What can cause parasitic oscillations in a stage?

For any electronic circuit to oscillate, it requires positive feedback. This means a portion of the amplifier's output signal is fed back into its input in phase with the original signal, causing it to self-sustain and amplify repeatedly. When this happens unintentionally in an amplifier stage—often due to stray capacitance between components or poor wiring layout—it causes parasitic oscillations. Students often guess 'unwanted negative feedback,' but negative feedback actually decreases an amplifier's gain and stabilizes it. Therefore, negative feedback suppresses oscillations, whereas positive feedback creates them.