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Old 19th November 2002, 05:18 AM   #1
Optical is offline Optical  New Zealand
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Default What is bootstrapped?

I hear its a patented design of some sort and results in low noise?

is it a circuit of some sort?
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Old 19th November 2002, 05:54 AM   #2
Taco is offline Taco  Netherlands
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I only know the bootstrap method from statistics . And then it is a aproximation of a dataset from a probability model.
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Old 19th November 2002, 05:55 AM   #3
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Default Bootstrap

The name comes from the expression "pulling oneself up on one's bootstraps".
Generally it means that a circuit node pulls itself up by itself (huh? did I say that?).

An example: suppose you have a preamp with an input resistor of 1k to ground. That preamp has then an input resistance of 1K, that's what the source sees as load. Now suppose you disconnect the ground pin of that 1k resistor and connect it to a signal point that has the same level as the input level. The current through that 1k goes to zero (both sides have the same voltage), and the source now sees an effective input resistance of infinity. You have now bootstrapped the input resistor: it has pulled itself up by itself (the input signal).
There are many other cases, and I'm sure members on this forum will be too happy to point it out, but the concept is always the same.

There is also an equivalent case in software: when your computer "boots", it is loading what used to be called a "bootstrap loader", which is a small piece of software that can load itself without having a program to load it in the first place, hence the name bootstrap loader. After that, the bootstrap loader loads the operating system.

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Old 19th November 2002, 09:25 AM   #4
Taco is offline Taco  Netherlands
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Yes was the story about bootstrap not from a famous writer? The story of a guy who pulled himself out of a swamp on his own bootstrap.
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Old 19th November 2002, 09:56 AM   #5
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Default Bootstrap

Yes I think you're right, I think it was the (in)famous Baron von Münchausen. I think he even did that while sitting on his horse, pulling up horse and all.
The same guy that jumped on a cannon ball fired from his own army to go over to the other side, do some reconnaissance, then came back by hitching a ride on a enemy cannon ball.

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Old 19th November 2002, 10:20 AM   #6
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Default Re: What is bootstrapped?

Quote:
Originally posted by Optical
I hear its a patented design of some sort and results in low noise?

is it a circuit of some sort?
It has nothing to to with low noise and I don't know anything about any patent.

A good example is here

Check the 100 µF cap. It serves like a battery if we talk AC-signals. This cap "lifts" up the potential in phase with the audio output.
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Old 19th November 2002, 11:00 AM   #7
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Default Little Niles, big Frasier.

Imagine you are using a crane to raise a load by 10 metres. All you really care about is how far the load is above the ground. As you start to raise it, your friend who has a MUCH bigger crane than you, decides to lift up *your* crane with *his* by the same amount that the load went upward, i.e. 10 metres. When everything has stopped, his crane has done almost all the moving and yours has moved hardly none. What use is that? Your crane has to do almost zero lifting but it is in control of the whole process.The bootstrapping part is where the little crane lifts the load and therefore causes *itself* to be lifted by *ideally* the same distance. I say "causes" because it doesn't actually lift itself, something else does.

In an amplifier cct, the bottom end of a load resistor is allowed to go upward and this point couples to the output stage. The output rail then lifts up the supply rail to the top of this resistor by the same amount that the bottom of the resistor went up. The voltage *across* the resistor has not changed therefore the current through it hasn't changed, so the transistor hanging of the bottom of this lovely load resistor thinks it is connected to an *infinite* resistance (the current doesn't change even though the voltage at the bottom end of the resistor does). Therefore it is in transistor heaven and makes LOTS of gain and LOW distortion.

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Old 20th November 2002, 12:37 AM   #8
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Thinking about it further, it's more like having a spring dangling from your ceiling and you have it pulled in a stationary position halfway down to the floor with a constant 10kg tension. As you pull it further down toward the floor, your friendly next-door-neighbour watches what you are doing and as you move first she co-operates by lowering your ceiling by *an equal amount*. Same goes for when you let the spring go upward.

The result is that you feel no change in spring tension because the length of the spring (theoretically) doesn't change. The good part is that if you now say to yourself "I shall *try* to apply an extra .0001kg to the spring even if my hand has to go beneath floor level" and the ceiling comes down on your head. Then you change your mind and *try* to reduce the spring tension by .0001kg and your ceiling goes all the way to the moon. You now have lots of gain. Next you invite your neighbour over and have tea and biscuits with her while congratulating each other on how clever you both are.
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Old 20th November 2002, 06:24 AM   #9
AKSA is offline AKSA  Australia
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What's a Bootstrap?

I think of a bootstrap this way. It is a means of applying identical voltage changes to the top and the bottom of a resistor to increase the dynamic impedance of that resistor. Effectively, it turns the resistor into a constant current source (or sink).

Let's say we have a 10K bias resistor from the base to ground on a power amplifier. Ignoring the input impedance of the base, which is very high anyway, the source sees a 10K load with respect to ground. This is a stiff load.

Let's disconnect the ground connection on our 10K resistor and hook it to a voltage divider coming off the output of the amp. (A separate resistive divider with the cold end grounded is often used in this situation.) Now, as the input goes up and down at the base, the other end of the resistor also goes up and down equally with it, since the bottom of the resistor is connected to a voltage divider off the output which mimics what is happening at the input.

The interesting thing is that the top and the bottom of the bias resistor are moving up and down in step. An approximate analog in the mechanical world is the Weston Differential Pulley - the endless chain pulley used to lift engines from autos. Thus there is no variation in current through the resistor, which actually defines a current source. Ordinarily, if the current does not vary with changing voltage input, we have a near infinite resistance, since by simple Ohms Law, R = deltaE/deltaI. This is boostrapping, and it's very effective.

The usual bootstrap application is voltage amplifiers in older SS PP designs, such as the Citation 12. Two resistors feed the collector of the NPN voltage amplifier, with a large electrolytic connected between the output of the amp and the midpoint of these two resistors. Thus, as the output of the amp hikes up and down, so too does the potential at the midpoint of the feed resistors. Thus the voltage across the lower of these resistors scarcely changes, leading to a constant current, and bingo, the virtual impedance of this lower resistor becomes very high.

And so the collector of the voltage amplifier sees almost an infinite value of resistance as load, and this in turn leads to the very high open loop gain so necessary for proper operation of a feedback amplifier.

What is not clear here is that the circuit which drives the bootstrap must have substantial power, so must be of a lower intrinsic impedance than the circuit being driven. Furthermore, the HF behaviour of the bootstrap electrolytic complicates things; as the frequency rises, so too does the ESR of the cap, reducing the bootstrap action.

Cheers,

Hugh

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Old 20th November 2002, 10:17 AM   #10
Optical is offline Optical  New Zealand
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Quote:
Originally posted by Circlotron
Next you invite your neighbour over and have tea and biscuits with her while congratulating each other on how clever you both are.
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