op-amp

94 Articles

Rethinking Your Jellybean Op Amps

January 6, 2025 by 43 Comments

Are your jellybeans getting stale? [lcamtuf] thinks so, and his guide to choosing op-amps makes a good case for rethinking what parts you should keep in stock.

For readers of a certain vintage, the term “operational amplifier” is almost synonymous with the LM741 or LM324, and with good reason. This is despite the limitations these chips have, including the need for bipolar power supplies at relatively high voltages and the need to limit the input voltage range lest clipping and distortion occur. These chips have appeared in countless designs over the nearly 60 years that they’ve been available, and the Internet is littered with examples of circuits using them.

For [lcamtuf], the abundance of designs for these dated chips is exactly the problem, as it leads to a “copy-paste” design culture despite the far more capable and modern op-amps that are readily available. His list of preferred jellybeans includes the OPA2323, favored thanks to its lower single-supply voltage range, rail-to-rail input and output, and decent output current. The article also discussed the pros and cons of FET input, frequency response and slew rate, and the relative unimportance of internal noise, pointing out that most modern op-amps will probably be the least thermally noisy part in your circuit.

None of this is to take away from how important the 741 and other early op-amps were, of course. They are venerable chips that still have their place, and we expect they’ll be showing up in designs for many decades to come. This is just food for thought, and [lcamtuf] makes a good case for rethinking your analog designs while cluing us in on what really matters when choosing an op-amp.

Homebrew Phosphorescence Detector Looks For The Glow In Everyday Objects

November 28, 2024 by 8 Comments

Spoiler alert: almond butter isn’t phosphorescent. But powdered milk is, at least to the limit of detection of this homebrew phosphorescence detector.

Why spend a bunch of time and money on such a thing? The obvious answer is “Why not?”, but more specifically, when [lcamtuf]’s son took a shine (lol) to making phosphorescent compounds, it just seemed natural for dad to tag along in his own way. The basic concept of the detector is to build a light-tight test chamber that can be periodically and briefly flooded with UV light, charging up the putatively phosphorescent compounds within. A high-speed photodiode is then used to detect the afterglow, which can be quantified and displayed.

The analog end of the circuit was the far fussier end of the design, with a high-speed transimpedance amplifier to provide the needed current gain. Another scaling amp and a low-pass filter boosts and cleans up the signal for a 14-bit ADC. [lcamtuf] went to great lengths to make the front end as low-noise as possible, including ferrite beads and short leads to prevent picking up RF interference. The digital side has an AVR microcontroller that talks to the ADC and runs an LCD panel, plus switches the 340 nm LEDs on and off rapidly via a low gate capacitance MOSFET.

Unfortunately, not many things found randomly around the average home are all that phosphorescent. We’re not sure what [lcamtuf] tried other than the aforementioned foodstuffs, but we’d have thought something like table salt would do the trick, at least the iodized stuff. But no matter, the lessons learned along the way were worth the trip.

Programming Tiny Blinkenlight Projects With Light

August 1, 2024 by 18 Comments

[mitxela] has a tiny problem, literally: some of his projects are so small as to defy easy programming. While most of us would probably solve the problem of having no physical space on a board to mount a connector with WiFi or Bluetooth, he took a different path and gave this clever light-based programming interface a go.

Part of the impetus for this approach comes from some of the LED-centric projects [mitxela] has tackled lately, particularly wearables such as his LED matrix earrings or these blinky industrial piercings. Since LEDs can serve as light sensors, albeit imperfect ones, he explored exactly how to make the scheme work.

For initial experiments he wisely chose his larger but still diminutive LED matrix badge, which sports a CH32V003 microcontroller, an 8×8 array of SMD LEDs, and not much else. The video below is a brief summary of the effort, while the link above provides a much more detailed account of the proceedings, which involved a couple of false starts and a lot of prototyping that eventually led to dividing the matrix in two and ganging all the LEDs in each half into separate sensors. This allows [mitxela] to connect each side of the array to the two inputs of an op-amp built into the CH32V003, making a differential sensor that’s less prone to interference from room light. A smartphone app alternately flashes two rectangles on and off with the matrix lying directly on the screen to send data to the badge — at a low bitrate, to be sure, but it’s more than enough to program the badge in a reasonable amount of time.

We find this to be an extremely clever way to leverage what’s already available and make a project even better than it was. Here’s hoping it spurs new and even smaller LED projects in the future.

