# BJT saturation

1. Dec 24, 2012

### likephysics

How do you design a BJT switch to ensure it is (Vce) saturated.

2. Dec 24, 2012

### yungman

It all depend what is the load at the collector, in general, drive more base current.

3. Dec 24, 2012

### likephysics

Drive with more current is the easy way out.
Let's say the load is a 100 ohm resistor. Vcc is 5V.
BJT is 2N2222.

4. Dec 24, 2012

### Staff: Mentor

What is the typical Vcesat for the 2222? So then what will be the voltage drop across the resistor Rc? So then what will be the current Ic? What is the minimum Beta for the 2222 at that Ic and your operating temperature? So what base current is needed to ensure saturation?

See how it's done? Show us your work...

5. Dec 24, 2012

### yungman

It is not the easy way out, it is the way. When saturated, β drop way low, you need way more base current. You ask how to make sure it is saturated!!!

5V 100Ω is 50mA. You want do drive at least 5mA to get good saturation. Read the data sheet, they drive 15mA for 150mA.

6. Dec 24, 2012

### Staff: Mentor

To ensure it is saturated you drive it with sufficient base current that under all foreseeable conditions, your base current IB > IC / β

Remember, β is far from a constant, it varies from batch to batch, from device to device, it varies with current, and it varies with temperature.

http://img803.imageshack.us/img803/4666/holly1756.gif [Broken]

Last edited by a moderator: May 6, 2017