Electrical Engineering ⇒ Topic : Output Characteristics

Output Characteristics

The curve between the collector current (I_C) and collector-base voltage (V_CB) at constant emitter current (I_E) gives the output characteristic. The collector current is taken along the Y-axis and the collector-base voltage magnitude is taken along the X-axis. Figure (a) shows the output characteristics of a typical transistor in common base configuration.

                                        Figure (a) CB output characteristics.

The following important conclusions can be drawn from the output characteristics:

• The output characteristics have three basic regions: Active, Cut-off and Saturation.In the active region, the emitter-base junction is forward-biased and the collector-base junction is reverse biased. In this region, the collector current is approximately equal to the emitter current.
• In the active region, the collector current is almost constant. This gives a very dynamic output resistance.
• The collector current is I_CBO when the emitter current is zero. This current is very small and can be neglected. Therefore, I_C = 0 and hence I_E = O. The region below I_E = 0 is known as cut-off region.
• The region of the characteristics which lies to the left of V_CB = 0 is known as saturation region. In this region, both the emitter-base junction and the collector-base junction are forward biased.
• As I_E increases, I_C also increases. Therefore, I_C depends on I_E and not on collector voltage.
• Thus input current controls the output current because transistor requires some current to drive it. Hence, it is called current operating device

The collector-base region is reverse biased in the active region. There is a limit on the maximum value for this reverse-bias voltage for every transistor. Figure (B) shows that the collector-base junction is reverse-biased. This reverse bias voltage must be within the maximum safe limits specified by the manufacturer. If this reverse bias voltage exceeds this limit, transistor breakdown occurs. This breakdown condition has been shown in Figure (B) This effect is known as punch-through effect.

             Figure (B) Maximum collector-base voltage rating in CB configuration.