Bias pin behavior on the QPL9547

RF Solutions RF Low Noise Amplifiers
1

Hello, Qorvo team!

I am designing a high-performance RF front-end for hobbyist SDR use, and I want to use the QPL9547 as the primary weak-signal LNA due to its excellent performance. This design has a wealth of high-quality digitally-adjustable current and voltage supplies available, and I would like to make use of these to allow the user to tune the behavior of LNAs and gain blocks in the system rather than relying on fixed set resistors.

I am curious about the behavior of the Vbias pin on this chip. The datasheet states that it “sets the LNA bias current for the device”, and in the reference designs it is simply connected through a resistor to Vcc, but not much more detail is provided as to how it behaves – for instance, is the bias current set directly by the current through this pin, or does the voltage control the bias current through the active biasing circuitry (or something else)?

As a side-note, if measurements showing how the RF characteristics of the device vary as bias current is adjusted are available, I would be interested in looking at these to get an understanding of how the linearity and immunity of this device can be tuned and what the specific trade-offs are.

Thank you, and I await your response ^^

2

The Vbias pin on the QPL9547 is a voltage input that feeds the internal active bias circuitry of the LNA. The resistor is used to drop the supply voltage down to give the voltage on Vbias that sets the required bias current. Increasing bias current improves linearity, until you top out at around 70mA. This LNA was designed to give best NF for currents in the 50-100mA range. The attached presentation gives further information for both 5V and 3V3 supply voltages.
QPL9547_Performance_vs_Bias_5V_3V3.pdf (580.7 KB)