# Choosing voltage opamp - Page 2

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Re: Choosing voltage opamp

Priceless. Where does the extra magical voltage come from?
You've described gain here, but failed to take into account that the
opamp has supply rails of +-5V, and hence its output will clip when it
gets near this value.

Re: Choosing voltage opamp

Dana wrote:

Not with +/-5V rails he won't.

Dave :)

Re: Choosing voltage opamp

a
you

So help me out here. If he has 5 and -5 rails, , and an input sinewave comes
in, so lonag as the op amp is properly biased, will not the output swing
between 5 and -5 (not exactly 5 do to loss), in essence giving a 10vpp
signal depending on how the output is referenced to ground.

Re: Choosing voltage opamp

Only rail-to-rail-output opamps come close to the rail
at the expense of some other parameter. Meaning RR-Output
is not exactly free.

Rene
--
Ing.Buero R.Tschaggelar - http://www.ibrtses.com
& commercial newsgroups - http://www.talkto.net

Re: Choosing voltage opamp

comes

True. But I think Dave was saying that with a +/- 5v supply, the output
would only be able to get a 5v p-p signal. I would think it would get close
to both the 5 and -5 supply giving close to a 10v p-p output.
Like the original poster mentioned when he observed an approx 8v p-p output.
And back to the original poster, yes if he required an absolute minimum of
10v p-p out, yes his rail supplies would have to be in line with that
requirement.

Re: Choosing voltage opamp

I was not saying a +/-5V rail will give you 5Vp-p, go have another
A regular op-amp will only give about 8Vp-p as the OP found out.
Only (output) rail-to-rail opamps will get close to the rail voltage.
Normal rule of thumb for ordinary op-amps is you design for a 2V margin
on the rails, unless you inspect the datasheet and can take it further.

Dave :)

Re: Choosing voltage opamp

Dana wrote:

Most op-amps clip the signal when the output is within 2 volts (or so)
of the + or - supply.  Hence the need for special "rail to rail" op
amps, which can get much closer to the supply voltage before clipping
(tens or hundreds of mV?)

The problem is worse the higher the output current, and the OP's desire
for 200 mA is a high current for an opamp.

Mark

Re: Choosing voltage opamp

It depends on the particular device.
As a general rule of thumb an opamp can get to within at least 2V of
either rail, so a +/-5V supply will give you at a minimum +/-3V. Your
opamp could aboviously get within 1V of the rail to give you 8Vp-p.
There are "rail-to-rail" opamps that claim to get very close to the
full output voltage rail, but usually not at any significant output
current.
You need a higher rail voltage regardless of the device you choose.

Dave :)

Re: Choosing voltage opamp

You should listen to David, he knows what he's saying - even the very best
of rail-to-rail op-amps won't go close to the supply rails, except if
they're very lightly loaded, 1M etc. They still usually fall short of the
negative rail by 5-50mV and 0.5-1.5V short of the positive rail.
... Johnny

Re: Choosing voltage opamp

OK, I doublechecked the requirements, I don't have to get the exact
10Vp-p. Maybe I was getting such high output voltages (for OPA657)

Jamie wrote:

But that would kill my BW (the BW on that is only 4MHz, and I need
40MHz), right?

redbelly wrote:

I have broadband light source w/ rapid scanning optical delay line
(RSOD) and phase modulator. Are you suggesting this would affect the
required slew rate?