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S parameter help required

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Abhishekabs

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what is the difference between large signal s parameter &
small signal s parameter??
S parameters depends on matching of ports
when we say large & small signal we talk about strenght of signal
& not matching.
please solve my confusion
 

A device's S parameter (for example amplifier) depend on the the signal strength, for example.FREESCALE'S device MHVIC915,when the output power is not the same, the S parameter is not the same, then the matched network is not the same
 

Hello Abhishekabs !

There's not difference between the small / large signal S-par.
This is due the fact the the S-par. are ratios value (dimensionless or in dB),
and they're not depnds on the amount of the signal.
They represents how much of the signal (large or small) went out from the port or reflected back from it (wasted and transmitted resp.).
This simple definition holds to the "matching" concept as well.

Let me give an example:
If we want to transmit, say 1 mW, but half reflected (ant half transmitted) so we say that the matching is 3dB (becuase 0.5 mW / 1 mW = 0.5 -> -20log(0.5)=3 dB).
Note that this is not depends on the quntity of the signal (small / large) but only to the ratio.
For other explanation and further discussion see the edaboard link:
 

Hi,
I think you both are right ...
I just try to give you my feedback:
definition of S-par is the same @large/small signal but the way you can use them changes; you can use small signal S-par if, for example, you are feeding an amplifier with a small signal that is typically inside device linearity and can use large signal S-par for the specified feeding condition.
BR,
Max
 

I found that S parameter depends on nonlinearity of the device.
But still i have doubt that why it is an issue while designing oscillator.
we always found in the book that oscillator is designed from small signal or large signal S parameters.
Datasheet of transistor do not comment about it they provide only one type.
 

Hello Abhishekabs,

The Large Signal S-Parameters are used to compute the equivalent of an S-parameter under large signal excitation conditions. The computation requires that the "From" port or input port be an excitation port.
For example, the measurement of the complete set of S-parameters for a two port would require two separate simulations where one simulation places an excitation on port one (for measuring S21 and S11) and the second simulation places and excitation on port two (for measuring S12 and S22).
This measurement also allows the selection of the harmonic frequency used for both the "From" port and the "To" port or output port, enabling the measurement of quantities like conversion loss or gain in mixers and multipliers.
This measurement can also be used to measure a large signal output return loss using two sources....


---manju---
 

Hi,

small signal S parameters are linear matrix parameters of a microwave network.

When working with non-linear networks the harmonic must be taken into account. Then every S parameter of the linear network is becoming a matrix itself relating the harmonic responses for all harmonics considered by the analysis. Sometimes there are very few non-linear S parameters that are sufficient to be used to describe accurately the circuit (oscillator is the typical example - only one), sometimes many harmonics have to be taken into account (mixer).

flyhigh
 

Small signal S parameters are derived from an active device which is operated in linear region. Large signal means nonlinear and more harmonic components.
 

company people provides datasheet for their transistor say GaAsFET with S parameters it is common for that perticular model.
Each device(transistors from same lot) has its own characteristic. so S parameters should vary from one fabrication lot to another .
it does not happen
can any one tell me how much tolerance is allowed in S parameter,
so that it will not affect design???
 

S paprameter is a method to figure out the out and input power

just to correct (itzikhaim)
-20log(0.5)=3 dB). it is 6 dB NOT as you wrote
 

The S parameters from an active device are based on the bias point. Different bias will make different S parameters for the same device. It's better for you to make your own measurements and find the average values.

Added after 4 minutes:

The S parameters from an active device are based on the bias point. Different bias will make different S parameters for the same device. It's better for you to make your own measurements and find the average values.
 

Measuring S parameter is not that easy. It requires special expensive equipments & jigs. It also requires experience & technical expertise to carry out S parameter testing.

hence only people with skill & equipments can do it.
 

When s-parameter is measured, it's assumed that operating point of the active device is constant.But in reality this is not so right.

Large signal s-parameters are used to characterize this phenomena and measurements of them are always difficult but in simulations, they can be used.

But in any case, nonlinear behaviours are not expressed with s-parameters because they are linear.Instead, large signal spice or similar models are used.
 

S parameters are totally depend on nonlinearity of transistor.
 

Abhishekabs said:
S parameters are totally depend on nonlinearity of transistor.

Indeed ???

Can you proof that ???
 

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