# difference between output conductance and output admittance

1. ## difference between output conductance and output admittance

What the difference between output conductance and output admittance?

Looking the equivalent model

I think rd is output admittance, so where is the output conductance?

2. ## Re: difference between output conductance and output admittance

The admittance is the reciprocal of the conductance.

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3. ## Re: difference between output conductance and output admittance

Originally Posted by julian403
I think rd is output admittance, so where is the output conductance?
rd is a resistor. Impossible to be admittance since admittance is measured in siemens.

Output admittance=Output conductance (just in this case) in your circuit is 1/(rd//RD)

Originally Posted by crutschow
The admittance is the reciprocal of the conductance.
I think is the reciprocal of impedance.

4. ## Re: difference between output conductance and output admittance

Let see. Conductance is admittance's real part. So I think there is the difference but the admittance (which in the parametter's table is the module) must be bigger than conductance and I do not know why it's not.
In the ac model which one I must use as rd?

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5. ## Re: difference between output conductance and output admittance

So I think there is the difference but the admittance (which in the parameter's table is the module) must be bigger than conductance and I do not know why it's not.
Did you notice the different measurement frequencies? Both specifications can't be compared at all.

In the ac model which one I must use as rd?
The simple model doesn't help because it ignores any reactant circuit elements.

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6. ## Re: difference between output conductance and output admittance

Did you notice the different measurement frequencies? Both specifications can't be compared at all.
You are right!

The simple model doesn't help because it ignores any reactant circuit elements.
Yes, it's just a simple model which I found in internet. I'm using all capacitance in it. The input capacitance which it's between gate and source, outpu capacitance which it's between drain and source and the reverse transfer capacitance which it's between gate and drain.

7. ## Re: difference between output conductance and output admittance

A realistic RF model will probably have bulk resistance elements in series with Cds and Cdg, resulting in an increasing real admittance component with rising frequency.

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8. ## Re: difference between output conductance and output admittance

Originally Posted by julian403
The input capacitance which it's between gate and source, outpu capacitance which it's between drain and source and the reverse transfer capacitance which it's between gate and drain.
Input capacitance is not Cgs, output capacitance is not Cds. Ciss = Cgs+Cgd, Coss=Cds+Cgd

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9. ## Re: difference between output conductance and output admittance

Oh thank! I forgot that, but I can find it with

Cgd=Crrs where Crrs is the reverse transffer capacitance

Cds= Cos - Crrs , where Cos is the output capacitance

Cgs = Ciss - Crrs , where Ciss is the input capacitance.

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