LvW said:
I´m sorry but most statements of your reply are definitely false.
Quote: again, it depends. there are two ways to define slew rates: SR=dv/dt. that by definition, is a small signal measurements.
No, this is not true. The formula is correct, but it has to measured under large signal conditiones. Otherwise it is a measure of the small signal rise time.
dv/dt is by definition an infinitesimal (read: infinitely small) change of voltage over an infinitesimal change in time.
if that doesn't constitute a "small" signal definition, what else does?
what you described is delta v / delta t: a finite change in voltage over a finite period of time.
this is just basic calculus. so if you agree that SR=dv/dt is correct (which you did), you have to disagree with yourself about whether SR is a small signal or a large signal definition.
Quote: in practice, most slew rates are measured with the time the output swings to 90% of its full output (aka when the amp settles), and the input is typically a pulse / step function.
According to the formula above dv/dt is the slope of the opamp response - independent on the 90% point.
a "slope", sir, is a small signal definition because it was talking about dv/dt.
the threshold is needed because almost all circuits require some settlement time.
Quote: as to what constitute a "large signal", there is no generally accepted consensus: should an amp swing to 90/80/50/20% of its maximum output? with what kind of load? etc.
I have mentioned already what "large signal" means. I repeat: It does NOT depend on any output level, but instead on the input stage condition. The first stage must go for a short time into saturation - until the feedback works and brings it back to the linear region. This effect determines the slew rate,
your are talking about group delay. read into TIM-D will help you in that regard.
Quote: I am not sure what "100%" negative feedback means: gain=+/-1?
This question touches basics of the feedback theory. 100% feedback means that 100% of the output are fed back. It´s quite logical. For an inverter with a gain of "-1" only 50% are fed back (opamp basics).
first of all, there are plenty of amps whose SRs are measured at anything over than gain of 1, +/-.
and 2ndly, there are plenty of amps who are not unity stable and cannot possibly work at unity gain.
your approach would have yielded no-meaningful SRs for those amps.
Partly, this is correct. Slew rate is specified sometimes for different test conditions (for example upwards/downwards).
very rarely SRs are specified on two directions.
typically, they are specified at different supply voltage, into different load, or under different gains, etc.
But always with feedback and large input signals (see above).
it is done with feedback because few amps, especially opamps, are used open loop.
on the 2nd point, you have to be able to separate how something is defined in theory and how that same thing is measured in real life. I said very early on that SRs are indeed measured in with large signal because of the difficulties in measuring it small signal - you would be measuring the slope of the waveform.
don't think so? take our your college calculus book and work out the formula for the minimum SR to reproduce a sine wave.
you can get some help from "Opamp application handbook" by Walter Jung. this very issue was talked early in the chapters.
OK, I cannot resist. the max slew rate for a sine signal V=Vp*sin(2pi()ft) is 2pi()fVp, which happens when t=n*/(2f), where n=0, 1, ......, pi()=3.14......
that signal will reach Vp, its max voltage, at t=1/4f. so the "large signal" SR would be 4fVp<2pi()fVp.
in other words, if you have an amp that can reproduce Vp in time, it wouldn't be fast enough to reproduce the sine wave.
hope this helps.