Multi-Stage GaN Doherty PA: Integrated RF Power Line-up

High-efficiency RF power modules require more than just a robust final stage; they demand a precisely tuned signal chain. For this project, the objective was to validate a customer’s proprietary GaN devices within a compact Doherty Power Amplifier (PA) topology. To ensure the GaN devices performed at peak efficiency, we engineered a complete driver and pre-stage line-up, integrating off-the-shelf control components with custom RF architecture.

The Challenge: Optimizing the Drive Train

Doherty amplifiers rely on precise phase and amplitude alignment between the carrier and peaking amplifiers to achieve high efficiency. Testing a custom GaN-based Doherty PA in isolation often fails to reveal system-level constraints. The engineering challenge was to build a compact, fully functional line-up that could drive the custom GaN stage to saturation while maintaining strict control over phase alignment and bias points.

System Architecture and Integration

We designed a multi-stage module that transitions from signal conditioning to high-power output. The system integrates three distinct technologies into a cohesive RF chain:

Pre-Stage (Phase & Amplitude Control):To manage the input signal characteristics, we integrated a Monolithic Phase & Amplitude Controller (MPAC) from pSemi. This stage allows for fine-tuning of the RF signal before amplification, which is critical for optimizing the linearity and efficiency of the downstream Doherty configuration.
Driver Stage (NXP LDMOS):For the intermediate gain stage, we selected an NXP LDMOS MMIC. LDMOS was chosen for its reliability and linearity in driving the final GaN stage. This ensures that the input to the Doherty PA remains clean and stable under varying load conditions.
Bias Control:Precise biasing is required to stabilize the GaN and LDMOS devices over temperature and operation. We outfitted the driver stage with Texas Instruments (TI) programmable bias controllers, allowing for dynamic adjustment of the quiescent current (Iq) and gate voltages.
Final Stage (Custom GaN Doherty):The core of the system is the compact Doherty PA, built using the customer’s own GaN devices. By controlling the preceding chain, we could characterize the GaN technology under optimal drive conditions.

Results

The resulting hardware provides a complete "left-to-right" evaluation platform: signal conditioning via pSemi MPAC, amplification via NXP LDMOS, and final power delivery via the custom GaN Doherty PA. This setup proved the viability of the customer's GaN technology in a high-efficiency Doherty configuration with a commercially viable footprint.