You want to analyze the stability of an op amp circuit in LTspice, but where do you begin? How do you configure a simulation schematic, and what do you plot and measure? Even though I have examined op amp stability in LTspice numerous times, I often have to revisit my notes. How did I do that again? And how do I trust that this method I’m using is giving me accurate results? In the next several posts, I will share all my notes on op-amp stability analysis in LTspice. That way, we will all have a quick reference to this information in the future!
(Note: this first post is heavy on theory, and light on LTspice tips. LTspice-specific content coming in future posts.)
Start with an Example that is Clearly Not Stable
Let’s begin with a transient simulation of a circuit that exhibits obvious stability issues. Figure 1 shows a fairly simple inverting op amp circuit using the ADA4896-2. I’m not doing anything strange or complicated in this circuit. Why is it ringing?
Figure 1: Transient Simulation of an Inverting Amplifier that is Excessively Ringy and Unstable
Download the example circuit to follow along:
Transient Simulation of a Clearly Unstable Circuit.asc
For this blog series, I will assume you have some basic knowledge about using LTspice. Please visit Getting Started with LTspice and the LTspice Basics video series to learn more about the basics of creating schematics, running simulations, and plotting results in LTspice.
Let’s modify the feedback network (Rf and Rg) and see what happens. Keeping the signal gain (-Rf/Rg) the same, let’s parameterize the resistor values by changing their value to {R} and run a simulation for Rf=Rg=250Ω as well as the original 5kΩ. See LTspice How-to: Using the .STEP Directive to Perform Repeated Analysis for more information about adding .STEP parameters to your simulation.
Figure 2: Transient Simulation of Both Unstable and Stable Examples
The ringing goes away when I reduce the values of Rf and Rg. But why? And how do I perform this analysis in the frequency domain to better understand and analyze?
A Crash Course on Stability Analysis in the Frequency Domain
Before reading any further, go watch our Stability 101 Whiteboard Series, which will provide you with the theory needed to understand the plots and analysis in LTspice. I will assume for the rest of this blog series that you have watched parts 1 (loop gain), 2 (Bode plots), and 5 (parasitic capacitance). When you’re done, you should understand everything on the whiteboard in Figure 3, which is important because our ringy ADA4896-2 example is explained perfectly with this theory.
Figure 3: Screenshot of Stability 101 Whiteboard Series, Part 5
Go ahead and watch; I’ll wait.
What Do I Need to Plot in LTspice to Analyze Stability?
In short, we need to plot the loop gain (Aβ) of the circuit, and determine if Aβ is approaching -1 (or 1∠180°). More on that in future blog posts.
How Do I Plot a Circuit’s Loop Gain in LTspice?
If you spend some time Googling, you will find many methods for using loop gain Bode plots to analyze stability. You’ll find methods with names like BigL/BigC, Middlebrook, and Tian. Be aware: there are many examples on the web using methods called Middlebrook, and they are not all the same. More about this and all these methods later.
Conclusion
In my next blog post, I will explore a technique for breaking up the circuit you want to analyze into isolated circuit segments to plot A and β separately. This will help us build intuition around what the A and β curves represent.
In future blog posts, I will use our ADA4896-2 example (and others) to demonstrate several techniques, as well as point out some shortcomings and things to watch for. I have my favorites, but my goal is to present them all so that you can form your own opinion on which method(s) you prefer. Stay tuned!
Read all the blogs in the LTspice Amplifier Stability series.