A variable gain amplifier with variable input impedance and auto-zero feedback for spin torque oscillators

This work presents a highly adaptive circuit for amplifying a signal of a spin torque oscillator (STO). For achieving power matching with respect to the STO´s variable output resistance, the input impedance of the amplifier can be tuned in the range of 50 Ω to more than 500 Ω. Multiple amplifier stages are deployed leading to accumulated gain of >60 dB at 1 GHz. The -3 dB bandwidth of 1.5 GHz makes this amplifier suitable for vortex STO. A fully integrated and adjustable auto-zero feedback for compensating the DC-offset has been implemented in order to convert the single ended signal into a differential one. The circuit was realized in a 180nm BiCMOS process and includes an I2C-slave interface to adjust on-chip currents and control signal switches.