Open-loop, Closed-loop and Feedback Questions and Answers

December 10th, 2012 by Adrian S. Nastase

One of my readers posted the following questions in the comment section of MasteringElectronicsDesign.com: An Op Amp Gain Bandwidth Product.

I am doing a work on fully differential Negative feedback op-amp with capacitive divider configuration. I have some questions and confusions, can you please clarify?

What is the difference between closed loop gain and open loop gain, and are they dependent to each other?

How can we calculate the unity gain frequency if I have a 3-dB frequency of 100Hz and closed loop gain of 40dB?

Does the feedback factor (BETA) has importance with respect to any other parameters?

How will it help in finding the closed transfer function of the system assuming the op-amp as a single pole system?

The answers needed some space, more than the comment section could offer, so here is a post on the topics of op amp open-loop, closed-loop and feedback.

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Categories: Analog Design

How to Derive the RMS Value of a Sine Wave with a DC Offset

February 20th, 2012 by Adrian S. Nastase

I noticed a question posted on one of Yahoo’s Q&A sites, asking what is the RMS value of a sine wave with a DC offset. The chosen answer as being “the best” was actually wrong. The next comment, which was trying to correct “the best” answer, was wrong too. I am not going to post the Yahoo link here. What I can do, is to show how to derive the RMS value of such waveform.

Let’s derive first the RMS value of a sine wave with no DC offset

Let’s start with the RMS value of a sine wave, with no DC offset, which is shown in Figure 1. It is well known that the RMS value of a sine wave is 0.707 times the signal peak level, but how can you prove this?

sine-wave-with-zero-offsetFigure 1

As shown in this article, MasteringElectronicsDesign.com: How to Derive the RMS Value of a Trapezoidal Waveform, or other RMS articles in this website , let’s start with the RMS definition.

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Categories: Analog Design

My Interview in EEWeb

December 29th, 2011 by Adrian S. Nastase

I was recently interviewed by EEWeb for the Featured Engineer edition. EEWeb, an electrical engineering publication, has many remarkable sections, including Analog Design, RF Design, tools, jobs, projects, and even a comic section. Of interest is the EEWeb forum, which is a growing discussion board organized in categories.

Here is an excerpt from the interview:
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Categories: Analog Design, Opinion

Become a Mentor

December 16th, 2011 by Adrian S. Nastase

For a few years now I mentor students at Ocean View High School in Huntington Beach, California. My activity is part of a program called Business Academy developed at Ocean High in the last years. Every spring, students meet accomplished professionals and business people who become their mentors. The goal of the program is to teach and show students the path to a professional life and what to do to pursue their goals.
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Categories: Education, Opinion

Measure a Bipolar Signal with an Arduino Board

August 14th, 2011 by Adrian S. Nastase

Arduino is a popular family of open source microcontroller boards. Hobbyists, students and engineers all over the world use this platform to quickly design and prototype a microcontroller driven circuit. One of its interfaces with the analog world is the ADC. Since these boards are mostly designed around an ATMEL ATmega32 or ATmega168 microcontroller, the ADC has 8 inputs and 10-bit resolution, making it suitable for many applications.

From time to time I receive a message through my Contact page with the question, how to interface a sensor, or an outside circuit with the Arduino ADC? In most cases the answer is an interface between a bipolar circuit and the Arduino board. As the bipolar circuit output varies from some negative to a positive level, the Arduino ADC cannot measure this signal directly, because the ADC inputs can only be between 0V and the reference voltage.

In one of these messages a reader asked me how to build an interface between a board that has an output voltage of -2.5V to +2.5V and the Arduino ADC. He told me that the Arduino reference voltage is AVCC = 5V. He would like to measure the +/-2.5V signal with the Arduino board and direct the microcontroller to take some action based on the result.

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Categories: Analog Design, Electronic Circuits Examples, Mixed-Signal Design, Summing Amplifier

Measure a Wheatstone Bridge Sensor Signal with an ADC

July 23rd, 2011 by Adrian S. Nastase

I received a message from one of my readers asking me to help with a Wheatstone bridge circuit. Since my response to him bounced back, and this being an interesting subject, I decided to write this article. Here is what he writes:

“I found a circuit to condition the output of the Wheatstone bridge in the National ADC1205 datasheet, page 16. It uses an Op Amp configured as follows: V1 from the bridge thru 10K resistor to (–) input of Op Amp, 1.5Meg feedback resistor and Vout connects to the V- of the 5V ADC. V2 from the bridge connects directly to (+) input of the op amp and the V+ of the ADC.

The bridge V2-V1 is 0 mV to 30 mV. This is both at 5.000 V (0 mV) and V1 = 4.985 V, V2 = 5.015 V (30 mV). Please advise the equations to calculate how this works. Since the ADC is 5 V, I cannot see how the Vout can exceed that voltage. Is it true that Vout = 5.015 V when V1 = V2 = 5.015 V and ADC Out = 0 V?”

The National ADC1205 is an obsolete component now, but the advice and application notes are still valid. We can use any ADC if we can correctly adjust Vref and the operating conditions. We can use an Arduino board as well, with its 10-bit ADC to achieve a complete system.

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Categories: Analog Design, Electronic Circuits Examples, Mixed-Signal Design

Come to My Electronics Class on June 9th, at UCR

May 30th, 2011 by Adrian S. Nastase

Every summer Mt. San Jacinto College has a program called STEM (Science, Technology, Engineering and Math). This year, the event will be on June 6 – 9 at Mt. San Jacinto College campus and also at University of California Riverside. Students in the Riverside district are invited to participate, and explore the program classes. The event is intended to expose students to science and technology and help them get more clarity in choosing their future profession.

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Categories: Education

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