A 280 W LDMOS broadband Doherty PA with 52% of fractional bandwidth based on a multi-line impedance inverter for DVB-T applications

Abstract

We introduce a new technique for the design of an output combiner for Doherty power amplifier (DPA) and its effective exploitation for the development of a wideband laterally diffused metal oxide semiconductor (LDMOS) DPA. The design is enabled by a two-line impedance inverter for the DPA back-off operation, which is capable of 52% of fractional bandwidth. The technique is validated by the development of a DPA prototype for Ultra high frequency terrestrial digital video broadcast (DVB-T) applications, with optimized peak power and efficiency over 470–806 MHz. The prototype delivers between 40 and 53% of average efficiency across the band, at 49 dBm output power in average across the bandwidth, and supporting DVB-T signals with 8 MHz bandwidth and a peak-to-average power ratio of 10.5 dB. It achieves the target adjacent channel power ratio of −52 dBc at 750 MHz if digital pre-distortion is applied, and provides 47.8 dBm of output power with a drain efficiency of 44.3%.

A 3-bit load-pulling digital power amplifier

“…When an amplifier switches from supplying current to the TL network, to appearing as only C DS , in this way, it will cause a change in the impedance presented to the output port of the other operating amplifiers, an effect which is known as load-modulation [7]. Load-modulation is the process that both Doherty [2] and Outphasing [3] PAs achieve high efficiency at back-off. By changing this impedance, the output current of an individual amplifier and therefore its own P OUT will change.…”

“…It therefore becomes necessary to develop PA architectures which can achieve a high efficiency at the average P OUT – a region often referred to as “back-off”. Several techniques have appeared over the last 80 years to meet this requirement including Envelope Tracking/Modulation [1], Doherty [2], Outphasing [3], and Dynamic Load Modulation (DLM) [4]. These can all be classed as analog techniques, and although capable of good performance, are limited in their dynamic range, and hence ability to efficiently amplify high PAPR signals.…”

High‐power UHF Doherty amplifier output combiner network optimization by 3‐port sub‐circuit X‐parameters characterization

“…The devices were terminated by multisection cascaded quarter wavelength TLs capable of providing the impedance transformation from the 50 Ohm load to the optimum impedance across the DPA operating band of 640 to 920 MHz. The technique of artificial transmission microstrip line synthesis, 8 allowed keeping the TLs' width limited, and at the same time absorbs the devices' drain parasitic capacitance, C p .…”

Large-signal vector characterization of LDMOS devices for analysis and design of broadband Doherty high-power amplifiers

“…A limitation of this approach consists in the maximum width of the taper, which is dictated by the device drain terminal spacing. In this case, a reduction of the width can be implemented by using artificial line synthesis [16]. By considering that during the operation of the DPA, the load modulation changes the impedance seen by both the peak and the main devices, we used 10 Ohm Ω as a trade off between the feasibility of the taper and the actual value for the terminations.…”

“…The purpose of the test fixture is to provide a connection between the calibrated ports of the three-port large-signal vector measurement setup [16] and the DPA sub-circuit. The measurement setup is based on a three ports nonlinear-VNA (NVNA), where two sources are actuated at operating frequencies and levels capable of stimulating the RF device pair with a large signal from the input network, and simultaneously injecting tick signals [18] for the extraction of the X -parameters.…”