Question

Sketch the static characteristics of a common emitter transistor and bring out the essential features of input and output characteristics.

Answer 1

Static characteristics of a NPN transistor in common emitter configuration

1. The circuit to study the static characteristics of NPN transistor is given in figure
2. Bias supply voltages V_BB and V_CC bias, base-emitter junction and collector-emitter junction.
3. Junction potentials are V_BE and Y_CE
4. R₁ and R₂ are used to vary base and collector currents respectively.

1. Input Characteristics:

1. Input Characteristics curve gives the relationship between I_B and V_BE at constant V_CE
2. For constant collector-emitter voltage V_CE, Base emitter voltage V_BE increases for corresponding Base current I_B which is recorded and graph is ploted.
3. The curve looks like forwarding characteristics.
4. Beyond knee voltage base current increases with an increase in base-emitter voltage for silicon 0.7 V and for Germanium 0.3 V.
5. Increase in V_CE decreases I_B. This shifts the curves outward.
6. Input resistance R_i = `((Δ"V"_"BE")/(Δ"I"_"B"))_("V"_"CE")`

2. Output characteristics:

1. The output characteristics give the relation between ∆I_C with respect to ∆V_CE at constant I_B
2. Initially base current I_B is set to a particular value. Increasing collector-emitter voltage V_CE corresponding collector current I_C increases. A graph is plotted.
3. Output resistance R_o = `((Δ"V"_"CE")/(Δ"I"_"C"))_"IB"`

The four important regions in the output characteristics are:

1. Saturation region: When V_CE increased above 0V, I_C increases rapidly almost independent of I_B called knee voltage Transistor operated above this knee voltage.
2. Cut-off region: A small I_C exists even after 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 the cut-off region.
3. Active region: In this region emitter-base junction is forward bias, collector-base junction is reverse bias. Transistors in this region can be used for voltage, current and power amplification.
4. breakdown region: If V_CE is increased beyond rated value, given I_c increases enormously leading to junction breakdown of the transistor. This avalanche breakdown can damage.