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Design >> Analog Design >> how to consider non-ideal characteristic for BPF design
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Message started by lhlbluesky_lhl on Oct 1^st, 2012, 7:05pm

Title: how to consider non-ideal characteristic for BPF design
Post by lhlbluesky_lhl on Oct 1^st, 2012, 7:05pm

in most active BPF design procedure, the opamp is assumed to be ideal; however, the actual opamp is non-ideal, first-order or second-order, finite DC gain, finite GBW, etc. i want to know, for a BPF design, how to consider the non-ideal characteristic of opamp? such as, what is the influence of dominant pole, GBW of opamp?how to decide the design parameter of opamp?

besides, i had seen a special BPF before, its center frequency can be adjusted only by a external resistor Ro:
Ro=8855/f0(khz)-13
what is the possible structure of this BPF? can anyone give me some advice?

thanks.

Title: Re: how to consider non-ideal characteristic for BPF design
Post by buddypoor on Oct 2^nd, 2012, 1:48am

Hi Ihlbluesky,

*You are right - the classical procedure is to calculate all parts assuming an ideal opamp. That means, for a practical realization an opamp must be used with a GBW much larger (at least factor 100) than the midband frequency.
*Nevertheless, there will be deviations from the desired behaviour: The center frequency as well as the bandwidth will be somewhat smaller (Q enlargement).
*It is the design goal to keep these deviations within the uncertainty limits of the parts tolerances.
*There are some filter design programs available, which are able to include a first-order finite gain amplifier in the parts calculation process.
* More than that, I rememeber an application note from Linear Tech, which describes how to do a kind of pre-distortion in order to consider a first-order opamp. If I find it, I will give notice.
* Finally, I like to mention that I have described and published a method how to reduce such errors (introduced by the opamp) using a circuit simulation program like PSpice and a specific optimization loop. In case you are interested I can give you a reference.

Title: Re: how to consider non-ideal characteristic for BPF design
Post by loose-electron on Oct 2^nd, 2012, 4:38am

The BPF that is tuned with a single resistor is probably a gmC filter not an op-amp architecture.

Title: Re: how to consider non-ideal characteristic for BPF design
Post by buddypoor on Oct 2^nd, 2012, 5:18am

Here are the application notes I was referring to (from National/TI, not from Linear):

http://www.ti.com/lit/an/snoa373b/snoa373b.pdf
http://www.ti.com/lit/an/snoa369a/snoa369a.pdf

Title: Re: how to consider non-ideal characteristic for BPF design
Post by lhlbluesky_lhl on Oct 5^th, 2012, 3:51am

hi, buddypoor, as you said 'I like to mention that I have described and published a method how to reduce such errors (introduced by the opamp) ', i want your reference, thanks.
and loose-electron, can you give me some advice or papers about gmC filter?
thanks all for your reply.

Title: Re: how to consider non-ideal characteristic for BPF design
Post by buddypoor on Oct 5^th, 2012, 9:23am

Hi Ihlbluesky,

I hope the link works:

http://electronicdesign.com/article/eda/spice-simulator-tunes-rc-active-filter-circuits627

Title: Re: how to consider non-ideal characteristic for BPF design
Post by lhlbluesky_lhl on Oct 5^th, 2012, 8:13pm

thanks buddypoor very much