Question

Transistor functions as a switch. Explain.

Answer 1

In saturation and cut-off region transistor works as an electronic switch that helps to turn ON or OFF a circuit.

                           Transistor as a switch

presence of dc source at the input (saturation region):

1. When high input voltage (V_in = + 5V) is applied, I_B increases and in turn increases I_C.
2. Transistor will move into saturation region (turned ON)
3. Increase in I_C increases the voltage drop across R_c lowering the output voltage close to zero.
4. Transistor acts like a closed switch (ON condition)

Absence of dc source at the input (cut-off region):

1. At low input voltage (V_in = 0V), decreases the I_B and in turn decreases I_C.
2. Transistor moves to cut off region (turned ON)
3. Decrease in I_C decreases voltage drop across R_c, increasing output voltage + 5V transistor acts as an open switch (OFF condition)
4. It is manifested that high input gives low output and low input gives high output.