Force amplifier into oscillation

Question

The Minicircuits RAM-8+ amplifier has been replaced by the pin compatible RAM-8A+ part. The old RAM-8+ part was conditionally stable and I had to use an extra output attenuator after the amplifier to ensure that it did not oscillate during open-circuit output conditions.

It appears from the Minicircuit supplied S-parameters plotted in terms of the stability factor k and the geometric stability factor μ that the new RAM-8A+ is completely stable across the operational band; however it does have sharp dip in μ at 50 MHz that approaches the unconditional stability limit line of 1.

I would like to test the new amplifier under stressed conditions to verify that it is indeed stable. My initial thought for a test setup would be a directional coupler on the RAM-8A+ output with the thru port alternated between open and short, while monitoring the coupled port for signs of instability. I'm curious if there are any other good setups for testing the stability of such an amplifier, e.g. monitoring the drain feed with a oscilloscope under open/short load conditions?

Answer 1

You are right on track. A spike in K or u are definitely cause for concern. Even if it doesn't cross the unconditional stability line, it probably will after considering part-to-part variation, temperature, ground quality, and other variables. Keep in mind that a well designed amplifier is not over stabilized. That would just be wasteful. I mention this because it's probably possible to make it oscillate if you try hard enough, so limit your stress tests to practical conditions.

I would start by plotting stability circles at 50MHz and any other questionable frequency. You should be able to link a stability circle to the "u" spike and figure out what impedance might cause an oscillation. Take note if the impedance needs to be presented to the source or the load. If that impedance is an open or a short, your method should work. Keep in mind, an open at the end of a directional coupler will transform to something else at the s-parameter plane of reference.

I prefer using a signal generator and looking at the output spectrum with a spectrum analyzer. As you mentioned, directional couplers can be used to add source/load pulls to the setup. You are also correct to consider monitoring the drain feed. Sometimes you can tell an amplifier is oscillating just by monitoring the power supply. The drain current might fluctuate randomly, or hold steady at an odd value.

A few extra thoughts... Low frequency oscillations can often be blamed on poor decoupling in the bias T. You could avoid adding an attenuator by adding some RC decoupling or an RLC on the output that resonates at the oscillation frequency.

Answer 2

i would use a directional coupler input and output and a short transmission line for delay and feed it back to the input. the TL and couplers are match properly thus it only generate oscillatory stresses and avoids DC instability induced oscillation modes.