Types of Configuration

Types of Configuration

A transistor can be connected in three configurations. They are:-

Common base configuration

Common emitter configuration

Common collector configuration
Common base configuration

This configuration is also known as grounded base configuration. The circuit diagram for determining the static characteristics of an NPN transistor in the common base configuration is given below.

Fig 4.5 Circuit used to determine the common base characteristics

Input characteristics:

In the active region, the input diode (emitter base) is biased in the forward direction. The input characteristics of fig. 4.6 represent simply the forward characteristics of an emitter - to- base diode for various collector voltages. To determine the input characteristics, the collector base voltage is V_CB is kept constant at zero in a suitable step by increasing V_EB. This is repeated for higher and fixed values of V_CB. A curve is drawn between emitter current I_E and emitter base voltage V_EB at constant collector base voltage V_CB. The input characteristic is shown in fig4.6. When V_CB is equal to zero and the emitter-base junction is forward biased as shown in characteristics, the junction behaves as a forward biased diode so that emitter current I_E increases rapidly with small increase in emitter base voltage V_EB. When V_CB is increased keeping V_EB a constant, the width of the base region will decrease. This effect results in an increase of I_E. Therefore, the curves shift towards the left as V_CB is increased.

Fig 4.6 Common Base Input Characteristics

Fig 4.7 Common Base Output Characteristics

Output characteristics

As in fig 4.7, it is customary to plot along the abscissa and to the right that polarity of V_CB which reverse biases the collector junction even if this polarity is negative. The collector-to-base diode is normally biased in reverse direction. If I_E = 0, the collector current I_C = I_CO. For the other values of I_E, the output diode reverse current is augmented by the fraction of the input-diode forward current which reaches the collector. Note that I_CO is negative for a p-n-p transistor and positive for an n-p-n transistor.

Active Region

In this region, the collector junction is reverse biased and the emitter junction is forward biased. Consider first that the emitter current is zero. Then the collector current is small and equals the reverse saturation current I_CO of the collector junction considered as a diode. Suppose now that a forward emitter current I_E is caused to flow in the emitter circuit. Then a fraction of -a I_E of this current will reach the collector, and I_E is therefore is given by I_E = I_B + I_C. In the active region, the collector current is essentially independent of collector voltage and depends on emitter current.

Saturation Region

The region to the left of the ordinate V_CB = 0, and above the I_E = 0 characteristics is called saturation region. The emitter and collector junctions are forward biased. We say that bottoming has taken place because the voltage has fallen near the bottom of the characteristics where V_CB ~ 0. Actually V_CB is slightly positive (for a p-n-p transistor) in this region, and the forward biasing in the collector accounts for the large change in collector current with small changes in collector voltage. For a forward bias, I_C increases exponentially with voltage according to the diode relationship. A forward bias means that the collector P material is made positive with respect to the base n side, and hence the hole current flows from the p side across the collector junction to the n material. This hole flow corresponds to a positive change in collector current. Hence the collector current increases rapidly and I_C may even become positive if the forward bias is sufficiently large.

Cut off Region

The characteristics for I_E = 0 passes through the origin, but is otherwise similar to the other characteristics. This characteristic is not coincident with the voltage axis though the separation is difficult to show because I_CO is only a few right of the I_E = 0 characteristics, for which the emitter and collector junctions are both reverse biased, is referred to as cut off region.

The Early Effect

As increase in magnitude of collector voltage increases the space charge width at the output junction diodes. Such action causes the effective base width w to decrease, the phenomenon known as 'Early effect'. The decrease in w has two consequences: first there is less chance for recombination with the base region. Hence the transport factor b and a increase with an increase in the magnitude of the collector junction voltage. Second, the charge gradient is increased within the base and consequently the current of minority carriers injected across the emitter junction increases.