Why a constant current source is used in a differential amplifier?

[Paul V. Sebastian]

I've read that it stabilizes the operating point..can somebody pls explain?..

 

[KlausST]

Hi,

By using a current source the input bias current is independent of common mode input voltage.
By using a current source the operating voltage of the second stage is constant. This ensures that noise and bandwith is constant and don´t deend on common mode input voltage.

Klaus

 

[Paul V. Sebastian]

So does it mean that with a simple emmitter resistance bias current varies with common mode input voltage ?...

 

[KlausST]

Hi,

So does it mean that with a simple emmitter resistance bias current varies with common mode input voltage ?...

yes, changing common mode input voltage means changing voltage across emitter resistor, thus changing resistor current, changing emitter current, changing base current = input bias current

 

[Paul V. Sebastian]

Thanks ...now I get it!