ccs impendance formula

[fastdata]

Hi
what is the formula for the impendance of ccs

regards
fastdata

 

[SY]

Which CCS?

 

[fastdata]

Which CCS?

Sorry, see attached pic,sorry about drawing

fastdata

 

[SY]

That will be (1/hoe) + (Re)(hfe).

 

[lineup]

Maybe there is a formula.
Knowing the actual values for a CCS, in a circuit,
it should be possible to Simulate / calculate what impedance.

 

[fastdata]

That will be (1/hoe) + (Re)(hfe).

thank you very much, but i dont understand the first part "hoe"
can you please explain what it is

fastdata

 

[SY]

hoe is the output admittance of the device (it's one of the hybrid parameters). It's not on every spec sheet, but it can be found for many common transistors.

 

[CBS240]

Here is a page that shows the AC model with the relationship of H parameters.

 

[AndrewT]

Hi,
one can measure the static impedance.
measure the current that passes at operating voltage. Increase voltage slightly and re-measure the passing current.
[V1-V2]/[I1-I2] = impedance at that current and that voltage.

As one changes the current and/or changes the voltage the impedance varies.

Is there a simple way to measure dynamic impedance?

 

[SY]

http://home.pacifier.com/~gpimm/ccs_performance.htm

 

[Juergen Knoop]

That will be (1/hoe) + (Re)(hfe).

see a decent textbook on this, like Tietze/Schenk, your local university library may have one exemplary
Things are slightly more complicated.
Regards

 

[SY]

Well, yes in that it assumes that Re >> rbe.

 

[oldeurope]

bjt ccs ...

Originally #11 posted by Juergen Knoop

see a decent textbook on this, like Tietze/Schenk, your local university library may have one exemplary
Things are slightly more complicated.
Regards

Hello Juergen Knoop,

read this one, please.

Kind regards,
Darius

 

[Juergen Knoop]

Well, yes in that it assumes that Re >> rbe.

if Re>>rbe then ra=hfe*rce
if Re<<rbe then ra=rce+µ *Re
Regards

 

[traderbam]

That will be (1/hoe) + (Re)(hfe).

The question was about impedance. What about the capacitance?

 

[SY]

That's why I use cascodes.

IIRC, Gary Pimm has some nice plots of impedance versus frequency for various CCS at his website.

 

[Juergen Knoop]

Re: bjt ccs ...

read this one, please.

a literature hint and a formula was given.
There is no need to pursuit it any further.
regards

 

[nigelwright7557]

Even tho the current stays the same the voltage across the CCS varies with the VAS voltage.

So the impedance will vary with voltage.

In a steady state the impedance will be the voltage across the CCS divided by the constant current.

 

[KSTR]

>> In a steady state the impedance will be the voltage across the CCS divided by the constant current.

Some misconception here. Impedance is per definition a complex value in the linearized small signal AC domain. While a shifted operating point may affect the impedance, the large signal values of voltage and current (which *are* just that operation point) are not part of the impedance.

http://en.wikipedia.org/wiki/Electrical_impedance

- Klaus

 

[traderbam]

Right Klaus.
Another way to look at it, is that the impedance is the instantaneous rate of change of voltage with current, or the slope of the tangent to the curve of voltage vs current.

Impedance is, of course, complex as it may comprise both resistive and reactive elements.
Brian