Current amplifying vs. voltage amplifying.

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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.
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.....!


roly94 said:

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!;) ;) ;)
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.
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.


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
About life
The sun
The current....
KWAKMANnl from GROningen
Lowlands;) [joke...];)
Guys thanks tons for the help.You guys are the best.It is gettin' clearer.
Well I have thought this through and have come up with,perhaps,a more apparent and obvious analogue.One doesn't need to refer to configurations of open or closed loading situations with transistors I hope.Merely keeping things at the conceptual level by referring to this water analogue.Here is the more accurate analogue I think.Please,please correct me if I am wrong:
This system involves 4 water tanks altogether.Two 5 gallon carboys(office water tanks ala crystal springs,alhambra,etc.) have water flowing,i.e.,oscillating back and forth between them through a pipe.At time T=0 one tank is completely full the other empty.Once the valve is opened the water flows back and forth continually(hypothetically) with no resistance(resistance is not really necessary in this analogue).In the middle of this pipe there is a butterfly valve that swings on its hinges like a Dutch(saloon) door back and forth with the water flow past it.Instantaneously open all the way in one direction,then at a very small increment of time afterwards open all the way in the other direction.Graphically this butterfly valve generates a sine wave which is the input signal.
The butterfly valve is directly coupled via some mechanical linkage to another larger butterfly valve in another pipe which is between 2 humongous "city" water towers that also have water oscillating back and forth between them.The small butterfly valve CONTROLS the swinging back and forth of the large butterfly valve thus controlling the oscillation of the larger water tank water flow.The 2 large water tanks represent the power available from double ended rails.The small butterfly valve then to the extent that it is directly coupled and controls the larger butterfly valve replicates the carboy water pulses(input oscillation) with much more power available with the large water tanks,hence producing an "amplified" output.The small input butterfly valve is,in essence,then a transitor.It controls the power of the double ended rails,the larger water tanks.
Now here is what I hope is the crux of the matter in making this system more "voltage oriented" vis a vis "current oriented" or current amplifying.
If the input butterfly valve has a spring attached to it with a very high spring constant or stiffness it would take alot of pressure(voltage) to get the input butterfly valve to swing back and forth,hence,the input system here is more oriented toward the pressure of the carboy tanks or voltage.Only a large pressure can get the input butterfly valve to swing back and forth.
Now the opposite.If the spring constant on the input butterfly valve were very slight and a very low stiffness(hair trigger as it were) then it wouldn't take much pressure at all to operate this system.Just the slightest amount of current flow or ripple would dictate things and thus the output system,oscillation of larger tanks, is controlled by a more current oriented system of the input.Thus one could say here the larger tanks amplify the input current.
I profusely apologize for the long winded attempt here.
If this analogue is correct,though, I will have it locked up watertight and perhaps have a way to explain to my tube loving friends just exactly what it is that a tube does vis a vis a transistor(bipolar).Then I can move on to the 3 basic configurations of transistors and later even the parameters.Thanks tons again,Roland.
current vs voltage

Lots of people get confused by these terms. The use of terms like current source as against voltage source, current amplification as against voltage amplification etc are just matters of convenience.

It is sometimes useful in thinking in terms of analogous mechanical concepts.

Voltage is mathematically and conceptually analogous to FORCE, and Current is in the same manner analogous to VELOCITY.

Force x velocity=Power, and Voltage x current = power, you get the idea ?

Now, is there something specific which can be called a Force Source as against a Velocity Source ?

One good example is the muscles of the upper arm of a human being.

have you tried throwing an apple ? I guess most guys can manage to throw it to a distance of 100-200 feet.

Now let us take a 16 lb shot (as in shotput). Try throwing it exactly the same manner as you threw the apple. Chances are that you will drop the shot on to your toe. Those muscles that you use to throw things are great at imparting velocity but not force.

Now let us try to heave that shot exactly like an athlete does. You should be able to throw it to 10-20 feet. Now let us heave the apple in the same manner. The apple is much much lighter. Yet I dont think you would be able to push it to greater than 30-40 ft.

You have been using muscles that can impart great force but not great velocity.

I have been a Naval Officer, so let me use an example that I a familiar with.

A warship, say a frigate can travel the sea at about 30 knots or about 60 kmph with ease. A Tug, on the other hand does hardly ten knots, but it can push and pull a supertanker or an aircraft carrier with ease.

The engines and architecture of a frigate give the characteristics of a velocity source, and those of a tug make it a force source.

Let us what in Diesel Engine can make such a difference ? An engine with thin cylinders and large stroke length get used in frigates. The cylinders in the engines of a tug have larger diameters but short stroke.

Force, velocity both are acted upon in all these cases. Calling something a force source or a velocity source is simply a matter of convenience.

it is similar with amplifiers and power sources.
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.

absolutely correct :)

People do get confused and I think it is because of well intentioned attempts to describe things in simple terms (e.g. "voltage amplifier" and "current buffer"). There are many examples where we use labels for circuit blocks and it's always a good idea to bear in mind that they are often just labels which can help in conceptualizing more complex collections of circuits but are not a 100% substitute for a deeper understanding.
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