After some Facebook chatter on last week’s blog, I think it deserves some follow-up. Here is the interview question that has bugged me for 41+ years:

A 1V AC source is connected to a 1Ω resistor in series with a 1Ω reactance capacitor. What is the AC voltage across the capacitor?

I’ve shared this question with various engineers through the years. The most common response is, “What’s the frequency…

Is it possible to parallel two op amps to get twice the output current?

We get this question periodically on our E2E forums. Though we may answer with a qualified “yes,” it tends to make us shudder just a bit. It can be done… but with great care. So let me come quickly to a key point. Don’t use the simple circuit on the left. Directly paralleling inputs and output of two op amps is sure to start…

It’s time for some fun! I’ve known a few folks who have tormented colleagues with a resistor cube—equal resistors on all sides. So in case you’ve solved that one, let’s add a twist. In this cube, not all the resistors are equal. The resistance from A to B is 1Ω. Resistor values are indicated, except for those marked “R?” in red.  What is the required value for R?

Spoiler…

SPICE is a useful tool to help check for potential circuit stability problems. Here is one simple way to do it:

Figure 1 shows a non-inverting amplifier using the OPA211 with a couple of minor variations that are common in many applications. R3-C1 is an input filter. R4 is an output resistor to protect against abuse when connected to the outside world. CL models a five-foot cable…

We often receive applications questions relating to the power supply, input and output voltage range capabilities of our op amps. It can be confusing so here is an attempt to sort it out:

First, common op amps don’t have ground terminals. A standard op amp does not “know” where ground is so it cannot know whether it is operating from a dual supply (±) or from a single power supply. As long as the power supply, input…

Did you see last week’s resistor puzzle? Check it out if you missed it. Here’s the solution:

We’re not accustomed to reading three-dimensional schematics so the first step is to redraw it clearly. There are three distinct paths from A to B, colored blue, green and red. Each has a series connection of 1Ω—R—1Ω.  The 3Ω resistors are effectively in parallel with the “R” resistors…

Have you noticed?...it’s rare to find new generation op amps and other ICs in DIPs (dual-inline packages). Without volume demand, it’s not economically viable to offer new ICs in DIPs. Breadboarding with these newer fine-pitch micro-packages can be a pain. What to do?

This DIP adapter kit eases the pain. For ten bucks you can mount SO-8, SOT23 (3, 5, 6, or 8-lead) MSOP-8, SC70-6, SOT563-6 packages. We won…

           August 22, 2012

Anyone who has read a few of my blogs has seen that I simulate circuits in SPICE. You may know that Bob Pease was pretty opinionated about SPICE and once wrote, “SPICE takes away your ability to get good insights on what is happening. Spice actually HURTS your understanding of how circuits function.” In honor of Bob’s birthday today, let’s consider some merits and pitfalls of SPICE.…

Jake, a respected veteran engineer in a power generation plant, retired with great congratulations and accolades. A few months later the plant suffered a major malfunction—a real meltdown with all systems involved. The engineering staff could not quickly diagnose the problem so with due urgency they called Jake for an emergency consultation.

He surveyed the situation, checking the condition of some indicator lights…

Bob Pease died a year ago yesterday. What a loss for us analog heads who struggle to make our circuits work. His wealth of knowledge and experience seemed limitless. His insight and intuitive explanations helped so many engineers understand their circuits better.

I can’t say that I read every one of his columns. He often strayed far off his mainstream analog topics, waxing into lengthy discussions on hiking,…

Welcome to “The Signal,” a technical blog focusing on analog signal processing circuits.

Op Amps used as Comparators—is it okay?

Many of you (and I, too) occasionally use an op amp as a comparator. Often, this is when you only need one simple comparator and you have a “spare” op amp in a quad op amp package. The…

A holiday week is a good excuse to stray a bit off topic so this is my chance to tell you about Goop. It is by far the most versatile glue I’ve found. Okay, I promise to tie this in (though weakly) to electronics. Goop comes in a fat toothpaste tube and squeezes out clear with the consistency of molasses.  It sticks to most anything, fills voids, sets up in an hour or two and cures overnight. Unlike epoxy, there…

As I travel this week for TI’s university recruiting I think back on some memorable interviews—ones on both sides of the table. One still haunts me. I was seeking my first industry job, one that I desperately wanted but was not offered. I’ve long wondered whether the way I handled a particular technical question made a difference. I’ll get to that question in a moment.

