Well, if your output stage on itself has 5% distortion, the 'predistortion' at the output stage input is also about 5%. You decide what is 'shocking'.
Jan
Believe it or not, I haven't heard it explained like that before! That is what I'd call shocking
sooooo obvious What many people find most 'shocking' about the intermediate stage signal is that it can sometimes include a roughly differentiated version of both the input signal and the distortion. This is caused by the HF rolloff in the forward gain causing an HF boost in the intermediate signal. Whether you see this depends on exactly where you look.
As a simple example, try a humble 741 open loop. Puajjjj, buy new ears before listening.
Well, OK, accept the joke. Butinject to it, say, 10mV of sine signal.
Or try a powerful TDA2003 open loop.
Saw a nice demonstration of feedback (and it limitations) today, here at the beach. You know these hoverboards (well they really are a Segway minus the stick), you just step on it and by leaning forward or backward, or sideways, you steer it. The internal negative feedback attempts - and succeeds! - to keep the center of gravity above the center of the board. They are so smooth, and completely still when not moving, if gives you a great appreciation of what feedback can do when well implemented.
But today it went wrong. One unit was ridden by a very obese girl with most of the weight located around and above the waistline. I saw her several times just keel over, for no apparent reason. She would smoothly move past where I was having my beer, then all of a sudden the system would slow unexpectedly and she would drop to the ground. Several times.
I have been thinking about how this could be and I think maybe the point of gravity was too high and the current slew limit to the motors was exceeded. But maybe it is something else, I don't really know. But very interesting (except for the girl).
Edit: could also simply have been an fault in the control unit of course.
Jan
No that's not what I mean. Both really will only output DC. Even with no input. Really.
Jan
No, my friend. It gives a pretty fine square wave. Several decades ago, I used an early At386 to decode RTTY transmissions over SW band. I received it via a SW tube receiver, and a simple 741 open loop receiving it signal, and powered from ±12V from the serial port acting a simple squarer. Then, a DOS program called HamCom decoded it via software. I listen (Bah, really read) lots of info via this method. Oh, pretty old times....
http://www.vu2fd.com/projects/images/hc/hamcom.pdf
http://www.vu2fd.com/projects/images/hc/hamcom.pdf
I imagine that!
The good part îs that durrig these some hundred on nanoseconds the input signal îs not changing to much because a low pass input filtre and because a frequency range of the input signal.
If you are not agree than please show a RF multistage amplifier with a global feedback.
You didn't understand his post. If almost everything in the world depends on feedback, jet fighters not crashing, tankers remaining afloat, nuclear plants not exploding, people not stopping to breath, everything literally depending on feedback, how can you then say 'it doesn't work'??
Jan
I did not say that.
If that was understood then it was not understood well.
In my last post I even explained way it still work.
The main idea is that the system and his feedback should be fast enought (maybe by ten's time faster ) that any perturbation that can apear.
If the perturbation is faster than the system, the result can be unexpected as in the example with the hoverboards when not the feedback but speed of the system was not enough to compensate the perturbation.
I'm not a fan of the no feedback amplifiers. I'm in searching of the fastest but stiil stable amplifiers.
If that was understood then it was not understood well.
In my last post I even explained way it still work.
The main idea is that the system and his feedback should be fast enought (maybe by ten's time faster ) that any perturbation that can apear.
If the perturbation is faster than the system, the result can be unexpected as in the example with the hoverboards when not the feedback but speed of the system was not enough to compensate the perturbation.
I'm not a fan of the no feedback amplifiers. I'm in searching of the fastest but stiil stable amplifiers.
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It's a good job they haven't far to go, is that why we have super short feedback loops a la some of the more "minimalist" gainclones?
> Speed of electrons in a wire is very low
We do not care about electrons. (There is a book which asserts they do not exist.)
We work on the electric field, which mostly moves near speed-of-light.
We don't deal with S-o-L in most electronics because Grace warned us. When she held up foot-long wires, computers were room-size, but far slower than nanoseconds. (My father was there.) As computers sped-up, they also shrank. The guy who came closest may have been Seymour Cray. Gate-chips packed close together in a circle, laid out so no critical path had to go very far. By contrast, a 11-inch NFB loop in an audio amplifier adds about 0.1% to the delay due to capacitances.
We do not care about electrons. (There is a book which asserts they do not exist.)
We work on the electric field, which mostly moves near speed-of-light.
We don't deal with S-o-L in most electronics because Grace warned us. When she held up foot-long wires, computers were room-size, but far slower than nanoseconds. (My father was there.) As computers sped-up, they also shrank. The guy who came closest may have been Seymour Cray. Gate-chips packed close together in a circle, laid out so no critical path had to go very far. By contrast, a 11-inch NFB loop in an audio amplifier adds about 0.1% to the delay due to capacitances.
When she is about to take the cable out of sac/bag I heard she says "microseconds" and "984 feet long".
Basicly that is the same with speed of light.
Depends on what you mean by "faster". Most audio circuits have an open loop bandwidth which is much smaller than audio bandwidth. They work fine. Problems come when the genuine delay is large compared with signal times. This is why RF people often have to use predistortion. The crazy thing is that they would love to use NFB, but they can't, yet ignorant audio people then want to copy them and use predistortion when NFB would work much better.sesebe said:
Stability is what matters. Ideally, a vehicle should gracefully and safely fail when it cannot cope - not dump its passenger on the floor.
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