When the base current Ib is constant, the relationship between Ic and Uce of the transistor is called an output characteristic curve. The curve is given by Ic(mA) as the ordinate and Uce(V) as the abscissa, which indicates the relationship between Ib, Uce and Ic when the transistor is working, which determines the working state of the transistor. As can be seen from the curve, the operating state of the transistor can be divided into three regions. Saturated area: Uce is small and Ic is large.
The collector and emitter are saturated and turn on as if they were shorted. The Uce at this time is called the saturation pressure drop. At this time, the emitter junction and collector junction of the transistor are both forward biased. Amplification area: A small change in Ib in this area can cause a large change in Ic, and the transistor works in this area to amplify.
In this area Ic is almost uncontrolled by Uce, and the curve is relatively flat. At this time, the tube's emitter junction is forward biased and the collector junction is reverse biased. Cut-off area: Ib=0, Ic is very small, the collector and emitter seem to be open (called cut-off), and the tube's emitter junction and collector junction are reverse biased.