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I have simulated a class A RF amplifier. When I simulated the output impedance, it is around 50 Ω for the frequency range that is of interest (50 kHz to 100 MHz), now at the input impedance is around 56 Ω and then it dips, how can I stabilize it to 50 Ω for the entire range?

Regarding the question asked by Tim Williams, I do not know how to ensure the flatness to the output...

Another good point that was raised is that the output impedance was simulated with the 50 Ω attached to it... Without the 50 Ω, the output is different. enter image description here

I am assuming that I will need a matching circuit at the output as well...

I believe maybe a Pi-network can work for this but I am unsure. Also, the characteristic impedance is around 65 Ω, but if I want to split it to a Resistance and Reactance, would that help?

Input Impedance: enter image description here

Here is the circuit in question. I modified it according to GodJihyo. enter image description here

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  • \$\begingroup\$ Do you just need to equalize Zin, or is there anything else to worry about e.g. flatness to the output? Also, did you measure Zout apart from the load resistor? \$\endgroup\$ Commented Aug 31, 2023 at 18:14
  • \$\begingroup\$ One possible problem I see right away is that you have both emitters connected together (putting the same label on two nodes is like connecting them with a wire), did you mean to do that? Also do you mean to have two separate power sources? As drawn you could eliminate V3 and power both transistors from V2. \$\endgroup\$ Commented Aug 31, 2023 at 18:51
  • \$\begingroup\$ Also, you haven't told us what your 'frequency range of interest' is. Is it the range of your simulation, 50kHz to 250MHz? \$\endgroup\$ Commented Aug 31, 2023 at 18:55
  • \$\begingroup\$ @TimWilliams your comments were addressed \$\endgroup\$ Commented Aug 31, 2023 at 20:40
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    \$\begingroup\$ I'm afraid you have too many issues in the circuit as posted. This is a design issue; the solution is to keep trying, and keep reading on how devices behave, how to design amplifiers, etc. You may find Starič & Margan's Wideband Amplifiers illuminating, as well as understanding the MF and HF cutoff characteristics of transistors, and device ratings. \$\endgroup\$ Commented Sep 1, 2023 at 13:52

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