Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.


resistor in parallel with transistor base_emitter

resistor in parallel with transistor base_emitter

why a resistor is added in parallel with a transistor base-emitter. e.g., a transistor is driven through a 10k at its base and between base and emitter another 10k is connected in parallel?

RE: resistor in parallel with transistor base_emitter

I'm guessing the base-emitter resistor is intended to speed up the turnoff by draining charge from the base.

At least, that's why some opto- transistors have a base terminal.


Mike Halloran
Pembroke Pines, FL, USA

RE: resistor in parallel with transistor base_emitter

Collector saturation is a direct function of the base current.  However, overdriving the base means that more space charge must be removed before the transistor can be turned off.  


FAQ731-376: Forum Policies

RE: resistor in parallel with transistor base_emitter

What is driving the base?  If it's something that may be disconnected, the resistor might be there to make sure that the base doesn't float when the driving source is removed. This will ensure that the transistor is off.

More details on your circuit will result in better answers.


RE: resistor in parallel with transistor base_emitter

If I understand your description, the Base-Emitter resistor is used to create a voltage divider with the base resistor.  This voltage divider will guarentee that the transistor is turned on hard.

RE: resistor in parallel with transistor base_emitter

Sounds like a PNP... the base-emitter resistor is to make sure the base is pulled high (transistor off) when nothing is driving it (the base driver is most likely open-collector).

Dan - Owner

RE: resistor in parallel with transistor base_emitter

I've seen this, and have done it in some industrial designs - frequently on transistors that buffer signals or control relays or solenoids. In an environment where arc, sparks, and high magnetics fields from nearby transformers are a norm, it insures the transistor is OFF unless it really does have base current. Also, if a module is disconnected, the wire/trace to the base is less likely to pick up enough stray EM to momentarily turn the transistor on.

RE: resistor in parallel with transistor base_emitter

Look at any transistor specification and the breakdown voltage is higher C to B than C to E.  Any leakage to B is amplified.   

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close