Hi,
I just read a tutorial about biasing a transistor. It says that we
cant use the formula Ic=Beta.Ib when it is in saturation region but in
active region we can use that. But in another tutorial, I found that
the transistor is in saturation region as a switch but it applied the
above formula, so which one is true ?
When you use a transistor as a switch, you normally want to saturate it.
So you calculate the maximum base current you could possibly need, then
multiply by 10 and design for that current.
For example, suppose I have a transistor with a gauranteed minimum DC
current gain of 100. I am using it as an inverter, and I have a 1k pullup
on the collector. I know that I need to drive it down to 0.8V to get a
valid LVTTL low.
So, I can go through the calculations to find the minimum collector current:
(3.3V - 0.8V) / 1k = 2.5 mA
This means that the minimum base current is:
2.5mA / 100 = 0.025mA
So, in theory, I would be safe if I can put 0.025 mA into the base. But I
want to be really safe, so I'll shoot for 0.25mA into the base. So that is
why I use the formula, to find the minimum current, which I then multiply
by 10.
Anyway, to finish, since the base will be at about 0.8 volts, and my input
voltage will probably be 3.3 volts, I need (3.3 - 0.8) / 0.25mA = 10k in
series with the base. Since I have a ten-fold safety margin, I don't need
to worry about 5% resistors or inputs that might be substantially less
than 3.3V.
HTH
Mac
--