Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Connecting resistors to BJT's base.

Status
Not open for further replies.

alphacat

New Member
Hi.

I was wondering why the below configuration is used?

Specifically, an engineer I met has used the following values:
R1 = 4.7KΩ, R2 = 47KΩ

I was wondering what R1 and R2 are for, and why R2 equals 10 times R1?

Thanks.

**broken link removed**
 

Hero999

Banned
It depends on what you're doing?

Are you using the transistor as a switch?

As a general rule of thumb it's good for the base current to be 1/10th of the collector current to ensure a low saturation voltage.

It depends on the transistor - check the datasheet.

EDIT:

How are we supposed to know which resistor is is R1 and R2? Post a schematic.
 
Last edited:

Jony130

Active Member
If we use BJT as a switch then we add R2 to create path to GND to avoid base from floating.
And sometimes the low level voltage is to high to ensure cut-off.
So we put R2 to proper cut-off the bjt.
 
Last edited:

Hero999

Banned
Again, a schematic might make what you've said make sense.
 

Jony130

Active Member
And I forgot to mention that R2 increase switching off speed and also pulls-down bjt leakage currents, particularly at higher temperatures.
 

Hero999

Banned
For the last time; POST A SCHEMATIC

Without a schematic you might as well be talking Martian - your post makes no sense so you're wasting your time.
 

audioguru

Well-Known Member
Most Helpful Member
Instead of posting the schematic here, he linked to it over there.
Here it is:

EDIT:
That website doesn't open half the time.
 

Attachments

  • schematic..PNG
    schematic..PNG
    3.3 KB · Views: 113
Last edited:

alphacat

New Member
Hi, thanks guys.
Sorry for the schematics problem.

If we use BJT as a switch then we add R2 to create path to GND to avoid base from floating.
Got it, its probably for cases where the input is floating as well.

So we put R2 to proper cut-off the bjt.
If R2 >> R1, then VBASE is just a little below VGPIO
 

Hero999

Banned
R1 depends on the transistor, the collector load and the voltage source driving the base. As a rule of thumb for a low saturation voltage the base current should be 1/10th of the collector current.

Assuming the emitter goes to 0V and it's not driven an op-amp with a high, low side saturation voltage, the value of R2 is not important and is just there to speed up the switch off, improve noise immunity and minimise the leakage current.
 
Status
Not open for further replies.

Latest threads

Top