What is the difference between an opamp becoming unstable and just oscillating?

An application note "AAN-2 Driving Capacative Loads" states that as a bigger capacative load is driven by an opamp, the frequency response changes such that the output oscillates and increasing it beyond a certain point shall cause the output to become unstable. It does not give scope output.

I want to understand for a square wave and a sine wave, how would the appearnce of the output on a scope change as the capacative load is increased until (including) the opamp becomes "unstable"? Could you describe it in words?

Hi,

I don´t know the exact definitions.

Generally there are "wanted" oscillations and "unwanted" oscillations.
For sure a sine wave oscillator is oscillation, but not considered to be unstable. (some may say this is a controlled unstable condition)

If an OPAMP input is a sinewave and it´s output is a sinewave, too, then it´s hard to say if it is stable or not.

If the opamp input is a square wave, then it is more "visible".
* on a stable OPAMP you may see no, or only a little overshoot at the output, but then soon becoming flat.
* a less stable OPAMP showing ringing at the output. The longer in time the ringing is, the closer you are to an unstable condition.
* a non stable OPAMP either shows heavy ringing (not becoming quiet) or a continous sine oscillation, or a continous square wave (clipped sine) oscillation at the output.

*****
There are many different cases....
Once I built a low power OPAMP circuit as a frontend for a SAR ADC.
The input to the OPAMP was very low frequency, about DC.
But the ADC builds a switched, capacitive load to the OPAMP output. Every time the ADC´s internal S/H circuit switched there was ringing at the OPAMP´s output. Causing a lot of error at the ADC readings.

Klaus

In general, there are two "forms of instability":
(a) Static instability: Instability of wanted DC conditions (operational point). In this case, we have a saturation effect (the amplifiers output voltage is at its upper limit, mostly supply voltage).
(b) Dynamic instability: The oscillation condition (unity loop gain) is fulfilled for one single frequency and the circuit oscillates at this frequency - either wanted (oscillator) or not (parasitic oscillations) .

When an opamp output is not clipping then as it becomes more and more unstable by a capacitive load then the edges of a square wave show ringing (damped oscillation).
A sinewave will fade with a damped oscillation when its input is abruptly stopped.

Many audio opamps become unstable when a normal shielded audio cable is connected to its output. Simply adding a series 100 ohm resistor isolates the opamp from the cable capacitance but allows audio to pass.