thanhthien283
Newbie
I designed an LNA circuit that exhibits a peculiar phenomenon during measurements:
The LNA circuit is designed to operate at 4V with a bias current of 40mA. I measured and observed the S-parameters while keeping the voltage VD fixed at 4V, then gradually increasing the current (tuned by the voltage VG with very fine steps). The S-parameters and noise were observed to be stable as the current increased gradually. S21 increased steadily with the bias current when adjusting the VG voltage slowly. However, when it reached around 20mA, the current surged straight to 120mA (the measuring device fixed the current), even though the VG step was still very fine. At this point, the observed S-parameters showed a decrease in gain, a significant increase in noise, but S11 and S22 remained good (below 10dB).
However, when I performed similar measurements at VD=3V, the circuit behaved entirely normally, without such rapid current surges.
In simulations, I tried to find the cause to determine if this could be oscillation, but all the simulated parameters showed that the circuit was not oscillating (the stability factor, K factor, Kurokawa condition, at various bias voltages). I have the following concerns:
The LNA circuit is designed to operate at 4V with a bias current of 40mA. I measured and observed the S-parameters while keeping the voltage VD fixed at 4V, then gradually increasing the current (tuned by the voltage VG with very fine steps). The S-parameters and noise were observed to be stable as the current increased gradually. S21 increased steadily with the bias current when adjusting the VG voltage slowly. However, when it reached around 20mA, the current surged straight to 120mA (the measuring device fixed the current), even though the VG step was still very fine. At this point, the observed S-parameters showed a decrease in gain, a significant increase in noise, but S11 and S22 remained good (below 10dB).
However, when I performed similar measurements at VD=3V, the circuit behaved entirely normally, without such rapid current surges.
In simulations, I tried to find the cause to determine if this could be oscillation, but all the simulated parameters showed that the circuit was not oscillating (the stability factor, K factor, Kurokawa condition, at various bias voltages). I have the following concerns:
- What is the phenomenon of rapid current increase while the S-parameters appear normal?
- How can I identify the cause and resolve this issue?
