several questions pertaining to these types of circuits, such as what is the common mode rejection ratio, virtual ground, how do you find the gain of an op amp circuit, etc. All of these questions and many more are now at our fingertips with Google. Or, now that it is back in print, I would suggest you purchase the new book by Les Atles as it contains the updated version of the Affinity Reference Guide. If you Google ISBN# 978-0-7575-4890-1, you will find Atles’ book, A Practicum for Biomedical Engineering & Technology Management Issues. The book contains a lot of information that may appear on the CBET exam.
Three sample questions that could appear on the ICC examination are in the section below.
John Noblitt, BS, CBET, is the BMET program director at Caldwell Community College and Technical Institute, Hudson, NC. For more information, contact .
- Refer to figure A above. When measuring the voltage with a DVM at the summing junction in an operational amplifier circuit with a gain of 200 and an input of 1 mv, you would expect to read what value?
- 200 mV
- 201 mV
- None of the above
- In figure A, if R1’s value was 100 ohms and RF’s value was 1K ohm, what would be the gain of the circuit?
- None of the above
- In figure A, with a 1 mV input signal and a gain of 10, the output would be____?
See the answer
See the answer
See the answer