Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Supply filter for low noise pre-amplifier

Status
Not open for further replies.

Troela

Junior Member level 1
Joined
Apr 17, 2006
Messages
17
Helped
1
Reputation
2
Reaction score
0
Trophy points
1,281
Activity points
1,415
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

Advanced Member level 5
Joined
Oct 5, 2004
Messages
7,942
Helped
2,311
Reputation
4,624
Reaction score
531
Trophy points
1,393
Location
West Coast
Activity points
66,416
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

Junior Member level 1
Joined
Apr 17, 2006
Messages
17
Helped
1
Reputation
2
Reaction score
0
Trophy points
1,281
Activity points
1,415
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

Junior Member level 3
Joined
Sep 30, 2004
Messages
26
Helped
1
Reputation
2
Reaction score
1
Trophy points
1,283
Activity points
191
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

Junior Member level 1
Joined
Apr 17, 2006
Messages
17
Helped
1
Reputation
2
Reaction score
0
Trophy points
1,281
Activity points
1,415
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

Advanced Member level 4
Joined
May 28, 2001
Messages
115
Helped
17
Reputation
36
Reaction score
5
Trophy points
1,298
Activity points
705
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

    Points: 2
    Helpful Answer Positive Rating

Troela

Junior Member level 1
Joined
Apr 17, 2006
Messages
17
Helped
1
Reputation
2
Reaction score
0
Trophy points
1,281
Activity points
1,415
Thanks man, the info I needed!!!
 

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top