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Static Characteristics of Power Transistor (V-I Characterstic)
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Out of the three possible circuit conf‌igurations for a transistor, common-emitter arrangement is more common in switching applications. So, henceforth, npn transistors will only be considered. -

Input characteristics : A graph between base current IB and base-emitter voltage VBE gives input characteristics. As the base-emitter junction of a transistor is like a diode, IB versus VBE graph resembles a diode curve. When collector-emitter voltage VCE2 is more than VCE1 , base current decreases as shown in Fig.1(b).

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Output characteristics : A graph between collector current Ic and collector-emitter voltage VCE gives output characteristics of a transistor. For zero base current, i.e. for IB=0 ,as VCE is increased, a small leakage (collector) current exists as shown in Fig.1(c). As the base current is increased from IB=0 to IB1,IB2 etc, collector current also rises as shown in Fig.1(c).

Fig.2(a) shows two of the output characteristic curves, 1 for IB=0 and 2 for IB0 . The initial part of curve 2 , characterized by low VCE, is called the saturation region. In this region, the transistor acts like a switch. The flat part of curve 2 indicated by increasing VCE and almost constant IC is the active region. In this region, transistor acts like an amplifier. Almost vertically rising curve is the breakdown region which must be avoided at all costs.

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For load resistor RC, Fig.1(a) the collector current IC is given by

IC=VCCVCERC

This is equation of load line. It is shown as line AB in above figure. A load line is the locus of all possible operating points. Ideally, when transistor is on, VCE is zero and IC=VCC/RC. This collector current is shown by point A on the vertical axis. When the transistor is off, or in the cut-off region, Vcc appears across collector-emitter terminals and there is no collector current. This value is indicated by point B on the horizontal axis. For the resistive load, the line joining points A and B is the load line.

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