stability vs gain testing in ltspice - Page 1
Forum section: electronics / projects

author: john23
author_rank: Frequent Contributor
timestamp: October 13, 2025, 09:00:17 am
content: Hello, there is the following structure (shown in the article )I want to simulate using ltspice tsmc180nm pdk.However as you can see there is a pole compensation which is basickaly a feedback structure .So I have a dillema:we have here a open loop circuit which cannot be unstable,so why are we doing a pole compensation if this structure cannot start oscilating.could you give me an update to a structure which I could test the closed loop stability vs gain ?Thanks.https://www.mdpi.com/2079-9292/12/11/2361

author: mtwieg
author_rank: Frequent Contributor
timestamp: October 13, 2025, 12:52:02 pm
content: I'm guessing this circuit is part of a larger system, and seeing that larger system would make the purpose of the compensation apparent.For analyzing stability, you typically define a feedback loop in the circuit, break it and inject a stimulus, then measure the voltage at either side of the stimulus to derive loop gain.If you provide the LTspice simulation files, I can show you how to make the measurement.

author: john23
author_rank: Frequent Contributor
timestamp: October 13, 2025, 06:41:57 pm
content: Hello mtwieg,I have built a basic structure of the circuit in tsmc 180nm ltspice.could you show me how to simulate stability for such structure?Thanks.

author: mtwieg
author_rank: Frequent Contributor
timestamp: October 14, 2025, 11:57:47 am
content: See attached. I made some modifications:Adjusted the bias voltages such that the MOS devices are all in saturation.Changed the load capacitor from 1uF to 100pF (seems more realistic).Also named nets similarly to the linked paper.Added another source V4 for injecting AC into the loop.Added some parameters to control whether V2 or V4 is injecting AC.The closed loop gain of the entire circuit (from vin to vo2) is given by vo2/vin while V2 is the AC source. In the attached plot, this is the upper green trace.The only real feedback path in this circuit is through M3, R1, and C2. So I added V4 in this path in order to probe the loop gain. The loop gain is the ratio of the voltages that appear on either side of V4 (vo1p/vo1). In the attached plot, this is the lower blue trace.Unsurprisingly, the phase of the loop transfer function only drops to -270 degrees at worst. So it can't be unstable, and its phase margin can't be worse than 90 degrees.

author: john23
author_rank: Frequent Contributor
timestamp: October 14, 2025, 02:24:32 pm
content: Hello Mtweig,few questions regarding what you built:1.Why did you put two AC sources in the circuit?2.why did you choose to check stability by the ratio of Vo2 to Vin to check the stability?our feedback loop is where you inject V4?so we need to see if there is +180 in V4 (positive feedback /oscilations).Thanks.

author: mtwieg
author_rank: Frequent Contributor
timestamp: October 14, 2025, 04:04:51 pm
content: Quote from: john23 on October 14, 2025, 02:24:32 pmV2 is for measuring the closed loop gain of the whole circuit. V4 is for measuring loop gain of the feedback loop.QuoteThat's for measuring closed loop gain, not loop gain (which is what indicates stability).QuoteInstability would be indicated by 360 phase in the loop gain (vo1/vo1p) with gain greater than unity.