The noise performance of amplifier circuits is greatly affected by the Johnson noise of resistors—the source resistance and feedback resistors. Most everyone seems to know that resistors have noise but may be a bit foggy on some of the details. Here’s a bite-sized review in preparation for future discussions on amplifier noise:
Click Here to read on EDN magazine web site.
Table of Contents for all The S…
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…
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…
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…
CMOS and JFET-input op amps are often selected for their low input bias current (IB). But there is more to this than the single line in the spec table—subtleties that you should be aware of.
Click Here to read on EDN web site.
Some circuits benefit when the characteristics of two more op amps are closely matched. So inside a dual or quad op package, how closely do their behaviors match? Hummm?
The most common request on our precision amplifier E2E forum is for matching offset voltage and offset voltage temperature drift. If you are making your own instrumentation amplifier, for example, matching op amp offsets would produce a net zero offset…
Have you ever tried to use a linear potentiometer (pot) as a volume control? Yikes! The volume jumps up much too rapidly. It requires a safe-cracker’s touch to adjust the volume to quiet listening levels. Thus the logarithmic potentiometer.
Our senses have extremely wide dynamic ranges. Our ears (actually, younger ones, not mine) have a useful range of 120dB or more, a 1,000,000-to-1 ratio. The origin of the decibel…
What is the input impedance of a transimpedance amplifier (TIA)? Infinite? Zero? No, what is it really? Nothing is really zero or infinite, right? The answer might surprise you—worth understanding, even if you don’t use TIAs. After all, an inverting amplifier is just a TIA with an input resistor, right?
Click here to read on EDN web site.
Index to all The Signal blogs.
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…
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…
When providing a sensitive amplifier input terminal to the outside world, designers wonder what someone might connect or how it might be treated. Will it be treated with care… or could they carelessly plug it into the AC mains? We all would like to make our equipment robust, able to sustain the most brutal treatment. How to protect against Electrical Over-Stress (EOS)?
The OPA320 is typical of most op amps; absolute…
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…
Potentiometers (pots) can be used as position sensors or to adjust circuits for proper operation. They function best as an adjustable voltage divider. They can also be used as an adjustable resistor but with some potential pitfalls. Do you know the difference?
Click here to read on EDN web site.
Index to all The Signal blogs.
My colleague Soufiane recently published an article, “Pushing the Precision Envelope.” In it, he discussed various technologies we use to “trim” or adjust the offset voltage of our amplifiers to very low values. It got me thinking about offset voltage trim pins—where did they go?
Most newer op amps lack the offset voltage trim pins once found on virtually all op amps. There are many factors…
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.…
After spending the last few months blogging weekly here on E2E, I’m pleased to announce that I’ve launched an affiliated blog on EDN that I’ll be posting to once a month. Also called The Signal, the content will be very similar to what you’ve seen from me so far. I’ll be discussing analog signal processing, including op amps, instrumentation amps and related circuitry. I’ll provide tips and cover common mistakes and misconceptions…
I’m surprised by the number of questions we get on our support forums regarding photodiodes and associated circuits. Here is a 10-minute quick-start—the stuff an analog designer should know.
A simple photodiode model shows the key elements—a diode in parallel with a current source that is proportional to the irradiance (light intensity). Parasitic components CD and RD can play a role in performance…
Offset Voltage and Open-Loop Gain—they’re cousins
Everyone knows what offset voltage is, right? In the simplest G=1 circuit of figure 1a, the output voltage is the offset voltage of the op amp. The offset voltage is modeled as a DC voltage in series with one input terminal. In unity gain the offset is passed directly to the output with G=1. In the high gain circuit on the right the output voltage is 1000…
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…
In helping to select op amps and instrumentation amps I frequently hear the comment, “I need really high input impedance.” Oh really?… are you sure?
It’s rare that input impedance, or more specifically, input resistance is an important issue. (Input capacitance, the reactive part of input impedance, is another matter so save that one for another day…
Rail-to-rail amplifiers are able to produce output voltages very close to ground… but how close? We’re talking about CMOS op amps that are often used on low voltage designs when you are trying to maximize output voltage swing. Our specifications for these devices generally look something like this:
This makes it appear that the output will never swing much closer than 15mV from ground and the last 15mV…
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…
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…
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,…
Designers sometimes find op amp data sheet specifications perplexing because not all performance characteristics have minimum or maximum specifications. You must occasionally rely on “typical” values in the specification table or typical performance graphs. But what does typical mean? How much can it vary?
There are no easy answers and it depends on the specification. Here are some guidelines on three characteristics…
