Re: gEDA-user: Current source rail blocking

[Dan McMahill <dan@xxxxxxxxxxxx>]

Karel Kulhavy wrote:
> Hello
>
> In my broadband high sensitivity amplifier I have a rail where bases of all
> current source transistors are hooked up. Each current source is a transistor
> with an emitter resistor which compensates variance in aplification so that
> even unmatched transistors produce matched currents.
>
> Do you have any recommendations how to block the current sources from picking
> up some garbage from the air and causing oscillation? Should I block the rail
> with a single big capacitor against the ground, or use individual capacitor for
> each transistor placed between emitter and base?

What sort of transistors are these?  Are you seeing oscillations in 
practice or is this just a concern prior to seeing any real hardware? 
I'd avoid using transistors which are faster than need be.  For example, 
sticking a 20 GHz device in there may not be a good idea.

If you can tolerate some additional noise, you can stick some resistance 
in series with the base if the transistors themselves are oscillating. 
If you have some extra capacitance on the emitter and some inductance in 
the base circuit (without much extra resistive loss), then it's not too 
hard to build an oscillator.  Without knowing some more details anything 
else would be pure speculation.

-Dan


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