Saved Bookmarks
| 1. |
Sketch the static characteristics of a common emitter transistor and bring out the essence of input and output characteristics. |
|
Answer» Solution :Static Characteristics of Transistor in Common Emitter Mode: The know-how of certain parameters like the input resistance, output resistance, and current gain of a transistor are very important for the effective use of transistors in circuits. The circuit to study the static characteristics of an NPN transistor in the common emitter mode is given in figure. The bias supply voltages `V_(BB)` and `V_(CC)` bias the base-emitter junction and collectoremitter junction respectively. The junction potential at the base-emitter is represented as VBE and the collector-emitter as `V_(CE)`: The rheostats `R_(1)` and `R_(2)` are used to vary the base and collector currents respectively. The static characteristics of the BJT are 1. Input characteristics 2. Output characteristics 3. Transfer characteristics  1. Input Characteristics: Input Characteristics curves give the relationship between the base current `(I_(B))` and base to emitter voltage `(V_(BE))` at constant collector to emitter voltage `(V_(CE))` and are shown in figure. Initially, the collector to emitter voltage `(V_CE))` is set to a particular voltage (above 0.7 V to reverse bias the junction). Then the base-emitter voltage `(V_(BE))` is increased in suitable steps and the corresponding base-current `(I_(B))` is recorded. A graph is plotted with `V_(BE)` along the x-axis and `I_(B)` along the y-axis. The procedure is repeated for different values of `V_(CE)`. The following observations are made from the graph: • The curve looks like the forward characteristics of an ordinary pun junction diode. • There exists a threshold voltage or knee voltage `(V_(K))` below which the base current is very small. The value is 0.7 V for Silicon and 0.3 V for Germanium transistors. Beyond the knee voltage, the base current increases with the increase in base-emitter voltage. • It is also noted that the increase in the collector-emitter voltage decreases a NPN transistor in common emitter the base current. This shifts the curve configuration outward. This is because the increase in collector-emitter voltage increases the width of the depletion region in turn, reduces the effective base width and thereby the base current.  Input resistance: The ratio of the change in base-emitter voltage `(DeltaV_(BE))` to the change in base current `(Delta_(IR))` at a constant collector-emitter voltage `(V_(CE))` is called the input resistance `(R_(i))`.The input resistance is not LINEAR in the lower region of the curve. `R_(i)=((DeltaV_(BE))/(DeltaI_(B)))_(V_(CB))` The input resistance is high for a transistor in common emitter configuration. Output Characteristics: The output characteristics give the relationship between the variation in the collector current `(DEltaI_(C))` with respect to the variation in collector-emitter voltage `(DeltaV_(CE))` at constant input current `(I_(B))` as shown in figure.Initially, the base current `(I_(B))` is set to a particular value. Then collector emitter voltage `(V_(CE))` is increased in suitable steps and the corresponding collector current `(I_(C))` is recorded. A graph is plotted with the `V_(CE)` along the x-axis and `I_(C)` along the y-axis. This procedure is repeated for different values of `I_(B)`.The four importantregions in the output characteristics configuration are:  (i) Saturation region: When `V_(CF)` is increased above 0 V, the `I_(C)` increases rapidly to a saturation value almost independent of `I_(B)` (Ohmic region, OA) called knee voltage. Transistors are always operated above this knee voltage. (ii) Cut-off region: A small collector current `(I_(C))` exists even after the base current `(I_(B))` is reduced to zero. This current is due to the presence of minority CARRIERS across the collector-base junction and the surface leakage current `(I_(CEO))`.This region is called as the cut-off region, because the main collector current is cut-off. (iii) Active region: In this region, the emitter-base junction is forward biased and the collector base junction is reverse biased. The transistor in this region can be used for voltage, current and power amplification. (iv) Breakdown region: If the collector-emitter voltage (VCP) is increased beyond the rated value given by manufacturer, the collector current (1) increases enormously leading to the jung breakdown of the transistor. This avalanche breakdown can damage the transistor. Output Resistance: The ratio of the change in the collector-emitter voltage `(DeltaV_(CE))`to the corresponding change in the collector current `(DeltaI_(C))` at constant base current `(I_(B))` is called output resistance `(R_(O))`. `R_(0)=((DeltaV_(CE))/(DelatI_(C)))_(I_(B))` The output resistance for transistor in common emitter configuration is very low. |
|