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BJT Bias Point - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement


Noise
Post subject: BJT bias point Posted: Sun Jan 07, 2007 7:47 am

Captain


Joined: Tue May 02, 2006 4:59 am
Posts: 15
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 ?


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IR
Post subject: Posted: Sun Jan 07, 2007 2:20 pm

Site Admin


Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
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.







Posted  11/12/2012

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