Analysis and Design of a Voltage–Current Transformer Feedback Neutralization Network With an Arbitrarily Chosen Coupling-Factor

One of the transformer neutralization conditions derived theoretically in previous publications is to make the coupling factor, k, of the transformer as close to one as possible, which is not easy to realize over millimeter-wave frequency range. To seek a method for neutralizing the transistor gate-drain capacitance to cope with the imperfect reverse isolation with the transformer coupling factor not necessarily being close to one, a voltage-current transformer feedback network is adopted in this paper. A phase noninverting transformer is added between gate and drain nodes of a transistor to form a voltage-current feedback. The neutralization condition of this network is derived. The effect of this network on the gain is predicted theoretically. The noise behavior of this network is also analyzed and compared theoretically with other transformer feedback structures reported in the millimeter-wave amplifier design. A Q-band three-stage amplifier using this network is designed to demonstrate the feasibility of this network. This amplifier is implemented in 90-nm low-power bulk CMOS technology. Measurement results show that this amplifier has the peak gain of 18.3 dB at 46 GHz and the reverse isolation better than 40 dB over the operation frequency range while consuming 20 mW at a supply voltage of 1.2 V, which verifies the unilateralization effect by using this network.