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Signal Chain Basics

# Signal Chain Basics #126: How to design active filters with different response types using circuit-transfer function equations

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CollinWells
8/3/2017 11:53:25 PM
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Re: OP AMP GBW
Thanks for your comments!  There is a nice explanation of the op amp gain-bandwidth requirements in Chapter 16.8.4 of the popular "Op Amps For Everyone" publication.  Basically the equations define that the op amp should have an open-loop gain magnitude at least 40dB (100V/V) larger than the peak gain (Q) of the filter which will keep the gain error <1%.

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sfierro53
8/3/2017 12:03:10 PM
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OP AMP GBW
I have read many articles on how to design active feedback filters which concentrate on the filter parameters and component values. I however have not seen one which explains the neccessary GBW product specification that will be required of the op amp. Filter pro will state the GBW specification but no explanation is given. For instance for a 2 pole 1MHz butterworth LPF with unity gain the GBW product called for is about 84MHz. This is for sallen key or MFB topologies. Can you explain why or give a link to an app note which does?

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silicon_researcher
8/2/2017 12:49:04 PM
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In general, the damping ratio changes when the natural frequency (f0) is changed. This is clearly seen in the equations for sallen key filter (both are dependent variable of the same independent variables, namely R1, R2, C1, C2). In tables 1 & 2 we see that the damping ratios are identical before and after application of correction factor k. In certain restricted cases of choosing cap/resistor values, I can see that one can make them identical. For example choosing Ci (new) = k*Ci (old), i = 1, 2 and thus f0(new) = 1/k *f0(old) but damping factor remains unchanged. Or are the damping ratios in table 2  values before correction? Can the authors clarify.

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