# bandpass active filter Current feedback

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#### dragon_03

##### Newbie level 2
CFA

i want to design bandpass active filter Current feedback (CFA) with the center frequency 50MHZ by using sallen-key topology.

i would like to know which OP amp i have to use and its possible to increase the center frequency to 100MHZ

#### LvW

Re: CFA

dragon_03 said:
i want to design bandpass active filter Current feedback (CFA) with the center frequency 50MHZ by using sallen-key topology.
i would like to know which OP amp i have to use and its possible to increase the center frequency to 100MHZ

As indicated by you (name of the topic) a CFA type seems to be appropriate with a UGB of at least 700...800 MHz.
Concerning the second question, do you mean frequency tuning or fixed design?

#### keith1200rs

##### Super Moderator
Staff member
Re: CFA

dragon_03 said:
i want to design bandpass active filter Current feedback (CFA) with the center frequency 50MHZ by using sallen-key topology.

i would like to know which OP amp i have to use and its possible to increase the center frequency to 100MHZ

You cannot put reactive components (capacitors or inductors) in the feedback of current feedback amps.

Keith.

#### LvW

Re: CFA

keith1200rs said:
You cannot put reactive components (capacitors or inductors) in the feedback of current feedback amps.
Keith.

Hi keith,

I am afraid you have overlooked the fact that in Sallen-Key (S+K) structures the CFA is used as a noninverting amplifier without any capacitive feedback. The S+K topology is the only one which allows CFAs .

#### keith1200rs

##### Super Moderator
Staff member
Re: CFA

LvW said:
keith1200rs said:
You cannot put reactive components (capacitors or inductors) in the feedback of current feedback amps.
Keith.

Hi keith,

I am afraid you have overlooked the fact that in Sallen-Key (S+K) structures the CFA is used as a noninverting amplifier without any capacitive feedback. The S+K topology is the only one which allows CFAs .

My mistake. Thanks. Sorry if I caused any confusion to anyone.

Keith.

#### LvW

Re: CFA

keith1200rs said:
................
My mistake. Thanks. Sorry if I caused any confusion to anyone.

Keith.

Don't mention it.
Regards

#### keith1200rs

##### Super Moderator
Staff member
Re: CFA

LvW said:
keith1200rs said:
................
My mistake. Thanks. Sorry if I caused any confusion to anyone.

Keith.

Don't mention it.
Regards

I must admit, when I get to 50MHz or above I would probably be looking at passive LC filters, but it depends on the applications.

Keith.

#### LvW

Re: CFA

keith1200rs said:
...........

I must admit, when I get to 50MHz or above I would probably be looking at passive LC filters, but it depends on the applications.

Keith.

Yes, I agree - 50...100 MHz are rather high for active filtering. However, its possible with high performance CFAs or OTAs.
vW

#### dragon_03

##### Newbie level 2
Re: CFA

i would like to know which "current feedback OP AMP" work at high frequency
:?:

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