From: matthews@eecs.ucdavis.edu (Thomas W. Matthews)
Newsgroups: rec.audio.high-end
Subject: Re: Amplifier specifications
Date: 14 Dec 92 22:11:00 GMT
Organization: Division of Electrical Engineering and Computer Science, UC Davis
NNTP-Posting-Host: 129.89.7.4
Originator: tjk@csd4.csd.uwm.edu


I'm sure there will be more postings to read on the slew rate issue,
but I'd like to throw in what I know at this time:

"Slew Rate" refers to the maximum rate of change of voltage with time.
It is possible to calculate the maximum rate of change (dv/dt) of a
certain signal (a sinewave for example), and if the slew rate of a
device does not meet or exceed the max. dv/dt of the signal then it
cannot reproduce that signal.  For a sinewave, max. dv/dt= 2 * pi *
freq * peak voltage.  Note that this rate of change may describe a
positive-going change or a negative-going change.

Example:  a 20 KHz. sine wave at one volt peak (.707 VRMS) has a max
dv/dt of 2 * pi * 20000 * 1 = 125.7 kV/sec or .1257 V/microsecond.
Note that one cannot draw a relation between maximum frequency and slew
rate required unless a signal amplitude is given.  Slew rate is not
directly related to bandwidth.  However, a parameter called "full power
bandwidth" implies a signal amplitude (that which results in the rated
maximum output signal), and this parameter can be related to slew
rate.  I have seen full power bandwidth calculated from the equation
above by plugging in the measured slew rate of a device and its maximum
output swing and solving for frequency.

I can't give the reference, but I have heard that an audio designer
might want to try for a slew rate in excess of 10 times the minimum
calculated value.  While an amplifier is slewing, it is essentially
unresponsive to changes in the input; the output is already doing all
it can to catch up to where it should be.  There has been a linking
between slew rate and transient intermodulation distortion (again, I
can't remember the reference; does anybody else have it?).   If an
amplifier has a slew rate far in excess of the max dv/dt of the signal,
then it will still have excess loop gain even when following the
fastest parts of the signal (that means that the amplifier will still
have the benefits of negative feedback).  I think that disregard for
that fact is one reason why some designs have too much transient
intermodulation distortion and hence some people have a bad feeling
about negative feedback.

When comparing slew rate specs, always bear in mind signal level.  If
an amplifier produces 100 Watts into 8 Ohms with a 1 VRMS input, then
it has a voltage gain of about 28.  Hence the amplifier would have to
have  a slew rate 28 time greater than the circuitry driving its input
to maintain equivalent performance.

The usual usages of "risetime" and "falltime" do not relate to slew
rate.  Any bandwidth limited system will not have instant response to
an input.  A simple low-pass filter will exhibit a risetime (the time
it takes for the output to go from 10% to 90% of the swing in response
to a step input).  However, the rate of change of the filter's output
voltage with respect to time is a function of the amplitude of the
input step.  Changes in the input that are within the bandwidth of the
filter still propagate through it, even while it is "rising" in
response to a transient.  If slewing is not present, an approximate
relation can be used:  risetime = 0.35/bandwidth.   Note that true
risetime does not depend on the amplitude of the input step.  The time
it takes an amplifier to slew to follow an input step *does* depend on
the amplitude of the step.

Note that "good sounding" op-amps have slew rates around 20
V/microsecond, and that they're handling signals that are nominally 1
Volt RMS.

Tom Matthews