we had a bank holiday here, nicely positioned close to the weekend and I took the free time to play with my multi-IF3601 low noise amplifier.
That was not so funny because it features a negative input impedance over most frequencies, so with a suitable inductance on the input it makes a stable oscillator.
- Zin is negative also with the feedback loop cut
- Choice op op amp does not matter: ADA4898 vs. THS4022
- There is no coupling via the bias system; base bias used to have a 2nd current source that shared the BAV99 string; that could produce startup problems (solved)
- type of cascode transistor did not make much of a difference, using an Infineon BFQ19S instead of FZT851/ZXT690B looked slightly better, but not good enough.
- Leaving the cascode transistor out remedied the situation, but I need the cascode transistor to introduce the bootstrap to eliminate Cgd of the paralleled FETs.
- The THS4022 suppresses the Miller effect quite nicely at least to 100 KHz, but the unmultiplied Cgate-drain is still easily a nF of more.
- The obvious solution of gate stoppers does not make sense because of the noise. A resistor in the base lead of the cascode makes things worse; 6R8 between the drains and the cascode emitter made no difference. (could not use more without messing up the DC operating point, it also would interfere with the bootstrap)
- ferrite beads in the gates do exactly nothing.
circuit snippet is at <
Currently, there are 4 FETs in parallel.
Does the combined expertise of the group have any idea on how to stabilize the cascode or an alternative way to introduce the bootstrap?