DC vs AC Parameters: Understanding the Difference

[Godwin Kessy]

Hello... The concept that resistance can be for a dc supply only and not for the ac supply coupled or superimposed to the same circuit, is something i don't understand to this moment... I first realized this in diode systems... where i saw bulk resistance for DC and dynamic resistance for AC... This doesn't get into mind at all.. they were both derived from the same I-V characteristic curve but bulk resistance utilized the curve made to diodes third approximation, while that of AC was obtained from the normal real curve nature of diode I-V characteristics...
Help on this...??

 

[crixus]

I didn't get "the curve made to diodes third approximation". Could you clarify...

 

[truesearch]

I have met this when dealing with diode curves of I against V. To get a particular current,I, requires a particular voltage,V. and it is logical to use this V and I to specify a resistance R = V/I, this is called DC resistance. Because the graph is a curve this value of resistance is only meaningful for the values of I and V given.
If the diode is operating at this point and the current changes by a small amount, ΔI, this will produce a small change, ΔV, in the voltage. The 'resistance' specified by ΔV/ΔI shows how the diode responds to a 'changing' I or V and the resistance is given by the gradient (or 1/gradient of I against Vgraph). This is called the 'AC' resistance because it shows what happens when changes are concerned.
You could meet it whenever the graph of I against V is not a straight line such as transistor characteristics and thermistor curves.

 

[jim hardy]

Hello... The concept that resistance can be for a dc supply only and not for the ac supply coupled or superimposed to the same circuit, is something i don't understand to this moment...

seems to me you explained it pretty well in your last post.
Resistance is not constant , as with normal linear reststors.
It varies with applied voltage and with temperature, too. Even normal resistors have a temperature coefficient.

diode: i = e ^(qv/kt))

so r = v/ (e ^(qv/kt)) which is a logarithmic function of voltage not a linear one

Which means you really need to specify at what voltage you assigned a value to r. Or at what current.

algebra will serve you better than words on this one.

Perhaps your difficulty stems from unfamiliarity with small signal AC amplifiers? They operate on small changes in signal , with DC typically blocked by capacitors. So it is the 'AC resistance' that's of interest .

old jim

 

[truesearch]

On the strength that a picture is worth a thousand words here is what I have met when the terms DC resistance and AC resistance have a meaning.
Strictly speaking it is not AC, it is fluctuating DC but the principle is the thing.

 

[Godwin Kessy]

I didn't get "the curve made to diodes third approximation". Could you clarify...

It's the graph that, approximated the diode as a switch, plus a resistor, which assumes the diode to be an open circuit until the voltage across is above the knee voltage... And at this voltage greater than the knee voltage... the diode acts as a normal resistance( the one called bulk resistance) for a diode...

 

[Godwin Kessy]

Okay.. thanks guys, but why we find that the dc resistance is not even an approximate of the ac resistance, from the graph and derivation it's clear that, we use some approximation with DC(Assuming straight line instead of a curve), but for AC we even utilize calculus, these values where supposed to be close to one another... Am i wrong??