What is transistor saturation?

Where does the output characteristic of BJT show the transistor in saturation?

• Seems basic, but I fail to see in this diagram how the transistor will fall in the saturation region.  For a given Ib, wouldn't the Ic value just trace the blue line for whatever value of Vce from start zero to Vce max? Say for about 1 volt Vce and Ib=80, I can trace about 45ma Ic(the blue curve), or should I just follow that green patch on the left and say Ic is near the red bar on top?

• Answer:

  The saturation region just refers to the transistor being operated in the region denoted by the green patch on the left. In this region, Ic varies with Vce. This is as opposed to the blue active region where Ic stays constant even with varying Vce. So for your example of Vce = 1 V and Ib = 80 uA, Ic is indeed about 45 mA and the transistor running in saturation since this point is in the green region. The DC Load Line (http://en.wikipedia.org/wiki/Load_line_%28electronics%29) effectively shows the IV curve of a DC supply. Thus, if you were driving the transistor with a DC line, the intersection of the DC Load Line and the blue curves gives you your operating point. In your example, with Ib = 80 uA, we see that the load line intersects the blue curves at Vce = 4 V and Ic = 50uA. Thus, we are in the active region. Notice that Vce is determined from the circuit based on the DC supply rather than an input we can specify. If instead Ib = 120 uA, then Vce = 1.5 V and Ic = 70 uA and we are in the saturation region.