Hey, resistive matching networks work only when the impedance transformation ratio is low (say half to two, this can be calculated exactly but I am just saying). When the ratio is as high as 5 which is the case here (1500/330), you will end up getting negative values for resistances, even if you allow to take say 6dB attenuation.
Agreed. A passive matching circuit has about 12 dB voltage loss in contrast. Also attenuation of possible subharmonic interferences is a good feature.The CE solution that I recommend above is a good choice because provides free gain, provides impedance matching to the IF filter, and also provides a band-pass characteristic using only RC components.
it is the circuit in post#4. The measurement of S11 was performed from the input side while the output was terminated into 330 ohm.Please clarify the exact measurement setup. If this is the post #1 circuit, we can hardly await 330 output impedance.
You Buffer is potentially unstable, it may oscillate.I implemented the CE proposed circuit on a PCB, and tried to measure the S11 using a nanoVNA. I got strange results which I am not able to interpret. I am seeing negative resistance, and part of the smith chart is getting out for a good portion of the frequency scan range.
Using the function generator at 10.7Mhz (100mv P2P) and oscilloscope probing at output, I see 300mv (P2P) and in fact the gain increases till 20MHz (400mV PP) then start going down.The BJT with its Cbe and C2 (emitter cap to gnd) can potentially give negative resistance looking from the base terminal. In fact, it is one of the types of of negative resistance circuits which is built for designing oscillators. Just using a high enough value for C2 (like few nF of cap or even higher if possible) should avoid oscillations or negative resistance.
Practically speaking:You Buffer is potentially unstable, it may oscillate.
Using such buffer will increase the nonlinearity therefore it should be avoidable.
Instead simple LC matching at a single frequency will be a simple and efficient solution.
Power consumption will also be low.
As a freelance, no student, I have no access or way for even trial versions of ADS or Microwave office to run similar RF simulation. I am mainly using LT Spice but obviously not an RF oriented simulator.Looks like is about Miller effect, due to improper transistor used (2N3904).
Placing the same schematic from post #4 in a real RF simulator, and using an RF transistor (BFR93), I got decent S11 (at 1.5k) and S22 (at 330 ohms). Stability K factor is above 1 for a wide frequency range.
Adjusting the DC bias of the transistor gets even better results.
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