Strange results of S-parameter simulation

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countmybones

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bfp405 unstable amplifier

Hi, I am doing a S-parameter simulation for a wideband LNA. But the problem is the simulation results is far from expectation. I am using Siemens BFP405 transistor. I have used some RF chokes for the DC biasing circuit and two DC blocking capacitors. What is the problem? There is no gain at all. Thank you.
 

s parameter simulation

about the gain , there is no gain , coz the resistor in the base of the transistor , it is very large which will attenuate RF singal very much

i have used ur amp , and changed this resistor with 100 ohm one , and the amplifier begin to give a gain the circuit and the results attached with this message

which this help
 
s parameter of 100 ohm resistor

Thanks a lot, it sure helped me. Now I wonder why is the resistor there in the first place since it will attenuate the RF signal.
 

s-parameter simulation results

usually this resistor is added to ensure stabilty of the amplifier , coz of the transistor high gain , the tarnsistor could be potentially unstable and may oscillate in some resistive loading

so this ressitor is to be added but it sould not disturbe or affect the bias point

i wish this help

khouly
 

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