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resistor in parallel with transistor base_emitter

fhuang (Automotive) (OP)
9 Dec 08 11:06
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?
MikeHalloran (Mechanical)
9 Dec 08 11:35
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

IRstuff (Aerospace)
9 Dec 08 12:04
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.  

TTFN

FAQ731-376: Eng-Tips.com Forum Policies

geekEE (Electrical)
9 Dec 08 13:55
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.

Glenn
melone (Electrical)
9 Dec 08 13:58
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.
MacGyverS2000 (Electrical)
9 Dec 08 14:23
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
http://www.Hi-TecDesigns.com

Comcokid (Electrical)
9 Dec 08 18:30
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.
 
OperaHouse (Electrical)
10 Dec 08 8:28
Look at any transistor specification and the breakdown voltage is higher C to B than C to E.  Any leakage to B is amplified.   

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