Source follower. Negative S21 why ?

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kavea

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Hello,
I'm playing with J310 in source follower and i would like to know why S21 response of a such amplifier is negative, whereas in theory the power gain should be high ? Biasing is composed of two resistors, gate resistor of 1 MOhm and source resistor of 200 Ohm. Source and load impedances are 50 Ohm, load is decoupled by a capacitor.
 

whereas in theory the power gain should be high
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How do you calculate power gain? It has to refer to the nominal impedance (50 ohm), not the FET input impedance. Considering a voltage gain below unity, 50 ohm referred power gain must be below 1 too (negative dB number).
 

I'm using HFSS Designer to simulate S parameters. Yes, voltage gain is below 1, but current gain should be high, so power gain for me should be more than 1, (S21 > 0 dB). I'm wrong ? Otherwise why we call it an amplifier ?
 

I think your problem is understanding S parameters.
 

A source follower and emitter follower cannot drive an ac couple load less than the R bias on source ( or emitter) without starving the device of current.
 

FvM said:
I think your problem is understanding S parameters.
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You mean that since both characteristic impedances (port1 and port2) are same, S21 is a simple ratio of voltages, so less than 1 ?
 

You mean that since both characteristic impedances (port1 and port2) are same, S21 is a simple ratio of voltages, so less than 1 ?
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Roughly yes. More exactly, S21 is the forward voltage gain, it corresponds to the voltage ratio if load and source impedance are equal to 50 ohms. S21 can be larger than 1, if the amplifier input impedance is high and the output impedance below 50 ohms.
 
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