Spin torque oscillator (STO) is a novel current-control-oscillator (CCO), based on two spintronic effects: spin momentum transfer torque and magneto-resistance (MR). It features large tunability, miniature size, high integration level, high quality factor, high operation frequency, etc., which makes it a promising technology for microwave and radar applications. However, the STO is still an immature technology, which requires intensive research for improving the spectrum purity and the output power performance . This paper proposes a wideband amplifier targeting magnetic tunnel junction (MTJ) type of STO device, which compensates the low output power of the STO.
The MTJ STO devices can cover a large part of ultra-wideband (UWB) from 3 - 8 GHz and provide an output power from -60 dBm to -40 dBm by tuning the bias DC current and the magnetic field . One important and potential application of STO device is a local oscillator (LO) in an RF transceiver. To achieve this task, the amplifier requires a gain of 45 - 65 dB. In addition, the source impedance of different MTJ STO devices varies from a dozen to several hundred Ohms, which makes the amplifier design challenging. An universal amplifier, which fulfills the extracted design requirements, is proposed. It is composed of two types of Balun-LNAs depending on the MR of STO devices as the input stages, a broadband limiting amplifier chain and an output buffer. A combination of a common source (CS) stage and a cross-coupled common gate (CG) stage is employed for the input Balun-LNA in the low impedance case while a cascoded CS stage is used in the high impedance case. The output of both LNAs is connected to a limiting amplifier chain, which provides enough voltage gain. An output buffer is used as the output stage to convert the balanced output to single-ended output and to match the output impedance to 50 Ohms.
The proposed wideband amplifier for MTJ STO is implemented in a 65nm CMOS process with 1.2 V supply. In the band of interest, it exhibits 55 dB gain with a maximum noise figure (NF) of 4.5 dB in the small MR case, and a 59 dB gain with a maximum NF of 3 dB in the large MR case. Besides the low noise performance and the high gain, the simulation results of the proposed amplifier also show that it has low power consumption and moderate impedance matching in the frequency range of 3 - 8 GHz, which is suitable for MTJ STO applications.