Through the years I’ve seen…

It’s a holiday week, Thanksgiving in USA—a good excuse for a break from techie topics. And it’s a chance to call your attention to some changes to our blogs. The title of this week’s blog is a common expression in my lexicon but these changes are good. I just thought I could get you to read if I sounded a bit grumpy.  :)

Perhaps most important is the new “subscribe” link on the right. This allows…

I posed a question a couple of weeks ago regarding film capacitors—what’s the meaning of the stripe on one end? Check the picture below.

These are non-polarized capacitors so it’s not a polarity marking. A reader, Richard, answered correctly—it identifies the outside conductive foil of the spiral wrapped innards. I’m finding that few engineers these days know this, and proper orientation can make a difference…

If you frequent our E2E forums they can be like detective mysteries. They are brain teasers that exercise and build your troubleshooting skills, solving analog problems with limited information. But not all these mysteries can be solved, given the information provided in the original posting.

“My amplifier circuit doesn’t work. Can you tell me what is wrong?” Seriously, that is nearly the extent of…

You have voted. Unity-gain-stable op amps won in a landslide—they’re far more popular than decompensated op amps. What’s this all about?

Click Here to read on EDN Magazine site.

It’s an easy concept—the inputs compare two voltages. The output is high or low. So, why all the chatter through the transition?

This effect usually occurs with slow changes through the transition voltage. Often it’s because the input signals have noise that jiggles through the transition voltage causing a chattering output. Even with very clean input voltages, comparators have their own noise—like an op…

Settling time is the time required for an op amp to respond to an input voltage step, enter and stay within specified error range of the final value. It’s important in applications that drive an a/d converter, digitizing rapidly changing inputs. But let’s look beyond the definition and focus on the character of settling waveforms.

Last week’s blog on slew rate showed how an op amp transitions from a…

In a previous blog on supply bypassing, I cautioned that poor bypassing could increase distortion of an amplifier. A reader, Walter, asked an interesting question… where should you connect the ground of a bypass capacitor to avoid problems?

This raises questions regarding proper grounding techniques. Wow. Big topic, but I may be able provide some insight with a couple of simple examples.

Figure 1 shows inverting…

Perhaps you’ve never used a thermocouple and think you have no reason to know how they work. I disagree. I believe that ten minutes of reading will be well spent. If you already know this much, please read and tell me if I got anything wrong.

Thermocouples are temperature measurement sensors made from at two different metals. They might be elements such as copper or iron or alloys made from a specific mixture of…

Last week we looked at two very common reasons for oscillations or instability in op amp circuits. The ultimate cause of both was delay or phase shift in the feedback path. Review it here. I confess that I had intended to discuss cures for both circuits this week. But to keep these blogs bite-size, I think it’s best to cover just one this week. (I must control my enthusiasm!)

A simple non-inverting amplifier can…

The instrumentation amplifier (IA) is an integrated combination of op amps and feedback resistors used to accurately acquire and amplify signals.

      Don’t know about instrumentation amps?  Get a very quick start here.

A common mistake in using these versatile amplifiers is to fail to provide a path for input bias current. For 25 years we’ve been showing a diagram to highlight…

I’m not referring to op amps in your parts bin. Those should be in anti-static bags or conductive foam. What about the one on your circuit board—the unused op amp in a quad or dual package. Hummm??

A recent question on our forums spurred me to address this subject but in the process, I ran across a great article by my colleague, Todd Toporski. He did an excellent job of covering the important issues and reasons…

Last week we looked at the source of input bias current in CMOS and JFET amplifiers, finding that it comes from the leakage of one or more reverse-biased P-N junctions. Check it out if you missed it. We ended with a caution that these leakages increase significantly with temperature.

The reverse-biased leakage of a P-N junction has a strong positive temperature coefficient, approximately doubling for each 10°C increment…