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I am trying to design an LNA with input impedance of 50 Ohms, I am considering the topology illustrated below. I have found an online calculator that computes this however when I try to work out the answer by hand I do not get the same answer for the impedance.

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I am very confused because when looking at the small signal according to enter image description here The answers do not match up: Note I am using a NPN 2n3904 transistor, also I am not sure If I am reading the datasheet correctly. My overall goal is to have input impedance of 50 and a decent gain of 20dB.

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  • \$\begingroup\$ Smells like an XY problem to me. Why are you wanting an input impedance of 50 ohms when you could make it high (ish) and put a 50 ohm resistor at the input to give you 50 ohms. \$\endgroup\$ – Andy aka Mar 17 '18 at 21:56
  • \$\begingroup\$ What exactly are you doing? The focus of an LNA is... low noise. And also, almost always, very weak signals that are just above the noise floor. So I mostly see this for satellite dishes and that kind of thing, where the frequencies are way over what a 2N3904 is going to handle. Talk more about what you are doing. \$\endgroup\$ – jonk Mar 18 '18 at 0:33
  • \$\begingroup\$ What about the parasitic capacitance in parallel with Rf; assume 0.5pF. \$\endgroup\$ – analogsystemsrf Mar 18 '18 at 3:29

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