Why do OPERATIONAL AMPLIFIERS go haywire? Explaining the principles of oscillation! | NEXTY Electronics, an electronic components and semiconductor trading company

What is OPERATIONAL AMPLIFIERS oscillation?

OPERATIONAL AMPLIFIERS use negative feedback, which feeds the output back to the input, to achieve functions such as amplification and FILTERS.
This feedback path generates a persistent oscillating signal in the output signal.
This state is called "oscillation."

Negative feedback block diagram and basic operation

The figure below is a block diagram showing negative feedback in OPERATIONAL AMPLIFIERS.
If Ao is OPERATIONAL AMPLIFIERS, then in the frequency band where Ao is sufficiently larger than β, the output V _O can be approximated to 1/β times the input V _I.

Conditions for Oscillation: Relationship between the Open-Loop Transfer Function and Gain/Phase

Ao×β is called the "loop transfer function."
Oscillation occurs when the gain is 1 or more at the frequency Ftt where the phase delay of the open-loop transfer function Ao×β is 180°.
For example, consider the case where the gain of the open-loop transfer function is doubled at the frequency Ftt where the phase delay is 180°.
Suppose you change the value of VI stepwise, and the value of Err at frequency Ftt becomes 0.1V@Ftt.
The value fed back through the feedback circuit is 0.2V@Ftt.
This fed back value is larger than the original 0.1V@Ftt, and each time it passes through the feedback circuit it becomes larger than the original value, never converging.

In reality, the value that OPERATIONAL AMPLIFIERS can output is limited by factors such as POWER SUPPLIES voltage, so when the limit is reached, Ao×β becomes less than 1 and the output of OPERATIONAL AMPLIFIERS decreases.
However, due to the decrease in the output of OPERATIONAL AMPLIFIERS, Ao×β becomes 1 or more again.
This phenomenon is repeated, resulting in sustained vibration or oscillation.

"Phase margin" is an index showing the stability of oscillation

We have explained that the condition for oscillation is "a gain of 1 or more at the frequency Ftt where the phase delay of the open-loop transfer function Ao × β is 180°," but we will now introduce the "phase margin," an index that indicates the level of stability that makes it less likely to reach oscillation.

The phase margin refers to the phase difference from -180° when the gain of the open-loop transfer function is 1 (0 dB).
The amount of phase margin required for a system to be considered stable varies depending on the variations and fluctuations of various factors that affect the system characteristics, but generally a phase margin of 45° or more is considered stable.

Example of phase margin of non-inverting OPERATIONAL AMPLIFIERS

The figure below is an example of the open-loop transfer function of a non-inverting OPERATIONAL AMPLIFIERS amplifier.

Comparison of characteristics between solid and dashed lines

Solid line characteristics: Phase margin 53°
Dashed line characteristics: Phase margin 0°

Causes of oscillation caused by capacitance and output resistance

The characteristics indicated by the solid line have a phase margin of 45° or more and can be said to be stable characteristics, but the characteristics indicated by the dashed line show that due to the electrostatic capacitance CL and output resistance RO connected to the output of OPERATIONAL AMPLIFIERS, the phase lag increases, the phase margin becomes 0°, and oscillation occurs.