BJT bias point - RF Cafe Forums
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Post subject: BJT bias point Posted: Sun Jan 07, 2007 7:47 am
Joined: Tue May 02, 2006 4:59 am
Can anyone explain how the Emitter resistance RE stablize the DC bias Point ? ( commom emitter ).
Something about negetive feedback ?
Is this stabilization is instead active bias ?
Post subject: Posted: Sun Jan 07, 2007 2:20 pm
Joined: Mon Jun 27, 2005 2:02 pm
RE is used as a negative feedback.
The current gain of Beta increases with the operating temperature of the BJT. Therefore Ic (Collector current) raises. This current flows through RE and as a result the voltage drop on RE increases - Ve becomes higher. Vb is usually kept constant in common emitter topology (By means of a voltage divider), hence Vbe decreases, which turns the BJT more to off state and reduces the Ic.
This mechanism can be used instead of active bias.