Sambuchi wrote:
Wow what a great response Mr Al!
I see what you are saying and now have a better idea of whats going on.
Soo.. how does the slew rate relate,or does it, to the gain bandwidth?
Hi again,
Well thank you
The gain bandwidth says something about the internal gain, the way it drops off with frequency.
The slew rate will tell you how high a frequency (sine wave) can be passed without distortion
at a given amplitude. The formula would be:
Freq=vpus*1e6/(2*pi*A)
where
Freq is the frequency in Hertz,
vpus is the voltage per us spec for the op amp,
A is the amplitude of the output (peak),
pi is the constant 3.14159265 approximately.
For example, say we have an op amp who's slew rate is 0.5v/us (like the LM358) and the output
has to change by 2 volts peak (sine wave). The maximum frequency we can get away with
would be:
Freq=0.5*1e6/(2*3.14159265*2)
where we made the volts per us equal to 0.5 and the amplitude A equal to 2 volts, so we get:
Freq=39789Hz
which is close to 40kHz.
If we try to go over 40kHz we will start to see distortion. If we try to increase the amplitude of the
output (by increasing either the input voltage peak or the gain of the circuit) we will also see some
distortion because amplitude is in the equation too.
Since you seem to want to be able to amplify a 50ns pulse and that represents a 20MHz signal,
you would have to go to at least three times that to 60MHz but even that probably wouldnt be
good enough depending on the accuracy you need.
For a 1v rise with a 0.5v/us op amp it takes 2us which is 10 percent of 20us, so that would work
if a 10 percent error in rise time is acceptable. The formula would be:
vpus=V/Tr
where
vpus is volts per us required,
V is the rise in voltage needed, and
Tr is the rise time needed.
Now for your app if a 10 percent rise time is acceptable that would mean you need to rise in 5ns which
is 0.005us, and say you only have to rise by 1 volt, then we have:
vpus=1/0.005
which of course means the volts per us spec would have to be 200 volts per us (200v/us).
You'd have to find an op amp that can do that.
If the voltage has to rise up to 5v instead of 1v then you would need 5 times that spec, or 1000v/us.
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