NPN working operation

[parth22]

Hi all,
I have a general question and confused with datasheet values.
I am using an npn transistor as a low side switch. Below is the data sheet of this transistor.

https://www.onsemi.com/pdf/datasheet/2n3903-d.pdf?msclkid=fb5d9851b78511ecbc76bce30cd18737

In the below image also you can see.
I would like to draw 10ma collector current. so for 10ma currnet VCE(sat.) is 0.2V (from data sheet). let's suppose VCC = 10 so i can calculate RC = (VCC- VCE(Sat))/10ma.

And same time for the calculation of base current...we need ß (dc gain) so for 10ma ß = 50 to 150 (from the data sheet )...in this way IB = 0.06 ma to 0.2 ma
but as per the data sheet for Ic= 10ma and VCE (sat) = 0.2V.....IB should be 1mA....HoW???
Whether I am opting wrong values or something else...
can anyone explain?

 

[Audioguru]

A transistor part number has some weak ones and some sensitive ones.
The datasheet guarantees that all of them saturate fairly well when the base current is 1/10th the collector current regardless of the maximum beta of some of them.

Instead, you can buy thousands of one transistor part number and test them all. You might find a sensitive one that can be used at "the edge of saturation" with a low base current.

I design circuits so that all of them work perfectly, not just a few of them.

 

[cupoftea]

Thanks thats great, i mean, next time i (or the OP) is doing a battery circuit where all currents must be absolute lowest.....i will say that in the above case where NPN is in common emitter, and with 100k collector resistor to 5v rail.....then ib must be a minimum of [(5-0.3)/100k]/10 Amps to garantee saturation.....and when they quibble for me to make the base current less than that.........just point out the ic/10 rule?

 

[Audioguru]

When designing a circuit where where all currents must be absolute lowest then you would select a transistor with a very high beta at very low currents. Then buy a few, measure them then select the most sensitive one.

(5V - 0.1V)/100k= 49uA. [(5-0.3)/100k]/10= 4.7uA. Then all transistors will saturate.
Most datasheets say that in writing but the graphs show transistors that have typical (halfway between weak and sensitive) specs.
A 2N3055 power transistor is guaranteed to saturate (fairly poorly) with a 10A collector current when its base current is 3.3A.

 

[KlausST]

Hi,

When designing a circuit where where all currents must be absolute lowest then

Maybe use a darlington BJT or a MOSFET..

Klaus