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'[OT] Op-Amp Capabilities'
2000\02\03@213439 by Anthony Clay

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Hello,

Anthony here again,  I was just wondering if anyone out there knows about
the most current op-amps.  I look in the catalogs and see op-amps that claim
to have "Megahertz of BW".  I am almost positive that this DOES NOT mean
that it can even buffer, say a 10.7MHz signal, much less amplify or filter
one.

Are there any that are capable of operating at 455kHz yet?  How well? Where
can I find info about how to interpret such specifications, (especially on
the net).  Should I find any I will gladly post an article about it on my
website.  I know this is a little OT, but I feel that Op-Amps play a very
important role when it comes to embedded processors such as the PIC, almost
as important as the 78L05 regulator and the 2K pull-up!

Anthony

2000\02\03@221641 by Sean Breheny

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Hi Anthony,

I take it that this is for your spectrum analyzer project? Sounds interesting!

You are correct that op-amps that have significant high-frequency
capability are kinda rare. You have to be careful with the specs, since
many op-amps will quote a "Gain-Bandwidth Product" which is usually in the
megahertz but that does NOT mean that the op-amp will work normally a 1
MHz, but it is supposed to give an idea (roughly) of how much gain you get
as frequency increases.

The main specs you want to take a look at are the "Open Loop Gain versus
Frequncy" graph and the maximum slew rate. You want the open loop gain to
be at least a factor of 10 or so higher than the closed-loop gain you
desire. You must also make sure that the maximum dV/dt of the op-amp output
won't exceede or come close to the maximum slew rate.

Are you looking for exact gain accuracy, or just a decent gain block? If
all you need is some gain and don't care if it is exactly correct, you can
always use discrete transistors or video/IF amplifier ICs. Even these could
probably deliver a few dB gain accuracy with careful design.

However, to return to your original question, yes, there are op-amps that
have pretty amazing capabilities. Maxim makes a few which will give at
least gains of 10 up into the 100MHz range( MAX4100/4101, MAX4106/4107).
The trade off is that they draw several uA input bias current, so you can't
have a really high input impedance, unless you add an input buffer. This is
not an issue for an IF amp, though.

Burr-Brown makes some really nice op-amps such as the OPA655, OPA634/635,
OPA686, and OPA603. The OPA603, while probably overkill for your
application, is AMAZING to me because it is a relatively normal,flexible
current-feedback op-amp which can drive a 50 ohm load to several volts
DIRECTLY at more than 10 MHz. I used it as the output stage in a function
generator. I would seriously consider the other OPAxxx models I listed for
your application.

If you want strict gain accuracy, it will still take careful design because
you need to minimize stray capacitance and as is always true with this
high-BW op-amps, you need to follow the datasheet's instructions as far as
choosing resistor values go. If you don't,it will end up oscillating due to
stray RC-networks being formed.

Don't be intimidated by current-feedback amps. They seem to work better for
high-freq stuff and the differences in using them vs. normal
voltage-feedback amps is minimal. Basically, you just need to use lower
resistor values in the feedback loop.

My guess is that for 455kHz, you could find a wide selection of options
including IF amp ICs AND many op-amps capable of this.

Sean



At 07:30 PM 2/3/00 -0600, you wrote:
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| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
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2000\02\04@033232 by Tom Handley

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  Anthony, there are a `wealth' of high-speed Op Amps out there with
full-power BWs of several hundred MHz. Check Analog Devices, Maxim, TI, and
others. I have a few here for my on-going logic analyzer/DSO project. As far
as current feedback Op Amps, they are easy to design with. A great reference
is Analog Devices "High Speed Design Techniques" book (ISBN-0-916550-17-6).
They were giving them away. Check their web site. Also check their Analog
Dialogue magazine. In volume 30, #3 "Ask the Application Engineer" they
discuss current feedback Op Amps. There are related articles in v29, #2.
Another reference is TI's Application Report (Nov 98, SLVA051)
"Voltage Feedback Vs Current Feedback Op Amps".

  Also look at back issues of Linear Technology magazine and Maxim's
Engineering Journal. Volume 34 has an article on ADC buffers.

     http://www.analog.com/
     http://www.ti.com/
     http://www.maxim-ic.com/
     http://www.linear-tech.com/

  - Tom

At 07:30 PM 2/3/00 -0600, Anthony Clay wrote:
{Quote hidden}

------------------------------------------------------------------------
Tom Handley
New Age Communications
Since '75 before "New Age" and no one around here is waiting for UFOs ;-)

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