Electronically reconfigurable wideband high-power amplifier architecture for modern RF systems (LMBA)

As mobile communications and other microwave systems continue to evolve designers and system architects are pushing for ever increasing bandwidth as multiple RF systems are increasingly sharing a common front-end amplifier to save space and reduce routing complexity and losses associated with having separate amplifier systems. The power amplifier in many RF systems typically accounts for the majority of the power consumption of the device or transmitter platform, it is therefore paramount that to improve the efficiency of these systems RFPA designs must be tailored to achieve the highest possible efficiency. RFPA modes of operation and architectures to achieve higher efficiency have been developed, but often come with compromises to other system aspects such as linearity, control complexity and most commonly bandwidth. With the next generation 5G communications specification including frequency bands of up to the Ka frequency spectrum and the high capacity multi-octave spectrum bands allocated at L-C band, traditional RFPA efficiency enhancement techniques struggle to be implementable due to either the high frequency requirements of the control systems needed or due to the bandwidth restrictions of such techniques. II Conventionally narrow bandwidth X-band radar systems that used to be operated at saturated output power conditions are starting to explore multimode operation that require more power back-off (PBO) and control of the RFPA, so are searching for techniques that are applicable at X-band and can achieve the same level of PBO requirements demanded by modern communication modulation standards while working to the power and cooling restraints that come from a limited application platform such as fighter aircraft. Similarly, such fighter platforms are demanding increased electronic warfare (EW) capability which are restrained to the same platform limitations but often need to cover multi-octave bandwidths where traditional efficiency enhancement techniques cannot be applied. This research will focus on wideband efficiency enhancement for both saturated and PBO scenarios that present a frequency agnostic technique of overcoming conventional limitations. The novel work presented is based around the quintessential, but relatively old, bandwidth extension architecture known as the balanced amplifier. The addition of a secondary control signal has been proposed whereby the operating impedance of the amplifier can be dramatically modulated while maintaining the fundamental advantage the balanced amplifier allow, that is multi-octave bandwidth. III The power of this architecture can draw similarities in impedance control afforded by load pull systems, in particular active load-pull. With the correct control signal, any impedance is able to be presented to the transistors to keep them operating at maximum efficiency, where passive matching alone is not able to achieve such efficiency due to fundamental matching theory. Due to the active element of this novel architecture, named the Load Modulated Balanced Amplifier (LMBA), frequency restrictive and thus band limiting elements present in other efficiency enhancement techniques; such as the quarter wave inverter present in the Doherty Amplifier or the difficulty of realizing the modulator in Envelope Tracking (ET) are not present. This thesis will present the fundamental theory driving the operation of an LMBA along with multiple implementations, each targeted at differing applications and different frequency bands to demonstrate the versatility and frequency independence of the technique

Electronically reconfigurable wideband high-power amplifier architecture for modern RF systems (LMBA)

As mobile communications and other microwave systems continue to evolve designers and system architects are pushing for ever increasing bandwidth as multiple RF systems are increasingly sharing a common front-end amplifier to save space and reduce routing complexity and losses associated with having separate amplifier systems. The power amplifier in many RF systems typically accounts for the majority of the power consumption of the device or transmitter platform, it is therefore paramount that to improve the efficiency of these systems RFPA designs must be tailored to achieve the highest possible efficiency. RFPA modes of operation and architectures to achieve higher efficiency have been developed, but often come with compromises to other system aspects such as linearity, control complexity and most commonly bandwidth. With the next generation 5G communications specification including frequency bands of up to the Ka frequency spectrum and the high capacity multi-octave spectrum bands allocated at L-C band, traditional RFPA efficiency enhancement techniques struggle to be implementable due to either the high frequency requirements of the control systems needed or due to the bandwidth restrictions of such techniques. II Conventionally narrow bandwidth X-band radar systems that used to be operated at saturated output power conditions are starting to explore multimode operation that require more power back-off (PBO) and control of the RFPA, so are searching for techniques that are applicable at X-band and can achieve the same level of PBO requirements demanded by modern communication modulation standards while working to the power and cooling restraints that come from a limited application platform such as fighter aircraft. Similarly, such fighter platforms are demanding increased electronic warfare (EW) capability which are restrained to the same platform limitations but often need to cover multi-octave bandwidths where traditional efficiency enhancement techniques cannot be applied. This research will focus on wideband efficiency enhancement for both saturated and PBO scenarios that present a frequency agnostic technique of overcoming conventional limitations. The novel work presented is based around the quintessential, but relatively old, bandwidth extension architecture known as the balanced amplifier. The addition of a secondary control signal has been proposed whereby the operating impedance of the amplifier can be dramatically modulated while maintaining the fundamental advantage the balanced amplifier allow, that is multi-octave bandwidth. III The power of this architecture can draw similarities in impedance control afforded by load pull systems, in particular active load-pull. With the correct control signal, any impedance is able to be presented to the transistors to keep them operating at maximum efficiency, where passive matching alone is not able to achieve such efficiency due to fundamental matching theory. Due to the active element of this novel architecture, named the Load Modulated Balanced Amplifier (LMBA), frequency restrictive and thus band limiting elements present in other efficiency enhancement techniques; such as the quarter wave inverter present in the Doherty Amplifier or the difficulty of realizing the modulator in Envelope Tracking (ET) are not present. This thesis will present the fundamental theory driving the operation of an LMBA along with multiple implementations, each targeted at differing applications and different frequency bands to demonstrate the versatility and frequency independence of the technique

