[SOLVED] ideal current source input/output/internal resistance

[fateme m]

This is from my textbook: "ideal current source
will have zero input resistance. An ideal current source has a high output
resistance"
I understand that an ideal current source has an infinite INTERNAL resistance, so that the current escapes out of it and move to the load, I just don't get the input and output resistance of an ideal ammeter... Why does the input resistance need to be zero and the output high? what does the internal and output/input resistance of an ideal ammeter exactly mean?? How are they different? I'm confused:-?:-?

 

[KlausST]

Hi,

an ammeter is not a current source. These are totally different things.

***
An ideal current source has infinite output resistance. But the input resistace is not of interest, but surely is far away from being zero.
(a current source may have only two connections = constant current circuit remote powered. Then you can´t differ input and output resistance)

***
An ammeter has only two connections. Therefore only one resistance. Ideally zero resistance.

Klaus

 

[srizbf]

[

There is a mix of references to a current source and then to an ammeter.
Which one is asked for?

 

[fateme m]

You are right, an ammeter and a current source are totally different:laugh:
Why does a CURRENT SOURCE have a high output resistance?
actually this is the complete part that I don't understand in my book: "Since current sensing is best done
with a low resistance, as in for example an ammeter, the ideal current source
will have zero input resistance. An ideal current source has a high output
resistance"

 

[fateme m]

a current source, I shouldn't have said ammeter:| sorry

 

[KlausST]

Hi,

Current source:

An example:
* power supply: 24V
* constant current circuit: 20mA (both give a current source)
* one LED (A) as load with V_f = 2V @20mA
* another LED (B) as load with V_f = 10V @ 20mA

Now we differentiate between "R" and "r".
R is the usual resistance = V / I
r is the differential resistance delta_V / delta_I

With LED (A):
Total voltage is 24V, total current is 20mA, therefore total resistance is 24V/20mA = 1200 Ohms
R of LED is: 2V/20mA = 100 Ohms.
R of constant current circuit is: (24V-2V)/20mA = 22V/20mA = 1100 Ohms
(Test: 100 Ohms + 1100 Ohms = 1200 Ohms. Correct)

With LED (B):
Total resistance is 24V/20mA = 1200 Ohms
R of LED = 10V/20mA = 500 Ohms
R of constant current circuit is: (24V -10V) / 20mA = 14V/20mA = 700 Ohms
(Test: 500 Ohms + 700 Ohms = 1200 Ohms. Correct)

Now to the differential resistance "r".
it combines both cases "A" and "B"
In case (A) the constant current source is 22V and 20mA
In case (B) the constant current source is 14V and 20mA
r = (V1 -V2) / (I1 - I2) = (22V -14V) / ( 20mA - 20mA) = 8V / 0mA --> infinite

***
The truth is: NOT "R" of a current source is infinite, BUT "r".

Klaus

 

[fateme m]

Ok, Thanks for answreing, But sorry it wasn't my question... I think I am being vague... for example imagine a very simple cmos current mirror, the input resistance is almost 1/gm which is small, and the output resistance is high, I mean before drawing the small signal model and calculating the Rin and Rout, for designing such a mirror current source why is it that the less input resistance and more output resistance make it a better current source? I hope I could explain better...

 

[d123]

Hi,
Not sure, probably not this: when I make current mirrors from BJTs, they are not practical current sources for my intentions, unlike the BJT/LED version which is stable with varying loads, because they cannot handle changes in load well. This will be due to my amateurish implementation. I'm guessing - and no doubt anyone who understands theory properly will correct this assumption - high output resistance may be so that the current is constant across loads? Input resistance is minimal to allow current flow into the source as needed without resistance? Probably not.

 

[KlausST]

Hi,

@OP: please provide a schematic, so we can discuss about it.

Klaus

 

[crutschow]

Ok, Thanks for answreing, But sorry it wasn't my question... I think I am being vague... for example imagine a very simple cmos current mirror, the input resistance is almost 1/gm which is small, and the output resistance is high, I mean before drawing the small signal model and calculating the Rin and Rout, for designing such a mirror current source why is it that the less input resistance and more output resistance make it a better current source? I hope I could explain better...

For the purposes of definition, a current-controlled current-source would ideally have a zero input impedance and an infinite output impedance.
A voltage-controlled current-source would have a zero input impedance and an infinite output impedance.

These ideal values are so that the source will not be affected by the external impedances of whatever is driving the source and the output current won't be affected by whatever the load impedance is.

The only place you will find such an ideal source is in a simulator such as Spice.
Practical sources would strive to those ideals but often have impedances significantly different from that.

Thus for example, take a BJT (transistor).
It basically acts as a current-controlled current-source with an input impedance of perhaps a couple thousand ohms and an output impedance of perhaps 50k ohms.
Not very ideal it would seem, but close enough for many applications.

 

[fateme m]

I almost got it, Thanks everyone!

 

[asdf44]

Personally I think the term impedance is overused in scenarios when a couple more words would describe the specific scenario or requirement. It's especially unintuitive when trying to teach a new subject such as a current source.

Current sources regulate current and ideal ones remain constant regardless of voltage. If you follow Klaus's example above that's all "infinite impedance" captures. Infinite impedance means that current doesn't change much in response to voltage just like an infinite resistor maintains the same current (almost zero) regardless of voltage.

What makes this case unintuitive is that current sources have non-zero current while infinite impedance (like resistors) should seemingly have near-zero current all the time (or would require infinite voltage to have any current). The point to grasp though is that impedance in this context is capturing the sources response to change.

 

[crutschow]

Personally I think the term impedance is overused in scenarios when a couple more words would describe the specific scenario or requirement. It's especially unintuitive when trying to teach a new subject such as a current source.
....................

You have a good point. A simple explanation may be more intuitive to a novice.
An ideal current source keeps the output current constant independent of the output load impedance or voltage.
An ideal voltage source keeps the output voltage constant independent of the output load impedance or current.

The output impedance required to do for each of the sources follows from the description.