The BPF between the two gain stages in this design is the Johanson 3750BP14D0900 [REF 2]. It comes in a 0603-package size and at a minimal cost while providing more than 50 dB of rejection at 1990 MHz. It is a very effective way to provide additional filtering of the blocker to reduce the IMD2/HD2 generated by the second stage amplifier. Again, referencing all calculations to the antenna input, the IMD2/HD2 product after the second stage amplifier is as follows (with two filters).
Blocker power (reference to antenna input) = 16 dBm – 70 dB – 50 dB = –104 dBm
IMD2 due to QPA9127 Amp = 2*Pwr_in –IIP2 = 2*(–104) – (12.5 dBm) = –220.5 dBm
The HD2 is typically 6 dB lower than the IMD2, and both are significantly below the target interference power level of –109 dBm. Without this BPF, the HD2/IMD2 is at ~–120.5 dBm using the above formula, which is also below the target level of –109 dBm, making this filter optional in this example RF lineup. It is useful for cases where a different second stage gain amplifier is chosen that has worse second order products, or if the designer wants to provide additional margin for component performance variation.