How to measure the input reactance and output reactance of a MOSFET/gain stage

1. How to measure the input reactance and output reactance of a MOSFET/gain stage

Hi, I found relative answers to the question but I want a little clarity.

I mentioned it as reactance as a general case but I know it is mostly capacitance. How do I find input/output capacitance of a gain stage/MOSFET/Network?. Usually it is the imaginary part of the total input/output impedance. So If I do SP simulation and plot the imaginary part of ZP from direct plot, It will give me the input/output capacitance of a gain stage. Correct? But why do some people use YP from direct plot for this purpose. I just want to know which parameter to use (Say ZP) and what modifications I should make if I use the other parameters(Say YP).

P.S I know if the network has series connection it is easier to use Z parameters and If it is parallel connections it is easier to use Y parameters. But If we consider the network as a block box we wouldn't know how the connections are inside.  Reply With Quote

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2. Re: How to measure the input reactance and output reactance of a MOSFET/gain stage Originally Posted by circuitslave So If I do SP simulation
You should not use S-parameters, since you can not understand RF. Originally Posted by circuitslave and plot the imaginary part of ZP from direct plot,
It will give me the input/output capacitance of a gain stage.
Correct?
Open state. Originally Posted by circuitslave But why do some people use YP from direct plot for this purpose.
Short state.

Consider a definition of Z11 and Y11. Originally Posted by circuitslave I just want to know which parameter to use (Say ZP)
and what modifications I should make if I use the other parameters(Say YP).
Surely learn a definition of Z and Y parameters. Originally Posted by circuitslave But If we consider the network as a block box we wouldn't know how the connections are inside.
It is very easy linear algebra whatever you use as 2-port parameters.  Reply With Quote

3. Re: How to measure the input reactance and output reactance of a MOSFET/gain stage Originally Posted by circuitking Hi, I found relative answers to the question but I want a little clarity.

I mentioned it as reactance as a general case but I know it is mostly capacitance. How do I find input/output capacitance of a gain stage/MOSFET/Network?. Usually it is the imaginary part of the total input/output impedance. So If I do SP simulation and plot the imaginary part of ZP from direct plot
No.. If you are interested in the input impedance, look at ZM1 and for output impedance, look at ZM2. Not Z parameters. V1 = Z11*I1 + Z12*I2. If you only look at Z11, then you don't see the effect of Z12 which is the effect of the feedforward capacitance (aka Cgd). ZM1 captures both of them.  Reply With Quote

4. Re: How to measure the input reactance and output reactance of a MOSFET/gain stage

Hi,

In my eyes the miller effect makes the system unlinear, you see this in the voltage plateau of V_gs.
Thus you can't apply S parameters.

You need to input power to get the gate charged. ... but you don't get the whole power back, because all the power for the miller effect is dissipated within the Mosfet.
It is a lossy unlinear system, depending on V_DS...and can't be simulated with an R C circuit.

Klaus  Reply With Quote

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5. Re: How to measure the input reactance and output reactance of a MOSFET/gain stage Originally Posted by vivekroy No.. If you are interested in the input impedance, look at ZM1 and for output impedance, look at ZM2. Not Z parameters. V1 = Z11*I1 + Z12*I2. If you only look at Z11, then you don't see the effect of Z12 which is the effect of the feedforward capacitance (aka Cgd). ZM1 captures both of them.
Yeah sure.
I used this
1.CinfromY=(1 / (abs(imag((1 / ypm('sp 1 1)))) * 6.28 * xval(ypm('sp 1 1))))
2.Cin=(abs(imag(zm(1 ?result "sp"))) / (2 * 3.14 * xval(zm(1 ?result "sp"))))

There is a lot of difference between two of the results and using 2. the Cin value is not constant over the frequency range. That is the my confusion as to which one to use and the Cgd effect would so much ?   Reply With Quote

6. Re: How to measure the input reactance and output reactance of a MOSFET/gain stage Originally Posted by circuitking Yeah sure.
I used this
1.CinfromY=(1 / (abs(imag((1 / ypm('sp 1 1)))) * 6.28 * xval(ypm('sp 1 1))))
2.Cin=(abs(imag(zm(1 ?result "sp"))) / (2 * 3.14 * xval(zm(1 ?result "sp"))))

There is a lot of difference between two of the results and using 2. the Cin value is not constant over the frequency range. That is the my confusion as to which one to use and the Cgd effect would so much ? The expression for Cin is incorrect.
Xc = abs(1/imag(1/zm(1 ?result "sp")) )
Cin = 1 / (Xc*(2*3.14159*xval(zm(1 ?result "sp"))))

Xc = 1/jwC ==> C = 1/(j*w*Xc)

This Cin is Cgs + Av*Cgd

You can check this value at very low frequencies. Your Cin is in nF which is really high.  Reply With Quote

7. Re: How to measure the input reactance and output reactance of a MOSFET/gain stage Originally Posted by circuitslave Yeah sure.
Are you truely sure ? Originally Posted by circuitslave 1.CinfromY=(1 / (abs(imag((1 / ypm('sp 1 1)))) * 6.28 * xval(ypm('sp 1 1))))
Very wrong. Can you understand imag(1/admittance) ? Originally Posted by circuitslave 2.Cin=(abs(imag(zm(1 ?result "sp"))) / (2 * 3.14 * xval(zm(1 ?result "sp"))))
Very wrong. Can you understand imag(impedance) ? Originally Posted by circuitslave There is a lot of difference between two of the results and using 2.
It is a very natural result. Originally Posted by circuitslave the Cin value is not constant over the frequency range.
It is a very natural result. Originally Posted by circuitslave That is the my confusion as to which one to use and the Cgd effect would so much ?
Both are very very very very wrong.
Simply you can not understand impedance and admittance at all.

Surely learn a definition of Z and Y parameters.

Y11 is a short state admittance.
Z11 is a open state impedance
ZM1 is a loaded impedance where load impedance value is reference impedance. Originally Posted by vivekroy If you only look at Z11,
then you don't see the effect of Z12 which is the effect of the feedforward capacitance (aka Cgd).
Wrong.

Z11 surely includes the effect of Cgd due to intrinsic load. Originally Posted by vivekroy The expression for Cin is incorrect.
Xc = abs(1/imag(1/zm(1 ?result "sp")) )
Cin = 1 / (Xc*(2*3.14159*xval(zm(1 ?result "sp"))))
I don't think Yin=1/zm(1 ?result "sp") is not appropriate, since circuitslave can not understand S-parameters at all.

I don't think reference impedance is matched to actual load resistance. Originally Posted by circuitslave So If I do SP simulation
Show me reference impedance and load impedance.  Reply With Quote

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8. Re: How to measure the input reactance and output reactance of a MOSFET/gain stage

Z11 surely includes the effect of Cgd due to intrinsic load.
Yes.. I see your point. I understand my mistake. Y11 = jw(Cgs+Cgd) (because the output would be shorted and both capacitances would come in parallel). Z11 expression will be longer but it will include the effect of Cgd. I apologize for misleading.  Reply With Quote

9. Re: How to measure the input reactance and output reactance of a MOSFET/gain stage Originally Posted by KlausST Hi,

Thus you can't apply S parameters.

Klaus
Then how do you find the input/output capacitances ?

- - - Updated - - - Originally Posted by vivekroy The expression for Cin is incorrect.
Thanks for correction.
So you suggest I use Zm instead of ZP. To me also Zm makes sense.  Reply With Quote

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