I believe this is the ac resistance of the current source.OK, the real deal is you want to look at delta V/ Delta I. e.g. Suppose you built a 1 mA current source and with nearly the same components or say a voltage change, you could make that source output 1.1 mA.
So, you will look at the 0.1 mA and the difference in voltage. You could achieve, say 1e12 ohms of output Z, but it's not infinate.
That is interesting. It is not really my question, though but I like to know that.In your first circuit, if you connect calculate the 'total resistance' 1v/1ma you get 1k.
So the current source sort of looks like a 1k resistor.
But wait, we can say the same thing about the voltage source because that also has 1v and 1ma.
So which is it that is the resistor, and which is the source?
Also, if we connect a 2k resistor in series with the two sources, we still have 1v and 1ma for the battery.
But now the current source has -1v and 1ma, and the 2k resistor has 2v and 1ma, so that's 2v across the resistor and -1v across the current source, so now the current source looks like a -1k resistor. So how did the current source go from being a 1k resistor to a -1k resistor?
I am wondering if this is the way we use to prove that voltage source has internal resistance zero and current source has internal resistance infinity.More exactly, if we connect the voltage source and current source to a circuit containing a few resistors and measure the voltage at some node N and measure 2.3 volts, then if we open circuit the current source and measure that same node and see 1.8 volts and then short the voltage source and reconnect the current source we see that same node measure 0.5 volts, and 1.8+0.5=2.3 volts, the same voltage we get with both sources active.
What this means is that the current source acts like an open circuit impedance that has the ability to force a current through the external circuit.
So we really have two completely different animals here, a source is not the same as a resistor, and so they are not handled the same way mathematically either.
To sum up:
1. The voltage source acts like a zero impedance.
2. The current source acts like an infinite impedance.
My confusion is about its definition:>I am really confused about current source resistance.
What are you confused about?
>An ideal current source has infinite reistance.
And the point is?
Yes, I mean incremental resistance.>As my understanding, the resistance said above is AC resistance, Rac.
Rac = dv/di
You say AC resistance. Don't you really mean incremental resistance? That is what Rac = dv/di indicates.
Yes, I mean total resistance.>But how about DC resistance of an ideal current sourrce?
You say DC resistance. Don't you really mean total resistance? That is what V/I means.
The bold part is what I am confused. You said with an ideal current source, the incremental and total resistance are the same but I don't see it.You seem to have a definiton problem. Incremental resistance is a small change in voltage divided by the corresponding change in current. Total resistance is the total voltage divided by the corresponding current. Incremental current is the slope of the V_I curve. A ideal current source has the same value for both incremental and total resistance, namely infinity. Any questions?
Interesting point. I feel a bit relieved now.Too bad you answered so fast. See the addendum to post #6. The incremental resistance of a ideal current source (ICS) is delta V/delta I . Since delta I does not change, its value is 0. So, delta v/0 is infinity. Using Thevenin's theoren for the total resistance of the ICS we have the open circuit voltage of infinity, and the closed circuit current of some finite value. Therefore, infinity/finite value is infinity.
Yes, as above.. Do you now see why the incremental value and total value of a ICS is the same?
Why we can't use the method to measure resistance of current source? Do we have to use Thevenin's theorem to calculate resistance for active devices?You are not measuring the resistance of the current source that way. You are measuring the resistance of the load. You would get the same value if you used a voltage source. If you want to measure the resistance of the ideal current source, use Thevenin's theorem instead.
Ah, with components that their voltage-current characteristics are linear then two types of resistance are the same. I can easy see it now.A V-I diode curve is not straight. Therefore the incremental resistance at a point of the curve is not the same as the total resistance at that point. Just apply the definition of the two types of resistances, and you can easily see the difference.
I know Thevenin's theorem very well but I don't see why Thevenin's resistace is the resistance of current source.As I said before, you are measuring the voltage across the load and the current through the load. That gives you the resistance of the load. You cannot measure the resistance of the current source while it is connect to a load by just measuring the voltage and current of both the load and source when they are the same. Study Thevenin's theorem to see why.
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?