Use of Transistor as switch

Use of Transistor as a switch

A transistor is a device that regulates current or voltage flow and acts as a switch or gate for electronic signals. It is composed of semiconductor material usually with at least three terminals for connection to an external circuit. It can act as a switch and an amplifier. It converts audio waves into electronic waves and resistor, controlling electronic current.

A transistor can be used as a switch. Transistor work as a switch when it is in cutoff and saturation regions. In this article, we will discuss how to use a transistor as a switch. In fig. (a) a ‘n-p-n’ transistor has been shown. It works as a switching circuit.

Working principle: In the figure, V_i = input voltage, V₀ = output voltage = V_CE =  potential difference between collector and emitter.

(1) When V_i = 0, then in the base-emitter junction there is no voltage i.e.. V_BE = 0. So, base current I_B = 0. Since I_B = 0, so I_C = 0. From figure (a) we get,

V₀ = V_S – I_CR_L.

Now since, I_C = 0, when V_i = 0

So, V₀ = V_S, V_S = supply voltage.

When V_i is increased slowly and until V_i > V₁ i.e., V_i ≤ V₁, then base current I_B increases very little, I_C also increases slightly. In this situations the transistor is said to be cut-off, then I_C = 0.

Now if V_i increases and becomes V₂ then collector current increases significantly and at that condition, the potential drop on R_L is almost equal to the supply voltage V_S. According to an equation, then V_O = 0. If the input voltage is greater than V₂, then, there does not occur a significant change in I_C, so no influence on output voltage V₀ is found. In this situation, the transistor is said to be saturated.

So, it is seen that by changing input voltage the transistor can be changed into two states. One is cut off state (V₀ + V_S) and the other one is the saturated state (V₀ = 0). The off and on a state of the transistor is used in a digital computer. A transistor can change state many hundred thousand times in a second. The output of a switch can be used as the input of the other switch and by connecting many switches together mathematical counting can be executed very rapidly.