Question

How does a transistor act as a switch?

Answer 1

Transistor as a switch

1. A three-terminal electronic component is a transistor. It has three terminals: an emitter terminal, a collector terminal, and a control ("gate") terminal. By using the control terminal, the current can be stopped in one direction through the emitter and into the collector. A positive voltage provided to the control terminal of the NPN transistor causes it to be switched on or off.
2. When a negative voltage is supplied to the control terminal of a PNP transistor, the transistor is either $ON$ or $OFF$ ($ON$ when the base-emitter junction has a forward bias $OFF$when it has a reverse bias) in the circuit. Four transistors are used in this. By employing the collector terminal as an output, the on/off state of the circuit can be managed. In the $ON$ state, for example, the circuit output is controlled in the high level ($ON$ state), and in the $OFF$state, the circuit output is controlled in the low level ($OFF$ state).

Terms in Diagram: $V_{CC}$ refers to Common Collector Voltage, $R_C$refers to Resistor Capacitor, $V_o$refers to Voltage Out, $R_b$refers to the Resistance of the Base Region,$V_{in}$ refers to Voltage In.

Cut Off State (Open Switch):

When the transistor operates in the cut-off region shows the following characteristics:

1. The input is grounded i.e. at zero potential.
2. The $V_{BE}$ is less that cut-in voltage $0.7V$ .
3. Both emitter-base junction and collector-base junction are reverse biased.
4. The transistor is full-an off acting as open switch.
5. The collector current $I_C=0A$ and output voltage $V_{out}=V_{CC}$.

Saturation State (Closed Switch):
The transistor operating in the saturation region exhibits the following characteristics −

1. The input is connected to $V_{CC}$.
2. Base-Emitter voltage is greater than cut-in voltage, $0.7V$.
3. Both the base-emitter junction and base-collector junction are forward biased.
4. The transistor is fully-ON and operates as a closed switch.
5. The collector current is maximum, $I_C=\frac{V_{CC}}{R_L}$ and $V_{out}=0 V$.