How to make transistor stay in saturation region and proper Rc value caluculation
How to analyze saturated switch circuits.
First, figure out how much collector current you want when the switch is on. Assume that the voltage from collector to emitter is Vce(sat) from the datasheet. It could be something like 0.2V for some transistors or 1V for others, depending on how much current and type of transistor.
Second, divide the desired collector current by 10 or 20. This number, 10 or 20 is called the forced beta (beta(forced)).
Third, arrange to drive the base with Ic/Beta(forced).
Full example. I have an LED that I want to drive with 20mA. VCC is 5V. The datasheet for the LED says Vf is 3V at If = 20mA. So I want Ic to be 20mA.
Vce(sat) for my 2N3904 is 0.2V at 20mA. So after I subtract Vce(sat) and Vf from VCC I have 1.8V left to drop across my resistor. So the resistor value needs to be 1.8V/20mA = 90 Ohms (pick the nearest value, e.g., 91 Ohms).
I am driving the base of the transistor with an IO pin from my microprocessor (VCC = 5V). I am going to use a forced beta of 20. So I want my base current to be 1mA. The base is going to be at around 0.7V with 1mA going through it. So the base resistor needs to drop 4.3V with current of 1mA. This means the base resistor needs to be 4.3k, which is a standard value.
This is how you handle a saturated switch. From the perspective of cause and effect, the way it works is when you drive the base hard, it pushes the transistor into saturation. To achieve saturation, you always want to supply an excess of current to the base to push Vce down lower. It is the base current that causes Vce to drop. By supplying excess current to the base you are forcing the transistor to operate in low beta. That is why they call it forced beta.



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