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Old 1st March 2010, 06:45 PM   #1
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Default XA100.5 Slew Rate

I'd like to understand how the XA100.5 can have a specified slew rate of 50v/us and yet an upper -2dB frequency of just 100KHz. Probably it is my understanding that is at fault but doesn't a slew rate of 50v/us for 100W output imply a frequency response to approx 200KHz? I base this on the peak amplitude for 100W being 40V and for a sine wave the max frequency = 50,000,000 / 2 * pi *40.

Where am I going wrong? Is it perhaps a case of the amplifier having a frequency response to 200KHz but with a filter at the input limiting the response to an input signal to 100KHz? Would this constitute an amplifier with 50V/us slew rate or would this be described as 25V/us based on what can be achieved from the (filtered) input?

Ignoring for a moment whether this really matters in practice, I remain curious both as to what this means and how it might be achieved. My prototype XA100.5 clone - well more something that shares the same general topology than a direct clone to be honest - seems unable to match this performance. I can just about stretch the response to 200KHz with sufficient feedback but then there is a peak in the response at around 114KHz, presumably due to lack of phase margin. Adding compensation to flatten the response brings the frequency response down to around 180KHz. The associated square wave response shows significant overshoot. Placing a filter at the input to roll off the high frequencies at 100KHz helps but does not completely solve the problem. The measured slew rate under these conditions is about 45V/us without the filter and 25V/us with.

Ian.
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Old 1st March 2010, 06:51 PM   #2
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You can have a high slew rate and a deliberately limited
bandwidth. To get the slew rate number, I look at the
maximum dv/dt I can see with a square wave.

You mileage may vary, of course.

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Old 1st March 2010, 11:31 PM   #3
a.wayne is offline a.wayne  United States
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Hello Nelson ,

What part of the design allows an amp to have a good or bad slew rate and how much is good, bad, enuff ?

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Old 2nd March 2010, 01:28 AM   #4
wensan is offline wensan  United States
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The "small signal bandwidth" and the "power bandwidth" are two total different things.
The "small signal bandwidth" is limited by stability.
But the "power bandwidth" is limited by slew rate and maximum output voltage.
They don't relate to each other directly.

Usually, when we say "bandwidth", it means the "small signal bandwidth".
Because we must designate the maximum output voltage,
Then we can calculate the "power bandwidth" by slew rate.

So, an amplifier's "small signal bandwidth" may higher or lower than its "full power bandwidth".
If an amplifier is working at a frequence higher than its "power bandwidth",
then the SID(Slew Induce Distortion) will occur because of slew rate limmit.
That means an amplifier's "small signal bandwidth" lower than its "full power bandwidth" will be better.

Last edited by wensan; 2nd March 2010 at 01:31 AM.
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Old 2nd March 2010, 04:17 PM   #5
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Quote:
Originally Posted by Nelson Pass View Post
You can have a high slew rate and a deliberately limited
bandwidth. To get the slew rate number, I look at the
maximum dv/dt I can see with a square wave.
In the case where you have a higher slew rate than a deliberately limited bandwidth, how do you measure the maximum dv/dt? Won’t this be impacted by the mechanism used to limit the bandwidth? Is it a case of taking the dv/dt measurement without the deliberate limitation in place?

I am using the same method to measure slew rate (rise time of the leading edge of a 10KHz square wave in my case) and I see around 1.6us at full output with feedback but without any compensation and around 3us with the bandwidth limitation in place. I believe these correspond roughly to 50V/us and 25V/us in my case.

Ian.
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Old 2nd March 2010, 10:12 PM   #6
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If you don't want to do anything elaborate, just clip the input
with a square wave.

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Old 3rd March 2010, 07:22 PM   #7
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Fascinating - I would never have thought to try something like this! Just ran some tests on my prototype and it does indeed work. Not that I doubted it would but I'm not entirely clear on how it does so. Anyway, I now have a really easy way to measure slew rate. Many thanks as always Nelson

Ian.
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Old 4th March 2010, 12:45 AM   #8
wensan is offline wensan  United States
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There are two ways to limit the "small signal bandwidth" of an amplifier,
One is put a low-pass filter in front of the amplifier,
The other is put a compensation capacitor in the feedback network.
Put a low-pass filter in front of the amplifier will slow down the input signal to prevent the amplifier driving at full speed.
Put a compensation capacitor in the feedback network will not slow down the input signal, so the amplifier still can drive at full speed.
That is the difference between them.
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Old 4th March 2010, 02:34 PM   #9
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Thanks for your input wensan and apologies for not replying to your previous post.

The low-pass filter at the input of an amplifier I can understand. High frequencies are blocked from entering the amplifier and hence cannot appear at the output.

A compensation capacitor in the feedback network is more difficult to grasp intuitively. This still acts to reduce the high frequency response of the amplifier, except in this case by increasing the feedback with increasing frequency. Although a high speed input signal can enter the input of the amplifier, the output will be attenuated which feels like a similar result to the other case. What does driving at full speed mean in this case?

Ian.
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Old 5th March 2010, 02:01 AM   #10
wensan is offline wensan  United States
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The slew rate limit of an amplifier is caused by the transient overload(or saturation) of the input stage.
An amplifier "driving at its full speed" means when input singal's slew rate too fast cause the transient overload of the input stage happen, the output singal of this amplifier will slew at its maximum slew rate.

Last edited by wensan; 5th March 2010 at 02:04 AM.
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