Variable gain rf amplifier

Abstract

A variable gain amplifier having an input node, a variable current source including 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 having first and second output terminals, one of the output terminals including an output node of the variable gain amplifier; and a potential divider having 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.

1 . A variable gain amplifier comprising:
 an input node;   a variable current source having a control input coupled to said input node;   first and second branches coupled in parallel between a first supply terminal and said variable current source, said 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 said output terminals comprising an output node of said variable gain amplifier; and   a potential divider having a middle node coupled to said first and second output terminals, wherein said middle node is also coupled to said input node by a capacitor.   
   
   
       2 . The variable gain amplifier of  claim 1 , wherein said potential divider comprises a first resistor coupling said middle node to said first output terminal and a second resistor coupling said middle node to said second output terminal. 
   
   
       3 . The variable gain amplifier of  claim 2 , wherein said first and second resistors have the same resistance value. 
   
   
       4 . The variable gain amplifier of  claim 1 , wherein said middle node is coupled to said input node by said capacitor coupled in series with a resistor. 
   
   
       5 . The variable gain amplifier of  claim 1 , wherein said current source comprises a transistor having a control terminal coupled to said input node, a first main current terminal coupled to a second supply terminal and a second main current terminal coupled to said first and second branches. 
   
   
       6 . The variable gain amplifier of  claim 1 , wherein:
 said first branch comprises a first transistor having a control terminal arranged to receive said first differential gain signal, a first main current terminal coupled to said current source and a second main current terminal coupled to said first supply terminal;   said second branch comprises a second transistor having a control terminal arranged to receive said second differential gain signal, a first main current terminal coupled to said current source, and a second main current terminal coupled to said first supply terminal; and   said middle node of said potential divider is coupled to said second main current terminals of said first and second transistors.   
   
   
       7 . The variable gain amplifier of  claim 1 , further comprising:
 a second input node;   a second variable current source having a control input coupled to said second input node;   third and fourth branches coupled in parallel between a first supply terminal and said variable current source, said third and fourth branches defining a differential pair arranged to be controlled by said first and second differential gain signals and comprising a second pair of output terminals, wherein said output node of said variable gain amplifier is a first output node of said variable gain amplifier and wherein one of said second pair of output terminals comprises a second output node of said variable gain amplifier; and   a second potential divider having a middle node coupled to said second pair of output terminals, wherein said middle node is also coupled to said input node.   
   
   
       8 . An RF device comprising:
 an input for receiving an RF signal;   the variable gain amplifier of  claim 1  coupled to said input.   
   
   
       9 . A mobile device comprising the variable gain amplifier of  claim 1 . 
   
   
       10 . A satellite receiver comprising the variable gain amplifier of  claim 1 . 
   
   
       11 . A system comprising:
 receiving circuitry for receiving an RF signal;   an input node coupled to said receiving circuitry;   a variable current source having a control input coupled to said input node;   first and second branches coupled in parallel between a first supply terminal and said variable current source, said 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 said output terminals comprising an output node of said variable gain amplifier; and   a potential divider having a middle node coupled to said first and second output terminals, wherein said middle node is also coupled to said input node by a capacitor