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?

Ignore this advice if the amplifiers are in coax packages....If you are talking about cascading MMIC chip amplifiers, 60 dB gain is enough so that a little radiation from wirebonds or microstrip can cause positive feedback and then oscillations (You need to have some space between them and preferably channelize the signal path with small enough cross-section to cut off any waveguide mode where gain is available (not a problem when you are at 2 GHz!) In a pinch some RF absorber over the top of the MMICs can do wonders, but I believe that if you put absorber over the first MMIC you might degrade the noise figure. Good luck and let us know how you solved this issue... even better if you supply some photos and text for a Microwaves101 page!

I was able to cascade two high gain amplifiers, one with a gain of 20 with a noise figure of 0.5 db and the 2nd stage with a gain of 55 and a noise figure of 5 db. Both amplifiers were obtained from EBay for less than $30. The problem I had regarding stability and oscillations were removed using a 3 dB attenuator between them. However, I found a problem after connecting a dual-Schottky diode envelope detector ( obtained also from EBay) to the 2nd stage amplifier. Unfortunately, output from the higher gain amplifier produced oscillations again. Fortunately, however, this problem was resolved after using a tunnel diode detector.

I was able to cascade two high gain amplifiers, one with a gain of 20 with a noise figure of 0.5 db and the 2nd stage with a gain of 55 and a noise figure of 5 db. Both amplifiers were obtained from EBay for less than $30. The problem I had regarding stability and oscillations were removed using a 3 dB attenuator between them. However, I found a problem after connecting a dual-Schottky diode envelope detector ( obtained also from EBay) to the 2nd stage amplifier. Unfortunately, output from the higher gain amplifier produced oscillations again. Fortunately, however, this problem was resolved after using a tunnel diode detector. I wish I knew why the use of the 3 dB attenuator and tunnel diode approach worked.

Just a thought. A sneak path seldom considered is the DC supply lines. Signal from the output after 75 dB of gain can sneak back to the input if isolation is less than 75 dB. The 3 dB pad could be just enough to bring a little gain margin back in to prevent oscillation. I suspect an isolation (rf out to dc at gain block 2 to rf in to dc at gain block 1 ) of about 72 dB. (You can measure it with a spectrum analyzer and a good bias-tee. A quick check could also involve distinct power supplies for the two gain blocks.

This could be a problem with the input match that the detector provides. Looking at a tunnel diode detector data sheet is spec'ed at 2:1 VSWR

https://www.pasternack.com/images/ProductPDF/PE80T6000.pdf

looing at a Schottky detector, the input match is not even specified

https://www.pasternack.com/images/ProductPDF/PE8004-P.pdf

I am not saying this is the smoking gun, but maybe the combined LNAs are conditionally stable. It would be worth looking at broadband input match on both detectors.

Steve

It's worth considering a somewhat overlooked pathway that could impact the performance of your system: the DC supply lines. Even after achieving significant gain, such as 75 dB, it's possible for signals to inadvertently travel from the output back to the input if the isolation between stages is not sufficiently robust. Specifically, if the isolation is less than 75 dB, there is a risk that some of the amplified signal could find its way back through the DC supply lines, potentially causing feedback and unintended interactions that could lead to instability or oscillation in the circuit.

One approach to mitigating this issue might involve implementing a 3 dB pad in the signal path. This pad could be just enough to provide a margin of gain reduction that helps to prevent oscillation by minimizing the risk of feedback through the supply lines. This slight adjustment could effectively maintain system stability by ensuring that any potential signal leakage is kept to a minimum.

To assess and address this concern, you might need to evaluate the isolation between different stages of your system. For example, you should measure the isolation from RF output to DC at gain block 2 and from RF input to DC at gain block 1. A spectrum analyzer, paired with a good bias-tee, can be useful tools for measuring this isolation. If the measured isolation falls short of the required 75 dB, you might observe that the isolation is closer to around 72 dB.

Another practical step for quickly testing this issue could be to use distinct power supplies for each of the gain blocks. By isolating the power supplies, you can minimize the risk of cross-talk and signal leakage between stages. This method not only helps in diagnosing the problem but also in preventing any potential interactions that could affect the stability of the overall system. <a href="CardeaConcrete.com">Cardea Concrete</a>

It's worth considering a somewhat overlooked pathway that could impact the performance of your system: the DC supply lines. Even after achieving significant gain, such as 75 dB, it's possible for signals to inadvertently travel from the output back to the input if the isolation between stages is not sufficiently robust. Specifically, if the isolation is less than 75 dB, there is a risk that some of the amplified signal could find its way back through the DC supply lines, potentially causing feedback and unintended interactions that could lead to instability or oscillation in the circuit.

One approach to mitigating this issue might involve implementing a 3 dB pad in the signal path. This pad could be just enough to provide a margin of gain reduction that helps to prevent oscillation by minimizing the risk of feedback through the supply lines. This slight adjustment could effectively maintain system stability by ensuring that any potential signal leakage is kept to a minimum.

To assess and address this concern, you might need to evaluate the isolation between different stages of your system. For example, you should measure the isolation from RF output to DC at gain block 2 and from RF input to DC at gain block 1. A spectrum analyzer, paired with a good bias-tee, can be useful tools for measuring this isolation. If the measured isolation falls short of the required 75 dB, you might observe that the isolation is closer to around 72 dB.

Another practical step for quickly testing this issue could be to use distinct power supplies for each of the gain blocks. By isolating the power supplies, you can minimize the risk of cross-talk and signal leakage between stages. This method not only helps in diagnosing the problem but also in preventing any potential interactions that could affect the stability of the overall system. http://cardeaconcrete.com