Instability found when cascading microwave amplifiers
  • Norman grody
    0
    I have been trying to construct a radiometer at around 2 GHz. This is more difficult than the ones I constructed between 4 and 22 GHz since there are no available LNA’s at high gain (60 dB). As such I was forced to cascade two rf amplifier, each having about 30 dB gain. However, I find that the direct coupling results in instability resulting in large amplitude oscillations around 2 GHz in addition to a large noise. I was able to remove the instability by placing an isolator between the two amplifiers. The noise still large however. I would like to know if this approach is the best way to cascade rf amplifiers?
    Norman grody Options
  • madengr
    1
    I've done this a few times. You can plot the stability circles and then adjust the transmission line length between the amplifiers to avoid the unstable region. So you'll end up with a forbidden length. Just watch it as things become more unstable as they get cold.

    Of course with that much gain, just put a 3 dB pad between the two, to accomplish the same regardless of phase. With 30 dB gain in the first stage, that will dominate the noise.

    What do you mean by "large noise"? If you terminate the input with 50 Ohms, do you get the expected -174 dBm/Hz + 60 dB at the output? Is the antenna impedance making the first stage oscillate?
    madengr Options
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