High-Output Current Gallium Nitride Transconductance Amplifier

In analog and microwave signal processing, transconductance amplifiers are used as building blocks for tunable filters, phase shifters, signal generation and other applications. These amplifiers are commonly known as operational transconductance amplifiers (OTAs) and deliver output currents in the milliampere range. Meanwhile, a different type of application space exists for transconductance amplifiers (TCAs) that yield output currents from Amperes to tens Amperes. These TCAs are systems onto themselves and are used for monitoring of power flow in electricity distribution networks, calibration of power transformers, power meters and high-current clamp meters among other applications. At present, commercial TCAs deliver up to 100 A of current from dc to 1 MHz to a shunt load. A standard TCA design uses a parallel arrangement smaller silicon-based TCA cells, each delivering 5 A of current. In this work, we report on the design, fabrication and measurement of a 25 MHz gallium nitride (GaN) transconductance amplifier (TCA) cell that delivers up to 6.45 A of ac current (12.9 A, peak-to-peak) to a shunt load. The TCA accepts input voltage signals of up to 3 V amplitude and yields a maximum transconductance gain of 2.15 Siemens. The TCA features a Class AB output stage driven by signal conditioning circuitry that converts a single-ended input to a differential signal. The gate bias for the dc-coupled GaN devices is provided through the driver circuits. The output current flows through a high-precision sensing resistor and terminates in a 10 m$\Omega$ load. The nominal dc power rating of the transistors is 35 Watts and this value is used to set the transistor dc bias voltages. For a target output current of 10 A peak-to-peak (5 A, peak), the VDSfor each transistor is set to 7 V. Considering the basic case of a balanced TCA output of 0 V, then $V_{S1} = V_{D2}$ = 0 V for the GaN devices and therefore, VDD= 7 V and VSS= −7 V. The GaN transistors used in this work are packaged Wolfspeed CGH40035F power HEMT operating from a 28 V rail supply. The total harmonic distortion (THD) of the TCA was calculated from measured time-domain data. The worst-case THD is 7% over the 25 MHz band while the minimum is 1% at 10 MHz and 10 A peak-to-peak output current. Table I below compares the frequency response of the Queen’s GaN TCA cell with industrial designs.

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