What is the difference between the gain voltage and the S21 parameter?

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HAJER LAHIANI

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what is the difference between the gain voltage and the S21 parameter?

NB: I find always a diffrent value of gain when I tried the S_param simulation and the AC simulation.
 

s21 gain

S21 is the insertion gain when the source and load impedances are Z0
 

s21 parameters

1. AC Simulation: You get linear small-signal response and you get Noise values.
Analyses includes:
*DC analysis performed (unseen)
*Nonlinear devices are linearized
*Kirchoff’s Law satisfied: sum of node current = 0
*Noise contributors defined and listed
*Budget analysis available (for named nodes)
*Signal voltages are peak - noise voltages are RMS

2 S-parameters are Power Ratios
*S21 - Forward Transmission (gain or loss)


These simulations sometimes introduce similar results, sometimes not and S21 is not voltage gain.
 

s21 parameter

S21 is the square of power gain.the ac analysis is the voltage gain.this is the most significant difference between them. In physics ,we have equation of Power=Voltage*current=woltage*voltage/resistance.so only under special input and output impedance the power gain equals the voltage gain.
usualy the S21 is used in RF designs in which the transmitted or received power is more important.the AC gain is used in Analog circuits,the small signal amplifying characteristic is more important.
 

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