Supply filter for low noise pre-amplifier

[Troela]

Hello, I'm gonna use the filter from the following application note:

h**p://www.national.com/an/AN/AN-222.pdf
page 4 figure 4

The filter is R7-C3-NPN transistor. They say that this filter supress the noise with 50dB. I'm wondering how this work. Does this filter hava a special name?

Grtz Troela

 

[IanP]

I don't think this filter can fulfill your expectations ..
Perhaps more efficient solution, as far as the noise attenuation is concerned, would be to introduce proper shielding + RF-choke(s) on both rails supported by decoupling capacitors (1-100µF electrolitic and 100pF+100nF ceramic) ..
Regards,
IanP

 

[Troela]

Thanks for the tips IanP.

Anyone who knows the theory behind the kind of filter that I mentioned in the first post? I never seen it before for using as a filter.

 

[fobyellow]

niose in what freq-range you concern most:?:
if low freq noise is concerned , the tradition linear regulator already provide good noise rejection(if built with low noise voltage reference).
if high freq noise is more concerned, try LC or CLC or even more complicated circuit.

 

[Troela]

Yes, I know that you can filter with LC, CLC pi,...
But do you know the working of the filter with the transistor? Why do they use a transistor?

 

[Encrypted]

R7, C3 and the transistor form a capacitance multiplier. The value of C3 is multiplied by the gain of the transitor.

Try a Google search for more info.

 

[Troela]

Thanks man, the info I needed!!!