Current Mode class-D Amplifier Research Articles

Multi-Turn Transmit Coil to IncreaseB1Efficiency in Current Source Amplification

A multi-turn transmit surface coil design was presented to improve B1 efficiency when used with current source amplification. Three different coil designs driven by an on-coil current-mode class-D amplifier with current envelope feedback were tested on the benchtop and through imaging in a 1.5 T scanner. Case temperature of the power field-effect transistor at the amplifier output stage was measured to evaluate heat dissipation for the different current levels and coil configurations. In addition, a lower power rated device was tested to exploit the potential gain in B1 obtained with the multi-turn coil. As shown both on the benchtop and in a 1.5 T scanner, B1 was increased by almost 3-fold without increasing heat dissipation on the power device at the amplifier's output using a multi-turn surface coil. Similar gain was obtained when connecting a lower power rated field-effect transistor to the multi-turn coil. In addition to reduce heat dissipation per B1 in the device, higher B1 per current efficiency allows the use of field-effect transistors with lower current ratings and lower port capacitances, which could improve the overall performance of the on-coil current source transmit system.

GaN HEMT based high‐efficiency current‐mode class‐D amplifier using chip‐on‐board technique

AbstractThis article presents a high efficiency current‐mode class‐D (CMCD) amplifier using a GaN high electron mobility transistor (HEMT) for class‐S system—a digitally controlled by delta‐sigma modulation. To enhance a drain efficiency of CMCD amplifier, the chip‐on‐board (COB) technique, which can reduce the external parasitic components of the packaged transistor and allow fast switching operation at high frequencies by minimizing distortion of the pulse waveform, is adopted. A LC‐resonance circuit with adequate Q values between each transistor and output matching network controls harmonic signal by using the lumped elements to extract pure fundamental frequency of the output signal. For a high efficiency push‐pull operation of CMCD amplifier, low‐loss rat‐race baluns are integrated at input and output circuits. From the measured results for a continuous wave of 3.35 GHz, the CMCD amplifier using COB technique shows a maximum output power of 35.88 dBm and a maximum power gain of about 10 dB. Also, the proposed CMCD amplifier provides a high power‐added‐efficiency (PAE) of 63.1% and a drain efficiency of 76.3% at maximum output power. The drain efficiency is still remained over 70% in the frequency range of 200 MHz. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett, 54:358–362, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26536

Highly efficient switched-mode transmitter using a current mode class-D RF amplifier

This article presents a switched-mode transmitter architecture using a current mode class-D (CMCD) amplifier. To achieve high average efficiency for a modulated signal the envelope of the complex baseband signal is transformed into pulses such that the CMCD amplifier is operated either at its peak efficiency or completely switched off. The CMCD amplifier has been designed based on single-tone active harmonic load-pull measurements to achieve a power-added efficiency (PAE) of 61.5% with 25 W output power at 900 MHz using LDMOS FETs. Removing the losses of the demodulation filter and of the amplifier a 10% higher efficiency than in an ideal class-B amplifier can be obtained for burst-mode operation with a peak-to-average power ratio of 10 dB. © 2010 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2010.

Enhanced architecture for microwave current-mode class-D amplifiers applied to the design of an S-band GaN-based power amplifier

An enhanced architecture for the design of current-mode class-D (CMCD) power amplifiers (PAs) at microwave frequencies is presented. In the proposed structure, the harmonic impedance loading conditions of CMCD amplifiers are realised by using multiharmonic output impedance transformation networks instead of a combination of balun and lumped element tanks, typically used in CMCD PA designs. The advantage of the proposed approach is to remove design complexity from the balun and lumped element tank. A 39 dBm GaN-based CMCD PA was designed at 2.35 GHz – to the best knowledge of the authors the highest frequency for a CMCD PA reported in open literature – to validate and to demonstrate the capabilities of this architecture. The fabricated PA achieves 68% DC-to-RF efficiency (η) and 65% power added efficiency. A comparison between this S-band amplifier and a conventional wideband-balun-resonant-tank CMCD PA at 1 GHz using a similar commercial active device demonstrates the frequency coverage and performance improvement of the proposed topology.

Design of high-efficiency current-mode class-D amplifiers for wireless handsets

Design considerations are discussed for current-mode class-D (CMCD) microwave power amplifiers. Factors affecting amplifier efficiency are described analytically and via simulation. Amplifiers are reported that incorporate parallel LC resonators alongside the switching transistors. To reduce parasitic resistance, bond-wires were utilized to implement a high Q inductor in the LC resonator. An experimental CMCD amplifier based on GaAs HBTs is reported, with collector efficiency of 78.5% at an output power of 29.5 dBm (0.89 W) at 700 MHz.

Current-mode class-D power amplifiers for high-efficiency RF applications

We show that current-mode class-D (CMCD) power amplifiers can achieve high efficiency at RF frequencies. In contrast with conventional voltage-mode class-D amplifiers, the CMCD features "zero voltage switching," which eliminates the output capacitance discharge loss. Experimental CMCD amplifiers with 76.3% power-added efficiency (PAE) at 290-mW output and 71.3% PAE at 870-mW output are demonstrated using GaAs FETs at 900 MHz.