A reappraisal of optimum output matching conditions in microwave power transistors

This paper presents a novel approach to the identification of output power and efficiency contours in microwave power transistors in compressed regime. The formulation is based on a polynomial representation of the drain-source voltage profile accounting for the knee region. Closed-form equations for the output power and efficiency as function of the fundamental load are demonstrated, enabling the plot of contours on a Smith Chart. From these, a further simplified drawing procedure for approximated contours is also derived, differentiating between two families of output characteristic. The first, with smooth knee, is usually experienced in GaN devices, while the second exhibits a steep knee which can be associated to GaAs devices’ typical behaviour. A 5W GaN HEMT, a 2.5W GaN HEMT, and a 0.7W GaAs pHEMT are characterized with load pull measurements. In all three cases, the proposed method results in a very accurate contour construction, despite being based on an approximated output current/voltage profile and on a rough estimate of output equivalent capacitance

An efficient broadband reconfigurable power amplifier using active load modulation

A novel power amplifier (PA) architecture, the Load Modulated Balanced PA (LMBA), is presented. The LMBA is able to modulate the impedance seen by a pair of RF power transistors in a quadrature balanced configuration, by varying the amplitude and phase of an external control signal. This enables power and efficiency to be optimized dynamically at specific power backoff levels and frequencies. Unlike the Doherty PA, the load seen by the active devices can be modulated upwards or downwards, both resistively and reactively, with minimal loss of power combination efficiency. The LMBA is presented as a potentially disruptive technique which enables any specific amplifier characteristic to be controlled dynamically over wide signal amplitude and frequency ranges

A broadband reconfigurable load modulated balanced amplifier (LMBA)

The Load Modulated Balanced Amplifier (LMBA) uses a control signal (CSP), injected to the normally terminated port at the output coupler of a balanced amplifier (BA), to modulate the BA transistor's impedance. The hybrid circuit demonstrator described here uses metal-backed multilayer organic substrate and GaN discrete devices. Maximum output power levels above 39.5 dBm are achieved at around P3dB. DE above 60% is seen between 4.5 and 7.5 GHz for power back-off to 7 dB with a fixed CSP of 1 W, and the bias between 18 to 28 V

High efficiency GaN 2.5 to 9 GHz power amplifiers realized in multilayer LCP hybrid technology

This letter presents wide bandwidth and high efficiency hybrid balanced and push-pull amplifier circuit topologies using Gallium Nitride (GaN) transistors and Liquid Crystal Polymer (LCP) multilayer circuit techniques. Push-pull circuit configurations allow differential and common mode impedances to be presented at even and odd harmonics independently, enabling the continuous mode (class BJ) design space to be exploited. Push-pull amplifiers display drain efficiencies > 40%, output power > 5 W over a bandwidth of 3.5 to 8.5 GHz at 2 dB compression levels, and under CW conditions. By comparison, although designed with identical performance goals, a conventional quadrature balanced amplifier performance is below that of the push-pull configuration

Fitting vast dimensional time-varying covariance models

Building models for high dimensional portfolios is important in risk management and asset allocation. Here we propose a novel and fast way of estimating models of time-varying covariances that overcome an undiagnosed incidental parameter problem which has troubled existing methods when applied to hundreds or even thousands of assets. Indeed we can handle the case where the cross-sectional dimension is larger than the time series one. The theory of this new strategy is developed in some detail, allowing formal hypothesis testing to be carried out on these models. Simulations are used to explore the performance of this inference strategy while empirical examples are reported which show the strength of this method. The out of sample hedging performance of various models estimated using this method are compared.ARCH models; composite likelihood; dynamic conditional correlations; incidental parameters; quasi-likelihood; time-varying covariances.