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Relation between feedback delay and oscillation of OpAmp

Electrical Engineering Asked by emnha on December 29, 2021

This is from TI Precision Labs – Op Amps: Stability – Introduction presentation here. He is trying to explain how delay can cause oscillation intuitively from time domain.

This is an exceprt from the presentation:

Too much delay between the amplifier output in the inverting feedback
node is a straightforward way to visualize the root cause of op amp
stability issues. The effects of feedback delay are displayed in this
example by observing the voltages at the amplifier output, Vopa, and
the amplifier inverting input, Vfb, when a step is applied to the
input, Vin. When Vin changes, Vopa responds in an effort to set Vfb
equal to Vin to reestablish the virtual short between the inputs.

However, the feedback delay results in an erroneous voltage at Vopa by
the time Vfb equals Vin at 0 volts
. As a result, Vfb continues to rise
and overshoots the input voltage, causing Vopa to reverse directions.
Depending on the severity of the delay, the output may settle, or may
result in a sustained oscillation as shown here.

Can anyone explain the bold part? How the feedback delay causes erroneous voltage at Vopa at that time?
What Vopa is supposed to be if the circuit is working correctly?

In this circuit how can you draw the waveform of Vopa or Vfb?
Because they’re inter-related so it’s hard to make drawing for oscillation case like that.

enter image description here

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