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Old 23rd July 2002, 06:35 AM   #1
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Default Current amplifying vs. voltage amplifying.

This is a very basic question.I am trying to understand the concept of "current amplifying" vs. "voltage amplifying".I wonder if anyone really understands that...I mean purely at the conceptual level.No need for figures or schematics.
I try to reduce things to the simples.Wittgenstein said that what could be understood could be understood simply.I try to conceptualize things with more palpable and accessible analogues.
Voltage is the big ol' water tank up town.Current is the rate of flow(whatever dimension per unit time).Transistors are the valves,the faucets.I know the water pressure or flow at the corporate doesn't oscillate...back and forth but it isn't a big conceptual step to imagine it does so.A mere extrapolation of the imagination.No biggy of course.
I can't conceptualize what exactly makes something more current oriented,i.e.,current amplifying instead of voltage oriented.Where there is one there is the other.I mean these concepts of voltage and current are of course mutually inclusive.One doesn't exist without the other,hence,taken individually they don't exist.Chimeras...Forgive me.
Oh if this post goes ignored I'll understand.You are engineers not philosophers.I just hate using things without knowing what they are.I guess this is the human disease.Thanks in advance.Roland.
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Old 23rd July 2002, 07:08 AM   #2
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Hi Roly,

Let's have a go at this one.

Voltage is line pressure in the hydraulic analogy.

Current is the flow rate.

So, we can have high pressure, but no flow. That's just a very high tank, couple of hundred feet above ground, with a tiny pipe, the diameter of a drinking straw.

The water will come out with quite a stream, but it won't do much damage. That's because the pipe has high resistance (or impedance if we talk about AC), and the flow is VERY restricted. This is almost analogous to static electricity, or say, a spark plug electrode on a gasoline engine.

Now, replace the very thin pipe with a 6" diameter pipe. Assume our tank high up holds a LOT of water.

If we turn on the faucet, a huge amount of flow results. At a height of 200 feet, the pressure will be around 87 psi at the exit. If you stand in front of this flow, it will knock you over, and quite possible concuss you. This is high voltage, and high current - like a street power line. When they fall to ground, you DON'T touch them!

In the early stages of power amplification, we need to increase the 'pressure' of the signal. So we use small devices, which don't draw much current and which produce little heat, to amplify the voltage at low current, or, put another way, to amplify the pressure at very small flow. This is voltage amplification.

Then we apply this greatly magnified 'pressure' to a device which amplifies flow - usually an emitter follower configuration if we use transistors - and this magifies the flow from a tiny straw to a 6" pipe suitable for driving a loudspeaker.

I hope this carries the analogy towards comprehension.....!

Cheers,

Hugh

www.printedelectronics.com
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Old 23rd July 2002, 07:53 AM   #3
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Talking Re: Current amplifying vs. voltage amplifying.

Quote:
Originally posted by roly94

I try to reduce things to the simples.Wittgenstein said that what could be understood could be understood simply.I try to conceptualize things with more palpable and accessible analogues.
You are engineers not philosophers.I just hate using things without knowing what they are.I guess this is the human disease.Thanks in advance.Roland.
Hi roly94
Ask Gromanswe!!!
Groman this one is for you. Take the challenge!
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Old 23rd July 2002, 08:17 AM   #4
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Default Re: Current amplifying vs. voltage amplifying.

Quote:
Originally posted by roly94
[snip]
Wittgenstein said that what could be understood could be understood simply.[snip]
Grossmann's Law:
"For every complicated problem, there is a simple, easy to understand, wrong answer".

Jan Didden
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Old 23rd July 2002, 08:23 AM   #5
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And then there are just times when the simple answer is correct but just seems to simple and therefore in order to please the academics and political bigwigs, an alternate answer is thought up and taught as truth thereby attempting to hold power over the masses and condemning those who see the real truth in the matter.
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Old 23rd July 2002, 09:20 AM   #6
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Hi Roland

I try to make it simple but still as accurate as possible.

Let's start with DC current sources and DC voltage sources:

A VOLTAGE SOURCE is someting that will output the same voltage (i.e. it's nominal voltage) into whatever load is connected to it. It shouldn't even matter if you source current out of it or sink current into it. To be capable of doing this it's internal resistance has to be ZERO.

Such a device doesn't exist in practice (just imagine to achieve this with short circuit as load !), so dont't try to order one at parts-espress or wherever. There are sources that are able to supply a reasonably constant voltage for loads that vary between an open output and a MINIMUM load resistance. A good approximation (!) to a DC voltage source is a beefy car battery whose voltage drops only very slightly under reasonable loads. Being an accumulator it is also capable of sinking current without causing it's output voltage to rise much.

Now we come to the CURRENT SOURCE. This is a device that is capable of feeding it's nominal current into any load connected to it. While it isn't a problem to feed a constant current into a short circuit it definitely is one to feed a constant current into an open output. The ideal current source has therefore an INFINITE internal resistance. There are some basic circuits that can be used as a current source within certain limits (i.e. between short circuit and a MAXIMUM load resistance).

A casual (and quite philosophical) definition used by our professor was the following: A voltage source is a short circuit that delivers an output voltage while a current source is an interruption that delivers a current.

None of them exist in practice. Every source that delivers electric power to a load is something inbetween both. Some of them behave more like a voltage source and some more like a current source.

For audio circuits DC sources are used as power supplies but to the user the whole "black boxes" look like AC sources (as far as music is concerned; I do of course know that a good power amp is capable of delivering a DC output voltage).

An audio amplifier is usually a good approximation for a voltage controlled voltage source. I.e. it has a reasonably high input impedance (drawing only a small amount of current) and a reasonably low output impedance (impedance means it isn't purely resistive). If you have an indepth look into the amplifier you will see that the circuit stages of the whole amplifier are also controlled sources.

The three basic circuits a bipolar transistor can used with can be regarded as follows (coarse approximation):

common emitter: a current controlled current source (with high voltage and current gain, depending on load of course)
common base: a current controlled current source with a gain of approx 1
common collector (i.e. emitter follower) : a voltage controlled voltage source with a gain of 1


I hope this is quite understandable while not being to simplistic.

Regards

Charles
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Old 23rd July 2002, 12:26 PM   #7
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Lightbulb Cycle of Elements

Hi all ,
River deep mountain high
Tina Turner
What a great person
Water; H2O flows down the mountain
The river
The higher the mountain
The higher the potential
The higher the current
The endless flow of the elements
What causes it?
The sun
When the sun stops glowing
The current stops flowing
We freeze to death
Better enjoy life
Listening to music
Have contact with the Gods
Zeus and Wodan
On top of the mountain
Having closer contact

Always helping friends
Making new friends
Here and on the AudioAsylum
Sitting on Mount Enos
Contemplating.........
About life
The sun
The current....
..........
...
.
--------------------------------------------------------
KWAKMANnl from GROningen
Lowlands [joke...]
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Old 23rd July 2002, 01:41 PM   #8
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Hi Elso

Do you get your dope from the same source as gromanswe or are you a follower of the same guru ?

Regards

Charles
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Old 23rd July 2002, 02:04 PM   #9
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Default Wow! Far out, man

The only thing missing is Eric Burdon going:

"In......the......be-gin-ning........"

Jocko
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Old 23rd July 2002, 02:22 PM   #10
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Talking Do you get your dope from the same source as gromanswe

I think not........ At least I hope not. One gro(ws his own,)man is one more than we need.

H.H.
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