Variable gain rf amplifier

This application claims the priority benefit of European patent application number 083095110.2, filed on Apr. 18, 2008, entitled “VARIABLE GAIN RF AMPLIFIER,” which is hereby incorporated by reference to the maximum extent allowable by law.

1. Field of the Invention

The present invention relates to a variable gain RF amplifier, and in particular to a low noise variable gain RF amplifier.

2. Discussion of the Related Art

RF (radio frequency) devices such as radio tuners, mobile telephones, satellite receivers, TV tuners etc. are devices that receive an RF signal. In such devices, input circuitry comprising a low noise amplifier (LNA) is generally provided for amplifying the received RF signal which is for example received via an antenna. The LNA should introduce as little noise as possible to the signal. To avoid unwanted reflections, the input impedance of the LNA is usually matched to the impedance of the antenna. Furthermore, the input circuitry generally includes a variable gain amplifier (VGA) coupled after the LNA, to adjust the amplitude of the RF signal to a range acceptable by downstream circuitry that is to process the RF signal.

A drawback with such RF input circuitry is that in providing a low noise amplifier coupled to a variable gain amplifier, the current consumption is relatively high, and a relatively large amount of area is needed. Furthermore, due to the number of components that are present through which the differential signal must pass, noise may be added to the system.

It is an aim of embodiments of the present invention to at least partially address one or more drawbacks in the prior art.

According to an aspect of the present invention, there is provided a variable gain amplifier comprising an input node; a variable current source comprising a control input coupled to the input node; first and second branches coupled in parallel between a first supply terminal and the variable current source, the first and second branches defining a differential pair arranged to be controlled by first and second differential gain signals and comprising first and second output terminals, one of the output terminals comprising an output node of the variable gain amplifier; and a potential divider comprising a middle node coupled to the first and second output terminals, wherein the middle node is also coupled to the input node by a capacitor.

According to an embodiment of the present invention, the potential divider comprises a first resistor coupling the middle node to the first output terminal and a second resistor coupling the middle node to the second output terminal.

According to another embodiment of the present invention, the first and second resistors have the same resistance value.

According to an embodiment of the present invention, the middle node is coupled to the input node by the capacitor coupled in series with a resistor.

According to an embodiment of the present invention, the current source comprises a transistor having a control terminal coupled to the input node, a first main current terminal coupled to a second supply terminal and a second main current terminal coupled to the first and second branches.

According to an embodiment of the present invention, the first branch comprises a first transistor having a control terminal arranged to receive the first differential gain signal, a first main current terminal coupled to the current source and a second main current terminal coupled to the first supply terminal; the second branch comprises a second transistor having a control terminal arranged to receive the second differential gain signal, a first main current terminal coupled to the current source, and a second main current terminal coupled to the first supply terminal; and the middle node of the potential divider is coupled to the second main current terminals of the first and second transistors.

According to an embodiment of the present invention, the variable gain amplifier further comprises a second input node; a second variable current source comprising a control input coupled to the second input node; third and fourth branches coupled in parallel between a first supply terminal and the variable current source, the third and fourth branches defining a differential pair arranged to be controlled by the first and second differential gain signals and comprising a second pair of output terminals, wherein the output node of the variable gain amplifier is a first output node of the variable gain amplifier and wherein one of the second pair of output terminals comprises a second output node of the variable gain amplifier; and a second potential divider comprising a middle node coupled to the second pair of output terminals, wherein the middle node is also coupled to the input node.

According to a further aspect of the present invention, there is provided an RF device comprising an input for receiving an RF signal and the above variable gain amplifier coupled to the input.

According to a further aspect of the present invention, there is provided a mobile device comprising the above variable gain amplifier.

According to a further aspect of the present invention, there is provided a satellite receiver comprising the above variable gain amplifier.

According to a further aspect of the present invention, there is provided a system comprising receiving circuitry for receiving an RF signal; an input node coupled to the receiving circuitry; a variable current source comprising a control input coupled to the input node; first and second branches coupled in parallel between a first supply terminal and the variable current source, the first and second branches defining a differential pair arranged to be controlled by first and second differential gain signals and comprising first and second output terminals; one of the output terminals comprising an output node of the variable gain amplifier; and a potential divider comprising a middle node coupled to the first and second output terminals, wherein the middle node is also coupled to the input node by a capacitor.