# What happens to input character as $V_{CE}$ is increased?

In a NPN transistor and its study of input character, we obtain a graph (Ib vs Vbe) similar to that of PN junction diode.

Now that is same for a same potential difference between Collector and emitter (Vce). What is going to happen to the curve on a different Vce, say when increased.

According to what I have understood, as Vce is increased, that'll mean that some electrons pulled into base will be pulled by Vce once it arrives at base decreasing base current. So as Vce is increased, Ib should decrease and the curve should shift to the right (i.e. More Vbe required for same base current). I'd appreciate insight in this matter.

• Would Electrical Engineering be a better home for this question? Feb 14, 2020 at 14:08

That’s broadly true in the base saturation region but, as you enter the linear region, increasing Vce does not result in an appreciable rise in collector current. See the typical characteristic below : -

Picture from here.

For a very low Vce (as opposed to an open collector) the base-collector junction is forward biased, so it's in parallel with the base-emitter junction, and will reduce the Vbe for a given current. The base-collector junction has a lower Vf for the same current as the base-emitter junction in a typical modern transistor (there are a few exceptions that are more symmetrical).

Once that's no longer a factor, the Vbe remains very constant.

• It starts out at 706.6mV at Vce = 0V
• It reaches 797.8mV at Vce = 400mV and remains there up to the limit of this simulation of Vce = 4V

See the simulation below for an example:

simulate this circuit – Schematic created using CircuitLab

Note that in this SPICE simulation, the junction is maintained at a theoretical constant temperature of something like 27°C. In a real transistor the base current would result in several hundred times more collector current and the junction would rapidly heat when Vce is more than a few tens of mV, causing the Vbe voltage to drop, but with a thermal time constant.