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You Can Use LEDs As Sensors, Too

July 23, 2024 by 63 Comments

LEDs are a wonderful technology. You put in a little bit of power, and you get out a wonderful amount of light. They’re efficient, cheap, and plentiful. We use them for so much!

What you might not have known is that these humble components have a secret feature, one largely undocumented in the datasheets. You can use an LED as a light source, sure, but did you know you can use one as a sensor?

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This Open Source Active Probe Won’t Break The Bank

June 13, 2024 by 19 Comments

If you’re like us, the oscilloscope on your bench is nothing special. The lower end of the market is filled with cheap but capable scopes that get the job done, as long as the job doesn’t get too far up the spectrum. That’s where fancier scopes with active probes might be required, and such things are budget-busters for mere mortals.

Then again, something like this open source 2 GHz active probe might be able to change the dynamics a bit. It comes to us from [James Wilson], who began tinkering with the design back in 2022. That’s when he learned about the chip at the center of this build: the BUF802. It’s a wide-bandwidth, high-input-impedance JFET buffer that seemed perfect for the job, and designed a high-impedance, low-capacitance probe covering DC to 2 GHz probe with 10:1 attenuation around it.

[James]’ blog post on the design and build reads like a lesson in high-frequency design. The specifics are a little above our pay grade, but the overall design uses both the BUF802 and an OPA140 precision op-amp. The low-offset op-amp buffers DC and lower frequencies, leaving higher frequencies to the BUF802. A lot of care was put into the four-layer PCB design, as well as ample use of simulation to make sure everything would work. Particularly interesting was the use of openEMS to tweak the width of the output trace to hit the desired 50 ohm impedance.

Op-Amp Drag Race Turns Out Poorly For 741

February 23, 2024 by 32 Comments

When it was first introduced in 1968, Fairchild’s 741 op-amp made quite a splash. And with good reason; it packed a bunch of components into a compact package, and the applications for it were nearly limitless. The chip became hugely popular, to the point where “741” is almost synonymous with “op-amp” in the minds of many.

But should it be? Perhaps not, as [More Than Electronics] reveals with this head-to-head speed test that compares the 741 with its FET-input cousin, the TL081. The test setup is pretty simple, just a quick breadboard oscillator with component values selected to create a square wave at approximately 1-kHz, with oscilloscope probes on the output and across the 47-nF timing capacitor. The 741 was first up, and it was quickly apparent that the op-amp’s slew rate, or the rate of change of the output, wasn’t too great. Additionally, the peaks on the trace across the capacitor were noticeably blunted, indicating slow switching on the 741’s output stage. The TL081 fared quite a bit better in the same circuit, with slew rates of about 13 V/μS, or about 17 times better than the 741, and nice sharp transitions on the discharge trace.

As [How To Electronics] points out, comparing the 741 to the TL081 is almost apples to oranges. The 741 is a bipolar device, and comparing it to a device with JFET inputs is a little unfair. Still, it’s a good reminder that not all op-amps are created equal, and that just becuase two jelly bean parts are pin compatible doesn’t make them interchangeable. And extra caution is in order in a world where fake op-amps are thing, too.

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No Inductors Needed For This Simple, Clean Twin-Tee Oscillator

February 18, 2024 by 11 Comments

If there’s one thing that amateur radio operators are passionate about, it’s the search for the perfect sine wave. Oscillators without any harmonics are an important part of spectrum hygiene, and while building a perfect oscillator with no distortion is a practical impossibility, this twin-tee audio frequency oscillator gets pretty close.

As [Alan Wolke (W2AEW)] explains, a twin-tee oscillator is quite simple in concept, and pretty simple to build too. It uses a twin-tee filter, which is just a low-pass RC filter in parallel with a high-pass RC filter. No inductors are required, which helps with low-frequency designs like this, which would call for bulky coils. His component value selections form an impressively sharp 1.6-kHz notch filter about 40 dB deep. He then plugs the notch filter into the feedback loop of an MCP6002 op-amp, which creates a high-impedance path at anything other than the notch filter frequency. The resulting sine wave is a thing of beauty, showing very little distortion on an FFT plot. Even on the total harmonic distortion meter, the oscillator performs, with a THD of only 0.125%.

This video is part of [Alan]’s “Circuit Fun” series, which we’ve really been enjoying. The way he breaks complex topics into simple steps that are easy to understand and then strings them all together has been quite valuable. We’ve covered tons of his stuff, everything from the basics of diodes to time-domain reflectometry.

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