What is the voltage when the transistor is turned off?

Definition of transistor cutoff state
The cut-off state of a transistor, in short, is the state in which the transistor is almost non-conductive under specific conditions. This state usually occurs when the base current is zero or extremely small, resulting in almost no current flowing between the collector and emitter. In BJTs (bipolar transistors), this usually means that both the emitter and collector junctions are in a reverse bias state, while in FETs (field-effect transistors), it is manifested as a channel being pinched off or no channel at all.
The concept of cut-off voltage
The voltage at which a transistor is turned off usually refers to the minimum voltage condition required to bring the transistor into the off state. This voltage value may vary for different types of transistors and different operating conditions. For BJTs, the cut-off voltage is mainly related to the voltage between the emitter and base (VBE). When VBE is less than or equal to a specific value (i.e. cut-off voltage VCEsat), the transistor will enter the cut-off state. For FET, especially MOSFET, the cut-off voltage is more related to the voltage between the gate and source (Vgs). When Vgs is less than the threshold voltage (Vt), the transistor is in the cut-off state.
Calculation and influencing factors of cut-off voltage
For BJTs, the calculation formula for the cut-off voltage VCEsat is usually VCEsat=IC/β+VBE, where IC is the collector current, β is the multiplication factor of the transistor, and VBE is the voltage between the emitter and base. However, this formula is mainly used for theoretical analysis and design calculations. In practice, the cut-off voltage is also affected by many other factors, such as the manufacturing process of transistors, temperature, and other components in the circuit.
For FET, especially MOSFET, its cut-off voltage is mainly determined by the threshold voltage Vt. The threshold voltage is the minimum gate voltage required to initiate conduction in the channel of a MOSFET. When Vgs is less than Vt, the channel is pinched off or non-existent, and the transistor is in an off state. The magnitude of the threshold voltage depends on factors such as the manufacturing process, doping concentration, channel length, and thickness of the gate oxide layer of the MOSFET.
The Application of Cut off Voltage in Circuit Design
Understanding and applying the cutoff voltage of transistors is crucial in circuit design. By controlling the base or gate voltage of a transistor, the switching state of the transistor can be precisely controlled, thereby achieving the logic function and signal processing of the circuit. For example, in digital circuits, transistors are often used as switching elements to achieve signal transmission and processing by controlling their off and on states.
In addition, in analog circuits, the cut-off voltage of transistors also plays an important role. By adjusting the cut-off voltage of the transistor, the operating point and gain of the circuit can be changed, thereby optimizing the performance of the circuit. For example, in an amplifier circuit, setting the cutoff voltage of the transistor reasonably can ensure that the circuit maintains a high gain when the input signal is small, while avoiding saturation distortion when the input signal is large.
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