Transistor functions as a switch. Explain.

Explanation

The transistor in saturation and cut-off regions functions like an electronic switch that helps to turn ON or OFF a given circuit by a small control signal.

Presence of dc source at the input (saturation region):
(i) When a high input voltage `(V_("in")=+5)` is applied, the base current `(I_(B))` increase and in turn increase the collector current.
(ii) The transistor will move into the saturation region (turned ON).
(iii) The increase in collector current `(I_(C))` increase the voltage drop across `R_(C)` thereby lowering the output voltage, close to zero.
(iv) The transistor acts like a closed switch and is equivalent to ON condition.
Absence of dc source at the input (cut-off region):
(i) A low input voltage `(V_("in")=OV)` decreases the base current .. and in turn decreases the collector current `(I_(C)).`
(ii) The transistor will move into the cut-off region (turned OFF).
(iii) The decrease in collector current `(I_(C))` decreases the drop across `R_(C)` thereby increasing the output voltage, close to +5 V.
(iv) The transistor acts as an open switch which is considered as the OFF condition.
(v) A high input gives a low output and a low input gives a high output.
(vi) In addition, we can say that the output voltage is opposite to the applied input voltage.
(vii) Therefore, a transition can be used as an inverter in computer logic circuitry.