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Active AF Low-Pass Filter Synthesis

Wed, 01/07/2009 - 06:54 — NT7S

Use the following formulae to synthesize an active AF low-pass filter for op amp or transistor use. The amplifier needs to have unity gain and be non-inverting. Set a Q of 0.707 for a flat frequency response up to the the cutoff frequency. Higher values of Q will cause a peaked response at fo. Lower Q values will cause a rolloff that occurs before the cutoff frequency. The design presented is a 2-pole filter; multiple stages can be cascaded together if more filtering is necessary.

R = {1 \over {2 \pi f_o\sqrt{C_1 C_2}}}

Q = {1 \over 2} \sqrt{C_1 \over C_2}

{V_o \over V_{in}} = {1 \over {R^2 s^2 C_1 C_2 + 1 + 2 s R C_2}

where

s = j \omega = j 2 \pi f

Schematic: 
Creator: 
Solid State Design for the Radio Amateur

Comments

Wed, 01/07/2009 - 12:46 — K3NG

Op Amp Gain

You show a "+1" on the op amp.  Does this mean unity gain and if so, is that a requirement?

Wed, 01/07/2009 - 13:37 — NT7S

Yes and yes

Definitely unity gain, and as far as I know, it's a requirement. I'll make this more clear in the documentation, thanks!