(a) Characteristic Curves: The circuit diagram for determining the static characteristic curves of an n-p-n transistor in common-emitter configuration is shown in figure.
Output characteristics: These characteristics are obtained by plotting collector current IC versus collector-emitter voltage VCE at a fixed value of base current IB. The base current is changed to some other fixed value and the observations of IC versus VCE are repeated. Fig. represents the output characteristics of a common-emitter circuit.
The characteristic curves show:
(i) When collector-emitter voltage VCE is increased from zero, the collector current IC increases as VCE increases from 0 to 1V only and then the collector current becomes almost constant and independent of VCE. The value of VCE upto which collector current IC changes is called the knee voltage Vknee.
We take the active region of output characteristics i, the region where collector current (IC) is almost independent of VCE.
Now we choose any two characteristic curves for given values of IB and find the two corresponding values of IC.
Using any two curves from output characteristics current amplification factor bac βac = ΔIC/ΔIB.
(b) A switch is a device which can turn ON and OFF current is an electrical circuit. A transistor can be used to turn current ON or OFF rapidly in electrical circuits.
Operation: The circuit diagram of n-p-n transistor in CE configuration working as a switch is shown in fig. VBB and VCC are two dc supplies which bias base-emitter and emitter collecter junctions respectively.
Let VBB be the input supply voltage. This is also input dc voltage (VC). The dc output voltage is taken across collector-emitter terminals, RL is the load resistance in output circuit.
Applying Kirchhoff’s second law to input and output meshes (1) and (2), we get
Let us see the change in V0 due to a change in Vi. In case of Si transistor; the barrier voltage across base-emitter junction is 0.6V. Therefore, when Vi is less than 0.6V, there is no collector current (IC = 0), so transistor will be in cut off state. Hence, from (iv) with IC = 0;V0 =VCC.
When Vi becomes greater than 0.6V, IC begins to flow and increase with increase of Vi. Thus, from (iv), V0 decreases upto Vi =1V; the increase in IC is linear and so decrease in output voltage V0 is linear. Beyond Vi =1 V, the change in collector current and hence in output voltage V0 is non-linear and the transistor goes into saturation. With further increase in Vi, the output voltage further decrease towards zero (though it never becomes zero). If we plot V0 versus Vi, we get the graph as shown in fig. [This characteristics curve is also called transfer characteristic curve of base biased transistor.] The curve shows that there are non-linear regions.
(i) between cut off state and active state and (ii) between active state and saturation state; thus showing that the transitions (i) from cut off to active state and from active to saturation state are not sharply defined.
Now we are in the position to explain the action of transistor as a switch. When transistor is non-conducting (IC = 0), it is said to be ‘switched off’ but when it is conducting (IC is not zero); it is said to be ‘switched ON’.
As long as input voltage Vi is low and unable to overcome the barrier voltage of the emitter base junction, V0 is high (IC = 0 and V0 =VCC), so the transistor is ‘switched OFF’ and if it is high enough to derive the transistor into saturation (IC is high and so V0 (=VCC - IC RL ) is low, very near to zero, so the transistor is ‘switched ON’. Thus we can say low input switches the transistor is OFF state and high input switches it ON.
The switching circuits are designed in such a way that the transistor does not remain in active state.
The transistor can operate as a switch in cut off region and saturation region.
