# Can collector current change when transistor is saturated?

We know that collector current doesn't change with base current anymore when a transistor is saturated. At the minimum base current to produce saturation: $$I_{C(sat)} = {\frac{V_{CC} - V_{CE(sat)}}{R_{C}}}$$ $$I_{B(min)} = {\frac{I_{C(sat)}}{\beta}}$$

$$Vcc = 10V, Vce(sat) = 0.2V, Rc = 1.0k\Omega, \beta = 200$$

Therefore, $$I_{C(sat)} = 9.8mA$$ $$I_{B(min)} = 0.049mA$$

My question is: if we increase the base current beyond the value obtained above, does collector current increase? I think yes because there is still a 0.2V drop across collector-emitter so when base current increases to 0.050mA, collector current will increase up to 10mA which will be the maximum collector current. And the minimum base current to produce saturation should actually be 0.050mA. What do you think?

## 2 Answers

Yes, it will change slightly with base current. Also you can't assume hFE will still be 200 with Vce=200mV. It's usually specified with at least 1V Vce, and will be dropping precipitously by the time Vce gets down to 200mV.

Assuming a collector load of a resistor to a fixed voltage, collector current will increase with increasing base current, and then decrease above a certain point (around where the base current is similar to collector current and Vce starts to increase because of all the excess base current).

We know that collector current doesn't change with base current anymore when a transistor is saturated.

That is incorrect. This graph from Confusion in interpreting NPN transistor's saturation region indicates a more accurate picture of the BJT saturation region: -

Image from here.

You might have been looking at websites or books where the graphs are drawn rather badly (plenty of incompetent examples on the net).

See the "confusion" link I gave above for an example of a badly drawn curve: -

So, when in saturation, we can use the base current to control the slope of the curve to perform analogue multiplication. A very important use of BJTs. If a BJT were like the picture immediately above this wouldn't be possible.

if we increase the base current beyond the value obtained above, does collector current increase?

I think you might be able to see what happens now. You can even prove this to yourself using a decent simulation tool. Here's an example for a BC547 BJT. Base current isn't explicitly show because of the way I drive the base using a voltage source and series resistor: -

I would encourage you to use tools like simulators to help formulate your thoughts.