It looks like a natural for ferrite beads on the gates of all those FET's. I have had to deal with internal oscillations there a lot.
There are 2 oscillation mechanisms in these amplifiers. One is local to the FET group.
The wiring of the 16 FETs allows a lot of feedback at VHF+ and gm is huge. This is helped against
by the beads in the gates and brutally by the capacitors from the drains to GND. The capacitors can
be huge in the 500 MHz world, but they won't limit bandwidth since the 2 Ohm input impedance
of the cascode is in parallel, at least a relatively low frequencies.
The other mechanism is feedback around the op amp back into the sources. Don't deal with it
befoe you have solved mechanism 1. Leave the feedback loop open until this is done.
The delay though the op amp creates a phase shift in the feedback loop that is too large for the attempted bandwidth. I have looked deeper into this in the single ended variation with feedback into the source. The differential version is probably the same.
Symptom is you get a negative real part of the input impedance at mid-frequencies, 100 KHz or so. That happens in LTspice as well as with the vector network analyzer. It goes away if you limit the bandwidth of the FET/cascode to very poor values or improve the feedback amplifier to "very very fast". A TI 3 GHz THS43?? op amp worked for me but its 1/f noise was so large that the 35 dB gain of the FET stage would not mask it. By far.
I could not find a solution to the feedback problem, and I tried exotic things. So I was fed up enough to go without feedback around the FETs. The gain of the FET stage varies with the square root of the drain current. If you stabilize the current, gain will stay where it is. I made a temp-stable IF3602 version that proved not to be necessary. The 45 mA for 16 pcs. 3910s was a lucky find in the simulator where a pos and a neg TC cancel. I did not verify this on real hardware because of operator fatigue.
BTW voltage noise of the FETs improves with the root of the gain, or the 4th root of drain current.
That makes it not so attractive to crank up the drain current.
I was doing some more testing and found that the circuit becomes susceptible to oscillation with increasing source impedance. This begins to show on the 60R calibration resistor and gets quite bad with the 1.5k resistor. The peak was originally at 1.78 MHz. Reducing the amplifier bandwidth by increasing feedback capacitance to 150 pF helped reduce the amplitude of the oscillation somewhat and the frequency of the main peaks is now 1.09 and 1.55 MHz. The -3 dB point is about 500 kHz for the first stage now. The behavior is the same when connecting sources with any significant length of coax (more than about 20 cm). A 50 ohm terminated length of coax is similar to the 60R calibration resistor. If I replace the termination with a short, the oscillation is still present, but some of the harmonics become larger in amplitude than the fundamental. For 1 m (with Cf=45 pF, with a fundamental of 1.78 MHz), a much larger oscillation occurs at 10.7 MHz. In this case the frequency of the oscillation depends upon the length of the cable used. The 10.7 MHz oscillation was about the same amplitude after the first stage (Av=201) as it was after the third stage (Av ~ 10k total).
...
Is there a general solution to this type of problem that is more elegant than reducing the bandwidth of the first stage further? My sense is that these oscillations are at too low a frequency for ferrite beads to be effective, but my experience is limited on this. I have also seen lead networks between the legs of the differential pair that I assumed were put in place to ensure stability. Are there general approaches to troubleshooting this problem or good resources to check out?
The beads behave like resistors at frequencies where they work. That means they also generate thermal noise like a resistor at the frequencies they work.
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