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…

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…

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…

Blogging from Wisconsin this week, attending my granddaughter’s second birthday.  :)

We recently received a question on our precision amplifiers E2E forum with a SPICE simulation schematic attached (thank you for providing a schematic!). It was an op amp circuit—the details aren’t important—the point is that it included bypass capacitors on the power supply pins. Of course, this may be because the engineer’s simulation…

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…

Last week we concluded that the very accurately matched resistors of an integrated difference amplifier were crucial in getting the needed common mode rejection. Review it here. But...

There is one relatively common case, however, where 1% resistors and a good op amp can make an adequate difference amplifier. When measuring current with a shunt on the “low side” of the load, the common…

In the design of monolithic ICs we routinely exploit the ability to accurately match internal components. Input transistors of op amps, for example, are accurately matched to provide low offset voltage. If we had to make our own op amps with discrete transistors we would have offset voltages of 30mV or more. This ability to accurately match components includes on-chip resistors.

Last week we reviewed the use of an input bias current cancellation resistor to balance the source resistance at the two inputs of an op amp. The conclusion was that this practice is often not necessary and may even be detrimental. This discussion builds on the previous blog so you may want to review it, first.

I ended last week saying that there are certain op amps for which this…

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…

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…

Last week we looked at the effect that internal differential input clamp diodes could have on op amps when used as a comparator. I posed a question--can these clamps affect op amp circuits. Op amps should have near zero volts between the two input terminals, right? So these diodes will never forward bias in normal op amp circuits... or will they?

Just a reminder, we’re talking…

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…