On 07/20/2012 12:33 AM, li...@lazygranch.com wrote:
Are you speaking of slew rate limiting in the strict sense of the word, that is
a current starved input stage due to the presence of a compensation cap? Or are
you using the term slew more vaguely.
I am speaking neither.
If you have a sine of a particular frequency and amplitude, then you
have a known slew-rate, it peaks at 2*pi*f*A, where A is the amplitude
of the sine. As you amplify this signal, the slew-rate will grow
proportionally. Recall that the jitter of a trigger point is noise
divided by slew-rate. This is why we want to increase the slew-rate to a
maximum while adding minimal noise.
Now, as the amplifiers has a gain, to increase the slew-rate by say 5
times, the bandwidth of the amplifier needs to be high enough to support
this, but in order to minimize the added noise, we want to keep the
bandwidth down. This may be best realized by also recalling that it is
the wideband noise at the trigger points which this first-degree
analysis depends on, and the RMS level. A 1 Hz amplifier bandwidth is
nice, but it won't support a high slew-rate...
In a two amp setup the later amp will have a higher bandwidth, but the
noise added of the first amp will also be gained up, so a tigther
bandwidth there will keep its contribution lower. You end up with having
high benefit for low noise amps in the beginning, but as you gain
slew-rate the amplifier slew-rate capability becomes more important over
it's noise properties. It's being balanced by amplifier feedback terms
for both gain and bandwidth. Also, diode limiters will maintain the
output as clipped sine, so we can continue to gain the output for slopes.
Cheers,
Magnus
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