TECHNICAL FIELD

Embodiments of the subject matter described herein relate generally to Doherty power amplifiers.

BACKGROUND

For many years, the Doherty power amplifier has been one of the most popular amplifiers for cellular infrastructure applications. A two-way Doherty amplifier includes a carrier amplifier and a peaking amplifier, which are connected in parallel between an amplifier input and an amplifier output. During operation, an input radio frequency (RF) signal is divided into carrier and peaking signals, and a phase difference of 90 degrees is applied between the carrier and peaking signals prior to amplification by the carrier and peaking amplifiers. On the output side, the amplified carrier and peaking signals are then phase-aligned before the amplified signals are combined together by a signal combiner.

A Doherty amplifier may have a symmetric or an asymmetric configuration, where each configuration type has its advantages and disadvantages. In a symmetric Doherty amplifier, the carrier and peaking amplifiers have substantially the same current-carrying capability, which may be achieved by utilizing carrier and peaking power amplifiers of the same transistor size (or periphery). Conversely, in an asymmetric Doherty amplifier, the peaking amplifier typically has a significantly greater current-carrying capacity than the carrier amplifier. This may be achieved by utilizing a significantly larger sized peaking power amplifier than the carrier power amplifier.

In comparison with asymmetric Doherty amplifiers, conventional symmetric Doherty amplifiers tend to have better gain, RF bandwidth, and linearity (i.e., the AM-AM/AM-PM behavior is favorable for linearization). However, the load modulation for a symmetric Doherty amplifier is limited only to 2 (i.e., the variable standing wave ratio (VSWR) is about 2:1). Accordingly, the efficiency boost obtained for a modulated load in the back-off Doherty condition tends to be better for a conventional symmetric Doherty amplifier than it is for a class-AB power amplifier. However, the efficiency boost for a conventional symmetric Doherty amplifier may be significantly poorer than for a conventional asymmetric Doherty amplifier. In contrast, because an asymmetric Doherty amplifier has a load modulation greater than 2 (i.e., VSWR>2:1), asymmetric Doherty amplifiers tend to have better back-off efficiency than both class-AB and conventional symmetric Doherty amplifiers. However, due to their nonlinear AM-AM/AM-PM behavior and the need for an uneven input RF signal split ratio, asymmetric Doherty amplifiers also are characterized by relatively lower back-off gain and poorer linearity, when compared with conventional symmetric Doherty amplifiers.

Amplifier designers strive to develop Doherty amplifier designs that achieve the advantages of both symmetric and asymmetric configurations, while avoiding the various disadvantages of the two configuration types. What is desired is a relatively simple Doherty amplifier design with reduced parasitic effects, relatively-simple impedance matching circuitry, relatively-high gain, and linearizable power added efficiency.

