Alice and Bob are mid-level engineers working on an analog-to-microcontroller interface. They were asked to give a short presentation to a class of freshman engineers.
In this narrative, we explore how Alice and Bob grew to understand Operational Amplifiers (op amps) and applications.
Tech Tip: For more information about op amps please refer to:
What was the first op amp you used?
Like most of you, we started with the classic Jellybean 741.
It was a great place to start, as the circuit is relatively simple. We could understand how transistor circuits such as the long-tailed pair and current mirror were implemented in the 741.
When did you know that you needed something better than the 741?
Believe it or not, this little Integrated Circuit (IC) is 62 years old! We have made some progress since Elvis and JFK.
It didn’t take very long to discover the 741 was lacking. In fact, our advanced labs demonstrated the low Gain-Bandwidth Product (GBW) of 1 MHz. This may seem like a high frequency, but remember that 1 MHz is the point when the gain falls to unity. In practice, the gain starts to drop in the low audio frequency range.
We really learned this lesson as we attempted to construct an op amp with a gain of 1000. It was a failure and acted more like a low-pass filter. We learned that it was better to cascade three op amps together, each with a gain of 10, to yield a total gain of 1000. That was fine, except for the nasty hiss. That led us to explore low-noise options and op amps with a higher GBW.
Alice interjects and talks about her passions for guitar amplifiers with vacuum tubes and a few low-noise NE5532 op amps.
What challenges did you face when connecting an op amp to a microcontroller?
That’s a very good question and a challenge we face today even when the op amp is embedded into the microcontroller die.
Bob cuts off Alice and says, “rail to rail.”
The microcontroller’s Analog to Digital Converter (ADC) is single ended with an input that scales from zero to the voltage reference. You may know this from your default Arduino experiments where an input voltage of 0 to 5 VDC yields an analogRead() result between 0 and 1023.
The 741 has a hard time amplifying this small voltage, especially when powered by 5 VDC source. In fact, the best it can do is pull down to about 1.5 VDC and pull up to about 3.5 VDC. In both cases, it required an overhead of about 1.5 VDC.
We could use high voltage rails such as the proper +/- 12 VDC. But this is an awkward and costly solution when much better options are available. An example is the LMV358 with the ability to swing rail to rail even on a 3.3 VDC source. It’s very simple and can even use the same power supply as the microcontroller.
Today, we continue looking for even better alternatives to find the best match between sensor, amplifier, and power supply. Cost is always an important consideration.
Parting thoughts?
We could talk for days about our favorite op amp circuits:
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Alice: instrumentation amplifier
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Bob: Wein bridge oscillator with a light bulb
Do you have any questions for Bob and Alice? Feel free to leave them in the comments.
Happy soldering!
— Alice and Bob
Related Information
Please follow these links to related and useful information:
- DigiKey’s product selection guides for a comprehensive index into DigiKey’s product families.
About this author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing articles such as this.