Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics.

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Re: Not quite..

Posted by Steve Eddy on January 13, 2004 at 17:14:12

In Reply to: Not quite.. posted by Commuteman on January 13, 2004 at 16:32:16:

Your statement is correct for slowly changing signals that are well within the amplifier's bandwidth, but is not true for high speed pulses.

The statement is correct for high speed pulses as well.

Since there is a time delay between input and output of an amplifier, any signal applied to the output will see only the amplifier's open loop output impedance until the signal has traversed the entire forward loop.

Which simply means that the correction signal becomes more and more out of phase with the output signal as frequency increases which reduces the amount of feedback as frequency increases which increases the amplifier's output impedance as frequency increases.

But the salient point is that you have essentially the same situation regardless of whether the voltage at the output of the amp is from the amp itself or some other source.

The correction signal isn't going to make it back to the input of the amp any sooner. The amp's not going to delay it any more.

At that time, a correction signal would appear causing the output to change and attempting to cancel the signal originally applied to the output. If the original signal has been removed (i.e. narrow pulse) I would expect some kind of damped overshoot as the loop re-stabilizes.

But you have the same situation even if the voltage is being applied to the amplifier's output.

This may be moot if we are considering signals that are arriving at the output terminals as a result of a musical signal, since they are (by definition) within the bandwidth of the amp.

Yes, that too. :)

And of course most of the energy will be well below even 20kHz.

se

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