Concept of transformer-feedback degeneration of low-noise amplifiers

Owing to its well-known properties of possible power-noise match as well as superior noise performance, the inductively-degenerated low-noise amplifier is nowadays the most-widely used RF preamplifier topology. However, there is not much flexibility when setting the amplifier’s input impedance to match the source impedance, as for a given biasing condition, matching is achievable only for a particular value of the degenerative inductance. Moreover, to cancel the imaginary part of the input impedance, a particular excess inductance is necessary at the input of the amplifier, directly degrading the over-all amplifier performance. On the other hand, the transformer-feedback degeneration, introduced in this paper, offers the possibility, for low-noise amplifiers, of simultaneous matching to the source impedance of both the imaginary and the real part of the input impedance, in an orthogonal way. What is more, by controlling the amount of feedback, the amplifier is relieved from the burden of having the lossy input inductor. Detailed analysis shows a novel representation of the input impedance as well as a novel power-matching scheme of the transformer-feedback degenerated low-noise amplifier.