Opamp - strange behavior

Opamp - strange behaviour

Hi guys,

I am simulating an opamp (connected as a buffer)and I have put a square wave at the input and I am looking at the output. There is two things that are happening:

1. At the negative edge the output has this kind of small spike at the middle of the negative edge.
2. When it reaches zero, it has a small ringing.

Does oanyone has an idea of what might be happening?

Regards.

Re: Opamp - strange behaviour

Hi,

why don´t you show us your configuration (scematic, waveforms, voltages...) and the simulation results?

Klaus

Re: Opamp - strange behaviour

As KlausST says - we need specifics about the circuit, and the op amp part (assuming you have a model of a commercial part), and picture of your simulation.
Some op amps will exhibit glitches on their outputs when they are requested to exceed their slew rate limit (but this depends on various internal factors of the particular amp).
Some op amps can exhibit a glitch during a transitional range when the output is switching to the negative rail input.
The models of op amps may or may not accurately depict the behavior of their physical counterparts, depending upon the source of the model.

Re: Opamp - strange behaviour

CAMALEAO said:

Hi guys,

I am simulating an opamp (connected as a buffer)and I have put a square wave at the input and I am looking at the output. There is two things that are happening:

1. At the negative edge the output has this kind of small spike at the middle of the negative edge.
2. When it reaches zero, it has a small ringing.

Does oanyone has an idea of what might be happening?

Regards.

Click to expand...

These kind of things generally come from non-proper models and/or approximate macro-models.Simulation setup can also have some problems such as integration method,convolution type etc.
If you post your simulation setup, we can say something more concrete.

Re: Opamp - strange behaviour

I betcha the opamp is a lousy old LM324 quad or LM358 dual with a poor high frequency response, very poor slew rate and lots of crossover distortion.

Re: Opamp - strange behaviour

Hi everyone,

Sorry for not posting the picture representing what I am talking about. I didn't have it with me at that time. Please find below:



The circuit is a basic 2-stage opamp with miller compensation.

That's what I am talking about. Concerning that small spike during the negative edge, I guess it has to do with the compensation capacitor? Regarding the ringing on the ground, don't know really. Maybe it has to do with the strengh of the nmos at the output.

@Audioguru, that's the problem that I find in posting threads in here. Each time I post at the right section, for me it's the right section (Analog Integrated Circuit (IC) Design, Layout and Fabrication) but erikl keeps moving my posts to this section.

Anyway, now you have the picture showing what I was referring to.

Re: Opamp - strange behaviour

It sounds like you do not have an opamp or a circuit to test. Instead it is just a guess by a SIM program. The "squarewave" looks like its high frequency is straining the opamp.

Re: Opamp - strange behaviour

High speed/high slew rates with a primarily capacitive load can certainly cause this sort of transient. I have often seen it in high power amplifiers driving piezo-based inkjet printheads, though usually the glitch is at or very near the top corner of the falling edge. This relates to the initial capacitive storage of the load and the reversal of the current through the load and coming back under control of the amplifier output. The delay from the top corner can be related to series impedance between the amp and the capacitive load.

I don't know what your simulation looks like, and this could be due to numerous model problems. But I have seen something similar in real world applications in industrial inkjet printing systems.

Re: Opamp - strange behaviour

Does your output become high impedance at the middle of the downward-going transition? With hardware circuits I've observed that such moments create an opportunity for oscillations to occur briefly (or for some other neighboring interference to take hold).

It does not occur while the output stage is conducting. So my explanation may not necessarily apply to the ringing at the trough of your waveform.

Re: Opamp - strange behaviour

Hi,

Post the schematic.

To me it looks like class B crossover distortion.

Maybe due to your circuit only in one direction.

Klaus