A switch is a BJT that toggles between OFF and SATURATED modes. A common emitter switch for an actuator (such as a DC motor) accepts a voltage signal at an input port and regulates the actuator's output current. The input voltage must be higher than the dropout voltage of the switch, but lower than the saturated collector-emitter voltage of the switch. When the switch is off, no current will flow through it; when it is on, all of its terminals are connected together so current flows through it.
The common emitter acts as a control circuit for another device such as a motor or solenoid. For example, an electromagnetic switch has a common emitter configuration. The switch toggles between off and on states by altering the bias of a base-emitter junction. This biases either the base or the emitter terminal high when the other is low. Thus, the only connections across the battery are made when the switch is closed, which allows the battery to charge up while the system is on.
In radio applications, especially AM radios, a common emitter amplifier stages are used to amplify the signals before they are sent into the speaker or headphone socket. The amplifier prevents feedback from the speakers or headphones back into the microphone. It also protects the speakers or headphones from being overloaded if the input signal has too much power.
Modes of operation for BJT In the bipolar transistor junction, there are three modes: cut-off mode, saturated mode, and active mode. To operate the transistor in one of these zones, we must give DC voltage to npn or pnp transistors. Based on the polarity of the DC voltage, a transistor can function in either of these zones. The emitter is always more positive than the base in npn transistors, so they will never enter saturation mode. Also, since the collector is always more negative than the base in pnp transistors, they will never enter cutoff mode.
In npn transistors, if the DC voltage is applied to the base while the collector is connected to ground (0 V), then it will enter saturated mode because the base will be biased with 0 V while the collector is sinking current from the source. If the voltage at the base is greater than 1.5 V above the emitter threshold voltage (Vth) then it will stay in this state forever. On the other hand, if the collector voltage is higher than the emitter voltage by more than 0.7 V then the transistor will enter cutoff mode. This happens when there is no current flowing through the collector because it is grounded.
In pnp transistors, if the DC voltage is applied to the base while the collector is open circuit (no connection) then it will enter saturated mode because the base will be biased with 0 V while the collector is sinking current from the source.
A bipolar junction transistor (BJT) is a current-controlled device, which implies that a tiny amount of current passing through the base results in a big current flowing from emitter to collector. Let's have a look at the events that occur in an n-p-n transistor and a p-n-p transistor in the following sections.
In an n-p-n transistor, there are three layers: the n-type layer, the p-type layer, and the n-type layer. Current flows from the emitter, through the base, into the collector. This is true for both the ON state and the OFF state.
In a p-n-p transistor, there are also three layers: the p-type layer, the n-type layer, and the p-type layer. But here current flows from the collector to the emitter instead of from the emitter to the collector.
So far we have discussed how bipolar transistors work in general and not specifically how they work as components in circuits. When looking at actual circuit diagrams it becomes clear that all practical bipolar transistors work in a similar way. Whether they are used as diodes, switches, or amplifiers depends on where they are placed in the circuit diagram.
In general, two types of bipolar transistors are used in logic circuits: NPN transistors and PNP transistors.