LNA Cascade

Here's an LNA cascade using BGA's

Block Description

The design of the cascade can be explained fairly simply in terms of the various blocks. Along the bottom path, the RF enters on at the port RF in, it traverses through the AC coupling capacitors, which also acts to block the DC bias voltage applied to the LNA transistor. To protect the LNAs from damage, high voltage signals are shorted to the ground by a pin diode, arranged with a Schottky diode in reverse bias, the pair act to self-bias one-another thereby attenuating both half-cycles of the incident signal. The RF signal is amplified by two such blocks after which the signal is conditioned by a top-coupled doublet filter. These filters offer good performance with a tight notch around the centre frequency. The signal is amplified again and then exits at port RF out.

Along the top of the schematic, we have the bias voltage supply. A regulated voltage is supplied at port Vbias, and it is split between three bias conditioning blocks. Two bias resistors drop the excess current, limiting the maximum voltage to one that can be safely handled by the LNAs. Two banks of de-coupling capacitors reduce any voltage ripple and buffer against transient current demands, and an RF choke injects the bias voltage onto the RF line.

The BGA transistors are provided power via their collectors, with their emitters tied to the ground. With the collector as the RF output port, we have to use an inductor which acting as an RF choke, blocks the RF signal from leaking into the bias circuitry.