1. Field of the Invention
This application relates to integrated circuits, in general, and more particularly to radio frequency circuits included on integrated circuits.
2. Description of the Related Art
A typical radio frequency (RF) receiver handles large blocking signals, i.e., signals other than a target received signal that may be spectrally near the target received signal, by including an attenuator in the received signal path prior to a low-noise amplifier. The low-noise amplifier is typically the first gain stage in the receiver and is used to amplify possibly very weak received signals (e.g., RF signals captured by an antenna). Use of a low-noise amplifier reduces the effect of noise from subsequent stages of the receiver, while noise of the low-noise amplifier itself is injected directly into the received signal and dominates the input-referred noise of a receiver front end. Thus, the low-noise amplifier should apply gain to achieve a target signal power level while adding as little noise and distortion as possible so that recovery of the received signal is possible in later stages of the system. Design parameters for low-noise amplifiers include gain, noise figure, non-linearity, and impedance matching at the input and/or output ports. Input and output impedance matching circuits may provide efficient power transfer, improve noise performance, and improve stability of the low-noise amplifier. A target attenuator-low-noise amplifier architecture requires a constant input-port reflection coefficient, i.e., a constant S11. In general, S11 is a two-port scattering parameter that is used to measure and quantify reflection at the input port of an RF receiver.
As referred to herein, a complex impedance is an electrical impedance represented by a complex quantity Z=R±jX, where R is the resistance of the complex impedance and X is the reactance of the complex impedance. A complex impedance has a magnitude, which represents the ratio of the voltage at a node to the current through the node, and a phase, which represents a phase difference between voltage at the node and current through the node. The impedance of an ideal resistor is a purely real, resistive impedance where Z=R. The phase relationship between voltage and current is exactly zero degrees. Ideal inductors and capacitors have purely imaginary, reactive impedances where Z=jωL and
      Z    =          1              jω        ⁢                                  ⁢        C              ,respectively. In both cases, for an applied sinusoidal voltage, the resulting current is also sinusoidal, but 90 degrees out of phase with the voltage. However, for an inductor, the current is lagging and for the capacitor, the current is leading.
Referring to FIG. 1, a conventional RF receiver includes attenuator 104 coupled to the input of low-noise amplifier 106. The input impedance and the output impedance of attenuator 104 are real impedances (i.e., ZINATT=R+j0, e.g., ZINATT=50 ohms, and ZOUTATT=R+j0, e.g., ZOUTATT=50 ohms) and the input impedance of low-noise amplifier 106 is also a real impedance (i.e., ZINLNA=R+j0, e.g., ZINLNA=50 ohms). Design of 50-ohm attenuators (i.e., attenuators having input and output impedances that are both approximately 50 ohms) is well understood and widely known. However, a 50-ohm attenuator will only perform as expected if the input impedance of the low-noise amplifier (i.e., ZINLNA) is approximately 50 ohms. A typical 50-ohm input impedance low-noise amplifier is a common-gate low-noise amplifier, or some variant thereof, or a feedback type low-noise amplifier. However, neither of those topologies achieves the low noise and low power performance of a complex-input-impedance low-noise amplifier, e.g., an inductively degenerated low-noise amplifier. In general, 50-ohm attenuators will not achieve target performance when used with a complex-input-impedance low-noise amplifier. Thus, typical complex-input-impedance low-noise amplifiers are not preceded by an attenuator and have poor large blocker handling characteristics. Accordingly, improved techniques for receivers are desired.