Opamp ringing - Page 1
Forum section: electronics / projects

author: Jan Audio
author_rank: Frequent Contributor
timestamp: June 03, 2019, 03:17:11 pm
content: Hi, i use opamps to build inverting amplifiers, normally there is a small capacitor needed between output and inverting input to avoid ringing.What do you call this ringing problem ?, u know the high frequency sinus wave that occurs without the capacitor sometimes.Do all opamp types need this capacitor against ringing ?When i use a 5volt rail to rail opamp i dont use this capacitor, i dont know why.When does this ringing occurs, i wont occur when there is a big enough signal it seems, maybe i am wrong.

author: DaJMasta
author_rank: Super Contributor
timestamp: June 03, 2019, 06:15:19 pm
content: A cap between the inverting input and the output acts as a low impedance feedback for high frequency content - as the frequency content of the signal goes up, the impedance of a capacitor goes down (provided you're not past its lowest impedance point), so more of the signal is let through to the input to cancel it out. This is useful if, for example, you want some low pass filtering on the signal coming in, as the higher frequency content on the input is reduced on the output because the feedback path is less impedance than just the feedback resistor.I woudln't expect this to cause ringing, though, so maybe there's something else going on? Are you actually seeing ringing on high frequency edges, or are you just seeing oscillation on the output? With the same consideration... what are your frequencies/waveforms like and how is your probing? If you're seeing inductive ringing on signals that are a few MHz, it could easily be that your probes are ringing and the circuit is fine. If it's actually ringing from the opamp, reducing the gain or increasing the value of the feedback capacitor (increasing high frequency rolloff) could help. If it's oscillations from the circuit, you can try to reduce capacitive loading of the output, or if the amp is driving a high impedance, you could add a load resistor on the output. Additionally, as a general guideline for ringing issues, decreasing trace/wire/lead length on the components used will help by reducing stray inductance.

author: dom0
author_rank: Super Contributor
timestamp: June 04, 2019, 12:13:28 pm
content: Do you use a ground lead with your oscilloscope probe or do you use a short ground connection close to the probe tip?The inductivity of ground leads often results in observing ringing where there is none.

author: David Hess
author_rank: Super Contributor
timestamp: June 04, 2019, 01:33:11 pm
content: Quote from: Jan Audio on June 03, 2019, 03:17:11 pmIt depends on the cause.QuoteNo, but this is also very application dependent.QuoteThe large and small signal responses of the operational amplifier are not the same.

author: exe
author_rank: Unknown
timestamp: June 04, 2019, 01:39:11 pm
content: I'd call it "to provide phase and gain margins for stability". Basically, an opamps adds a delay to the signal, at some frequency this becomes 180degree delay, so negative feedback turns into positive and it starts oscillate, unless the signal at this frequency is attenuated so the gain is below 1. So, there is a so-called "compensation network" (or just "compensation" for short) that ensures that signals that cause opamp oscillation are attenuated.Sorry if this all doesn't make sense, it's a quite a big topic, squezzing into a small paragraph hard. Google "opamp stability"

author: Jan Audio
author_rank: Frequent Contributor
timestamp: June 04, 2019, 01:51:06 pm
content: Quote from: dom0 on June 04, 2019, 12:13:28 pmNo, also without scope with a finished product in the studio all the sudden there is no sound, only volume on the mixer, after triggering a sound its gone.Quote from: exe on June 04, 2019, 01:39:11 pmOk, will try.Quote from: DaJMasta on June 03, 2019, 06:15:19 pmWith cap prevents ringing, without cap it rings in the megahertz about right i guess.It happens more often with the TL07X then the NE5532.

author: David Hess
author_rank: Super Contributor
timestamp: June 04, 2019, 02:57:47 pm
content: Quote from: Jan Audio on June 04, 2019, 01:51:06 pmCommon causes include:1. Poor power supply decoupling.2. Load capacitance combined with the open loop output resistance causes phase lag in the feedback loop.3. Capacitance from the inverting input to common/ground combined with the impedance at the inverting input causes phase lag in the feedback loop. This is a major problem at high impedances which requires a compensating capacitor across the feedback network.

author: exe
author_rank: Unknown
timestamp: June 04, 2019, 03:11:01 pm
content: Quote from: Jan Audio on June 04, 2019, 01:51:06 pmI don't think so. The former is a slower opamp that can tolerate even no decoupling and breadboard wiring. The NE5532 is a higher speed opamp, that oscillates without decoupling. The thing is, this may even not be noticeable, it manifests as excessive power consumption and bigger offset voltage. I experienced this myself, although didn't connect it to the oscilloscope, so I don't know how big is the impact on the output waveform.