Switching circuit

In addition to being used as an AC signal amplifier, a triode can also be used as a switch. Strictly speaking, the action of a triode is not exactly the same as that of a conventional mechanical contact switch, but it has some characteristics that mechanical switches do not have. The characteristic of switching triodes in saturated conduction state is that both the emitter junction and the collector junction are in forward bias. The characteristic of a transistor in the amplified state is that the emitter junction is in a forward bias and the collector junction is in a reverse bias. This is also the principle that you can use a voltmeter to test the voltage value of the emitter junction and collector junction to determine the working condition of the triode. Switching triodes work precisely based on the switching characteristics of triodes.

The shape of a switching triode is the same as that of a normal triode. It operates in the cutoff and saturation regions, equivalent to cutting off and conducting a circuit. Due to its ability to complete the opening and closing of circuits, it is widely used in various switching circuits, such as commonly used switching power supply circuits, drive circuits, high-frequency oscillation circuits, analog-to-digital conversion circuits, pulse circuits, and output circuits. When the voltage applied to the emitter junction of the triode is greater than the conduction voltage of the PN junction, and when the current at the base increases to a certain extent, the collector current no longer increases with the increase of the base current, but does not change much near a certain value. At this time, the triode loses its current amplification effect, the voltage between the collector and emitter is very small, and the conduction state between the collector and emitter is equivalent to that of a switch, This is the conduction state of the triode. The load resistor is directly connected between the collector of the transistor and the power supply, and is located on the main current circuit of the transistor. The input voltage Vin controls the open and closed actions of the transistor switch. When the transistor is in the open state, the load current is blocked, and conversely, when the transistor is in the closed state, the current can flow.