• Op Amp Voltage Ranges—input and output, clearing some confusion

    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…

  • Taming the Oscillating Op Amp

    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…

  • Why Op Amps Oscillate—an intuitive look at two frequent causes

    Bode plots are great analytical tools but you may not find them intuitive. This is a purely intuitive look at frequently encountered causes for op amp instability and oscillations.

    The perfectly damped response in figure 1 occurs with no delay in the feedback signal reaching the inverting input. The op amp responds by ramping toward the final value, gently slowing down as the feedback signal detects closure on the proper…

  • The Unused Op Amp—what to do?

    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…

  • Paralleling Op Amps—is it possible?

    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…

  • PCB Layout Tricks—striped capacitors and more

    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…

  • Input Bias Current Cancelation Resistors—do you really need them?

    Do you add a resistor to match the DC resistance at the inputs of your op amp circuits?  Check the circuits in figure 1 below. Many of us were instructed to add Rb as “good practice,” making its value equal to the parallel combination of R1 and R2. Let’s look at the reason for this resistor and consider when it’s appropriate and when not.

    The purpose of Rb is to reduce the voltage offset caused…

  • Op Amps used as Comparators—is it okay?

    Other Parts Discussed in Post: UA741, OP07C, OPA277, OPA227, OPA244, LM324, OPA2251, LM358

    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…

  • Input Capacitance—common-mode?...differential?… huh?

    The input capacitance specifications of op amps are often confused or ignored. Let’s clarify how these specs can best be used.

    Stability of an op amp circuit can be affected by input capacitance at the inverting input by causing phase shift—a delay of the feedback reaching the inverting input. The feedback network reacts with input capacitance to create an unwanted pole. Scaling the impedance of the feedback network…

  • Bypass Capacitors… yes, but why?

    Everyone knows that op amps should have power supply bypass capacitors located near the IC’s terminals, right? But why? Why, for example, is an amplifier more apt to oscillate without proper bypassing? The reasons will increase your understanding and awareness.

    Power supply rejection is an amplifier’s ability to reject variations in the power supply voltage. Figure 1, an example, shows that this rejection capability…

  • 1/f Noise—the flickering candle

    The 1/f (one-over-f) low frequency noise region of amplifiers seems just a bit mysterious. Reader “tweet” asked for a discussion of 1/f noise—a challenging topic for a short blog.

       Click Here to read on EDN magazine web site.

    Excel noise calculation file here… e2e.ti.com/.../4812.Flicker-Noise-v1.xlsx

    Note:  This file is different than the one posted a month ago.

     

  • SPICEing Op Amp Stability

    Other Parts Discussed in Post: OPA211

    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…

  • Illuminating Photodiodes ;-)

    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…

  • Taming Oscillations—the capacitive load problem

    Other Parts Discussed in Post: OPA320

    We’ve been looking at stability of op amps, considering how phase shift (or call it delay) in the feedback path can cause problems. Picking up from last week, stability with a capacitive load is a tricky case. If you’re joining us late, you may want to first read the previous two blogs, Why Oscillations and Taming Oscillations.

    The trouble maker, open-loop output resistance…

  • Grounding Principles

    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…

  • Thermocouples—stuff that every analog designer should know

    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…

  • Offset Voltage and Open-Loop Gain—they’re cousins

    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…

  • Interview Questions—memorable times on both sides

    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…

  • Goop—a sticky topic

    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…

  • Simulating Gain-Bandwidth—the generic op amp model

    It may not always be obvious how the gain-bandwidth product (GBW) of an op amp may affect your circuits. Macro-models have a fixed GBW. Though you can look inside these models, it’s best not to tinker with them. What to do?

    You can use a generic op amp model in SPICE to check your circuits for sensitivity to GBW. Most SPICE-based circuit simulators have a simple op amp model that you can easily modify. TINA’s is…

  • Protecting Inputs from Damage— EOS

    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…

  • Comparators—what’s all the chatter?

    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…

  • Resistor Puzzle—the sequel

    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…

  • Controlling Volume—log pots

    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…

  • “Typical”—what does it mean in a data sheet specification?

    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…