As stated previously, a small amount of additional emitter inductance can help stabilize the BFP620 at lower frequencies, but larger amounts of added emitter inductance can jeopardize stability at higher frequencies (for this particular amplifier, in the region above about 8 GHz). As the BFP620 is a very high gain device with an extremely high transition frequency, the potential for high frequency instability exists, and thus great care must be exercised.

A large amount of inductive emitter degeneration was required in order to meet the linearity requirement target of an input IP ₃ of +10 dBm with less than 10 mA of current. Due to this large amount of required emitter inductance, it was found that the LNA could oscillate at 10 GHz if both LNA input and output were left open-circuited. The 10 GHz oscillation could be observed on a spectrum analyzer if the area near the LNA were sniffed with an RF probe while amplifier input and output were left open-circuited.

[Note: A cable with a PC board mount SMA connector inserted into one end may be used as a makeshift RF probe, with the center pin of the SMA connector used as an antenna. Be sure to include a DC block at the spectrum analyzer input!]

Another observable symptom of the 10 GHz oscillation was a shift in DC operating current. As the input and output of the LNA is changed from a 50 ohm termination to an open circuit, the DC...