# BJT Saturation Characteristic

From what I understand a saturation for an BJT is when the B-E voltage doesnt change the collector current anymore. In other words, Saturation simply means that an increase in base current results in no (or very little) increase in collector current. However, the section for Saturation in the graph is when the current and voltage rise almost linearly together for a fixed base current. Isnt this contradicting? Shouldn't the slope be somewhat similar to the 'active region' and be flat?

I meant if we change B-E voltage, the collector still should be the same. Am I missing something here? Thanks

I understand a saturation for an BJT is when the B-E voltage doesnt change the collector current anymore.

In the graph you presented, the x-axis is $V_{ce}$, not $V_{be}$. Changes in $V_{be}$ are represented by jumping from one parameterized curve to another.

You can see in the saturation reason, the parametric curves overlap, meaning changing $V_{be}$ doesn't result in a change in collector current, just as stated in your proposed definition.

• Can you elaborate on the overlap parametric curves? For each curve in the saturation zone, as you move the x value the y value is higher compare to the last value. – Ace8888 Nov 5 '17 at 2:32
• @Ace8888, in the (lower current part of the) saturation zone, the curve for Ib1 is the same as for Ib2 is the same as for Ib3. Ib is one-to-one related to $V_{be}$. So the chart is telling you that changing $V_{be}$ doesn't change $I_c$. – The Photon Nov 5 '17 at 2:37
• @Ace8888, look at your chart. What is the label of the x-axis? Is it $V_{be}$? or is it something else? – The Photon Nov 5 '17 at 2:38

Saturation is well defined. It is when both base-emitter junction and base-collector junction are forward biased. The way you said it is approximately correct also. In this graph, the base-emitter junction is always forward biased. So Vbe is around 0.6V or so. Varying slightly with actual base current.

Over on the left side of the graph is where Vce is <= Vbe, so that is the saturation region.

Think of it this way. As you move to the left on any one of the curves, Vce goes down, down, down, but Ic is relatively constant (constant beta). Only when Vce gets lower than Vbe does Ic start to collapse. This is the beginning of the saturation region.

So there is no contradiction. Everything is copacetic.