Transistor - Saturation and Active Regions

Question

I am using this BC817-40 transistor as a switch.

In the above image, we can see that inorder to operate my transistor in the saturation region, I need to make sure my Ic/Ib ratio is less than 40.

And to make sure it is in the active/linear region, I need to make sure my Ic/Ib ratio is more than 40. This can be done by selecting appropriate resistor value for a given Base and Collector voltage.

My actual questions :

1. In the above DC Current gain section, when they say Vce=1V and Ic=100mA, are they saying that the voltage difference between the collector and emitter should be less than 1V or more than 1V in the active region? And this dc current gain section is especially to be checked when we need to use the transistor as an amplifier,right?

2. In my application, I am using BC817 transistor as a switch. I refer the Figure5. to calculate my Base emitter voltage and Figure6. to calculate the Collector emitter voltage. In both the graphs, the Ic/Ib=10. I take the Vbe and Vce values from the graph and calculate my Base current and collector current. Now, my calculated Ic/Ib ratio comes around 18. Since 18>10, do I need to adjust my base and emitter resistance values to make it less than 10 so that my transistor is in saturation mode when it is turned ON. Or should I just leave it at 18, since 18<40. (40 being the minimum Hfe given at the Table 8)

FIGURE 5 : Ic Vs Vbe(sat)

FIGURE 6 : Ic Vs Vce(sat)

3. Since the figures 5 and 6 plots, specifically mentioned the Vbe and Vce parameters for Ic/Ib=10, and 10 being less than minimum Hfe of 40, does it mean, that these two plots should be referred only when we need to use these transistors in saturation mode? Or is there a case, when we can use these plots in active/linear mode also?

Answer 1

1. These are testing points, assuming you have a 1V drop between Vce and 100mA flowing through, you have X amount of gain, so for your particular transistor the BC817, at 100mA being switched, you have a beta between 250 to 600, If your use case is at a different current, the gain may be different (transistors tend to have a fairly wide range of gain in manufacturing) the lowest voltage across the Vce junction is drawn out in Figure 6. when saturated at a given current, it will have that voltage.

2. Focus on the current, instead of the voltage, the transistor itself will define the voltage, e.g. if your switching from 5V, assuming your switching 500mA, and you don't know where in that beta range you lie, you need atleast 12.5mA (500mA / beta of 40) flowing into the base, The worst case Vbe is about 1V, so 4V needs to be dropped at 12.5mA, meaning a 320 Ohm resistor to satuarate it reliably in all use cases. If your switching a higher current, you would adjust as required

3. These plots are showing how it behaves when significantly saturated. they only specify this particular case so unless graphed elsewhere, that is all they are promising, in reality the saturation voltage will be fairly low for most transistors, but with how widely the manufacturing tolerance is of units, I presume they did not want to specify anything too close to what a bad unit may not be able to meet.