Static Characteristics of Transistor in Common Emitter Mode

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 9.30. The bias supply voltages V_BB and V_CC bias the base-emitter junction and collector- emitter junction respectively. The junction potential at the base-emitter is represented as V_BE and the collector-emitter as V_CE. The rheostats R₁and R₂ 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 9.31.

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 p-n 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 the base current. This shifts the curve 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 impedance

The ratio of the change in base-emitter voltage (∆V_BE ) to the change in base current (∆I_B ) at a constant collector-emitter voltage (V_CE) is called the input impedance (r_i). The input impedance is not linear in the lower region of the curve.

The input impedance is high for a transistor in common emitter configuration.

2. Output Characteristics

The output characteristics give the relationship between the variation in the collector current (∆ I_C ) with respect to the variation in collector-emitter voltage (∆V_CE ) at constant input current (I_B ) as shown in Figure 9.32.

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 important regions in the output characteristics are:

(i) Saturation region

When V_CE is increased above 0 V, the L_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 (V_CE ) is increased beyond the rated value given by the manufacturer, the collector current (I_C ) increases enormously leading to the junction breakdown of the transistor. This avalanche breakdown can damage the transistor.

Output impedance

The ratio of the change in the collector-emitter voltage (∆V_CE ) to the corresponding change in the collector current (∆ I_C ) at constant base current (I_B) is called output impedance (r_O).

The output impedance for transistor in common emitter configuration is very low.

3. Current transfer characteristics

This gives the variation of collector current (I_C) with changes in base current (I_B ) at constant collector-emitter voltage (V_CE ) as shown in Figure 9.33. It is seen that a small I_C flows even when I_B is zero. This current is called the common emitter leakage current (I_CEO ), which is due to the flow of minority charge carriers.

Forward current gain

The ratio of the change in collector current (∆I_C ) to the change in base current (∆I_B ) at constant collector-emitter voltage (V_CE) is called forward current gain (β).

Its value is very high and it generally ranges from 50 to 200. It depends on the construction of the transistor and will be provided by the manufacturer. There are transistors with β as high as 1000 as well.

Relation between α and β

There is a relation between current gain in the common base configuration α and current gain in the common emitter configuration β which is given below.

α = β / 1+ β

(or) β = α / 1 - α

EXAMPLE 9. 6

The output characteristics of a transistor connected in common emitter mode is shown in the figure. Determine the value of I_C when V_CE = 15 V. Also determine the value of I_C when V_CE is changed to 10 V.

When V_CE = 15 V, I_C = 1.5 μA

When V_CE is changed to 10 V,  I_C = 1.4 μA

The collector current is independent of the collector- emitter voltage in the active region.

EXAMPLE 9.7

In the circuit shown in the figure, the input voltage V_i is 20 V, V_BE = 0 V and V_CE = 0 V. What are the values of I_B , I_C , β?