BJT saturation

  • #1
likephysics
636
2
How do you design a BJT switch to ensure it is (Vce) saturated.
 

Answers and Replies

  • #2
yungman
5,624
226
It all depend what is the load at the collector, in general, drive more base current.
 
  • #3
likephysics
636
2
It all depend what is the load at the collector, in general, drive more base current.
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
berkeman
Mentor
64,124
15,327
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.

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
yungman
5,624
226
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.

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
NascentOxygen
Staff Emeritus
Science Advisor
9,242
1,073
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.
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:

Suggested for: BJT saturation

  • Last Post
Replies
5
Views
1K
Replies
1
Views
1K
Replies
4
Views
553
Replies
19
Views
791
  • Last Post
Replies
10
Views
526
Replies
43
Views
3K
Replies
2
Views
459
Replies
16
Views
1K
  • Last Post
Replies
9
Views
1K
  • Last Post
Replies
4
Views
954
Top