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'[OT] Current Feedback Op-amps'
1999\02\12@200422 by Sean Breheny

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Hi all,

       I'm looking for a primer on current feedback op-amps. I can't find
anything on them in Horowitz & Hill's "Art of Electronics". From diagrams
I've seen, it looks to me as if they are hooked up rahter similarly to
voltage feedback ones,but I'd like to know how to properly analyze them.

       I'd like to learn about them in general,but in this specific application
,
I need an approx. 5 times gain buffer which swings almost rail to rail on
+/- 5V supplies (or can handle +/- 30V supplies and swing +/- 5 V) while
driving a 100 ohm load (50 ohms back terminated) and provides output short
circuit protection, all such that it maintains almost 5 times gain up to
20MHz. I have found a Linear Tech. part (OP681) that seems to fulfill my
requirements,but it is current feedback. I am of course willing to consider
other parts if anyone has a suggestion.

Thanks,

Sean

|
| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
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1999\02\12@200843 by Sean Breheny

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I wrote:

>20MHz. I have found a Linear Tech. part (OP681) that seems to fulfill my
requirements,but it is >current feedback. I am of course willing to
consider other parts if anyone has a suggestion.

I meant Burr-Brown part, OPA681



|
| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
\--------------=----------------
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1999\02\13@024756 by electme

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try the opamp cookbook.
it has a section on current control using opamps, current scaling etc.
it should put you in the right direction
glen

Sean Breheny wrote:

> Hi all,
>
>         I'm looking for a primer on current feedback op-amps. I can't find
> anything on them in Horowitz & Hill's "Art of Electronics". From diagrams
> I've seen, it looks to me as if they are hooked up rahter similarly to
> voltage feedback ones,but I'd like to know how to properly analyze them.
>
>         I'd like to learn about them in general,but in this specific applicati
on,
{Quote hidden}

1999\02\15@092058 by Matt Bennett

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At 05:03 PM 2/12/99 , you wrote:
>Hi all,
>
>        I'm looking for a primer on current feedback op-amps. I can't find
>anything on them in Horowitz & Hill's "Art of Electronics". From diagrams
>I've seen, it looks to me as if they are hooked up rahter similarly to
>voltage feedback ones,but I'd like to know how to properly analyze them.

I copied a couple of application notes out of an old Comlinear data book,
the description is pretty thorough, and there are lots of good example
circuits.

A much shortened version of the description:

1. Input stage is a unity gain voltage buffer connected across the inputs
of the
op-amp- to force V- to follow V+

2. Amplification is provided by a transimpedance amplifier which senses
current
delivered by the buffer (above) to the external feedback network, and
produces an output
voltage such that Vo =z(jf)In, where z(jf) represents the transimpedance
gain of the
amplifier in V/A or ohms, and In is the current out of the inverting input.

>        I'd like to learn about them in general,but in this specific
application,
>I need an approx. 5 times gain buffer which swings almost rail to rail on
>+/- 5V supplies (or can handle +/- 30V supplies and swing +/- 5 V) while
>driving a 100 ohm load (50 ohms back terminated) and provides output short
>circuit protection, all such that it maintains almost 5 times gain up to
>20MHz. I have found a Linear Tech. part (OP681) that seems to fulfill my
>requirements,but it is current feedback. I am of course willing to consider
>other parts if anyone has a suggestion.

They are used somewhat like an op-amp, where the op-amp is not as "ideal"
as the
traditional op-amp, and the CFOA is missing the internal "roll off"
capacitor.

You can also look for the data sheet to the Analog devices AD811, another CFOA

Matt

1999\02\15@093937 by Harrison Cooper
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Another good source, for many analog design issues, is a publication put out
by Analog Devices, called the "Analog Dialog".  They had a 30th Anniversary
Issue from 1997, that had a two part series on current feedback op-amps.

Might try getting ahold of them and seeing if they either have this online,
or could send you a copy.

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