0
\$\begingroup\$

What happens if the collector-emitter current is less than the base-emitter current? Do I need to increase the base resistor to lower the base current or is it okay to keep the transitor operating in this state?

The background is that I did connect a LED with a 2KOhm resistor to the collector side (5V) and I am using a 1kOhm resistor at the base side (3.3V, Arduino). Originally I used a 220Ohm resistor for the LED, but I just needed to reduce the LED brightness. I am not sure if I actually need to increase the base resistor value, just to keep the base current always lower than the collector-emitter current.

\$\endgroup\$
4
  • 1
    \$\begingroup\$ It isn't a good practice to force too much base current in comparison with collector current. Usually it does nothing like in simple driving Leds but sometimes...it causes an ugly spikes. \$\endgroup\$
    – user208862
    Nov 20, 2021 at 2:36
  • 1
    \$\begingroup\$ You should not forget that under saturation condtions the current into the base is NOT identical with the base-emitter current. The B-C junction is forward biased and a remarkable portion of the base current goes into the collector. \$\endgroup\$
    – LvW
    Nov 20, 2021 at 8:34
  • \$\begingroup\$ @LvW: I dont understand this. Do you mean the current flows in the opposite direction, from base to collector? \$\endgroup\$ Nov 21, 2021 at 2:02
  • \$\begingroup\$ When both pn junctions are forward biased there are two currents going through the base node: Ib=Ibe+Ibc. Does it not sound logical? That is the reason Kevin White wrote: The collector current can even be zero: Ic=Ice - Ibc (opposite directions). \$\endgroup\$
    – LvW
    Nov 21, 2021 at 9:51

1 Answer 1

4
\$\begingroup\$

There is no requirement that the base current be smaller than the collector current.

If you are driving more than is needed you are, however, wasting power so in a battery operated circuit it would be useful to drive a lower current into the base.

The usual design technique for driving a transistor into saturation is to assume an Hfe of 10-25 and select the base resistor appropriately. This ensures that even a worst case transistor will be driven adequately.

It is acceptable for the collector current to be zero, or it can even be negative in some circuits, especially where the transistor is switching AC.

The base current should be within the transistor's ratings and you may not want to drive so much that it causes significant thermal dissipation.

One disadvantage with driving the base very hard is that it will increase the amount of charge stored in the base that will cause the transistor to be slower to turn-off when the base current is reduced to zero. For many applications this is not a problem. If faster turn-off is required a capacitor in parallel with the base resistor can help by driving negative current into the base when turning off.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.