With the help of labelled circuit diagram, explainthe working of transistor as a switch.

Explanation

The operation mode of a transistor as a switch depends upon operating conditions of transistor. A transistor acts as switch in cut off and saturation region. In cut off region when input voltage is very low, then voltage across output is very high and in this state transistor does not conduct and it acts as switch in cut off state and when input voltage is very high, then output voltage is very small, so it behaves as switch in on state. In this way transistor acts as switch. For the working of transistor as switch, input voltage is so chosen such that transistor never operates in active region of operation. As a switch, a transistor operates in the shaded parts of above Figure and changes over rapidly from the .off. state in which `I_(C )= 0` to the .on. state in which `I_(c)` is maximum (saturation).
Working Applying Kirchhoff.s rule to the input and output circuits, we have
`V_(B B)= I_(B) R_(B) + V_(BE)`
and `V_(CE )= V_(C C)- I_(C )R_(C )`
We shall take `V_(B B)` as d.c input voltage `V_(1) + V_(CE)` as the d.c output voltage `V_(0)`. So
`V_(i) = I_(B) R_(B) + V_(BE)`
`V_(0)= V_(C C)- I_(C )R_(1)`
If we study the variation of `V_(i)` versus `V_(0)` the graph obtained is shown in figure. The graph between `V_(0) and V_(i)` is known as the transfer characteristics of base biased transistor. The portion of graph CD is known as cut off region, De known as active region and portion beyond E is known as saturation region.
(i) When `V_(i)` is less than A, the transistor is in cut off region and `I_(C )` will be zero. Hence `V_(o)= V_(C C)`
(ii) As `V_(i)` becomes greater than A, the current `I_(C )` starts increasing and `V_(o)` decreases as the term `I_(C )R_(1)` increases.
(iii) WIth further increase in `V_(i), I_(C )` increases linearly, So `V_(0)` decreases linearly.
(iv) Further increase in `V_(i), V_(o)` decreases and never becomes zero.

From graph it is observed that transition from cut off state to active state and from active state to saturation state are not sharply defined.