The triode has three working states: cut-off state, amplification state and saturation state. The working state of the triode used for different purposes is different.

1. Transistor cut-off working state

The triode used to amplify the signal should not work in the cut-off state. If the input signal part enters the cut-off region of the triode characteristic, the output will produce nonlinear distortion.

Non-linear distortion means that a sinusoidal signal is input to the triode, and the output signal from the triode is no longer a complete sinusoidal signal, and the output signal is different from the input signal.

If the negative half cycle of the input signal on the base of the triode enters the cutoff region of the triode, it will cause clipping distortion. Note that the negative half cycle of the signal at the base of the triode corresponds to the positive half cycle of the signal at the collector of the triode, so the positive half cycle of the input signal at the collector of the triode is removed by the cutoff region of the triode.

When the triode is used in a switching circuit, one of the working states of the triode is the cut-off state. Note that the triode in the switching circuit works in the switching state, so there is no such clipping distortion.

2. Transistor amplification working state

In the linear state, if a sinusoidal signal is input to the triode, the output is also a sinusoidal signal. At this time, the amplitude of the output signal is larger than the input signal, indicating that the triode has amplified the input signal, but the characteristics of the sinusoidal signal have not changed. , so there is no nonlinear distortion. The amplitude of the output signal becomes larger, which is also a kind of distortion, which is called linear distortion. This kind of linear distortion is needed in the amplifier. Without this linear distortion, the amplifier will not have the ability to amplify.

In order to make the triode enter the amplification area, whether it is an NPN transistor or a PNP transistor, a suitable DC voltage must be given to each electrode of the triode. In summary, there are two conditions: the collector junction is reverse-biased, and the emitter junction is forward-biased.

3. Transistor saturation working state.

On the basis of amplifying the working state of the triode, if the base current increases a lot further, the triode will enter a saturated state. At this time, the triode current amplification factor β will drop a lot. The deeper the saturation, the smaller the value, and the current amplification factor can always be small. To a degree less than 1, the triode has no amplification capability at this time.

When the triode is in a saturated state, the signal input to the triode will enter the saturation region, which is also a nonlinear region. The signal distortion caused by the triode entering the saturation region is different from the signal distortion in the cut-off region. The positive half cycle of the signal added to the base of the triode enters the saturation region, and the negative half cycle is cut off in the output signal of the collector, so the signal is amplified When the triode can not enter the saturation region.

In the switching circuit, another working state of the triode is the saturation state. Since the triode switching circuit does not amplify the signal, there is no such distortion.

Among the three working states of the triode, the working current of the triode has a certain range. Among them, the current range of the cut-off area is the smallest, the range of the amplification area is the largest, and the saturation area is second. Of course, the adjustment of the external circuit can also change the current range of each working area. .

In the three working states of the triode, the magnification β is also different. The β in the cut-off region and the saturation region is very small, and the β in the amplification region is large and basically unchanged.