What occurs when the base becomes so negative with respect to the emitter that changes in the signal are not reflected in collector-current flow?

When the base of a bipolar junction transistor (BJT) becomes significantly negative relative to the emitter, the transistor enters a state known as cut-off. In this condition, the base-emitter junction becomes reverse-biased, preventing current from flowing into the base from the emitter. As a result, the transistor does not conduct; therefore, there is no collector current in response to changes in the signal. This effectively means that the transistor is turned off, and it behaves as an open switch for the purposes of signal amplification.

In the cut-off region, even if the input signal attempts to produce changes in current, these changes will not affect the collector current because the required base-emitter junction conditions for conductivity are not met. Understanding this behavior is crucial for designing circuits that utilize transistors for switching and amplification in various applications.

The active region refers to the operational mode where the transistor can amplify signals; saturation refers to the state of maximum current flow through the transistor, indicating it is fully on. Mode-switching would suggest transitioning between states but does not directly address the scenario presented in the question.