Balun With Improved Common Mode Rejection Ratio

Abstract

A balun includes a first winding which has a first terminal coupled to an input, and a second terminal coupled to a reference potential terminal. The balun includes a second winding magnetically coupled to the first winding. The second winding has a first terminal coupled to a first differential output, a second terminal coupled to a second differential output, and a tap coupled to the reference potential terminal. The balun includes a first parasitic capacitor which has a first terminal coupled to the first winding and a second terminal coupled to the second winding. The balun includes a third winding which has a first terminal coupled to the reference potential terminal and a floating second terminal. The balun includes a second parasitic capacitor which has a first terminal coupled to the third winding and a second terminal coupled to the second winding.

What is claimed is: 
     
         1 . A balun comprising:
 a first winding having a first terminal coupled to an input, and having a second terminal coupled to a reference potential terminal;   a second winding magnetically coupled to the first winding and having a first terminal coupled to a first differential output, a second terminal coupled to a second differential output, and a tap coupled to the reference potential terminal; and   a third winding magnetically coupled to the first winding and having a first terminal coupled to the reference potential and a floating second terminal.   
     
     
         2 . The balun of  claim 1 , further comprising:
 a first parasitic capacitor having a first terminal coupled to the first winding and a second terminal coupled to the second winding; and   a second parasitic capacitor having a first terminal coupled to the third winding and a second terminal coupled to the second winding.   
     
     
         3 . The balun of  claim 1 , wherein the tap is coupled to the center of the second winding. 
     
     
         4 . The balun of  claim 1 , wherein the first and second parasitic capacitors have equal capacitances. 
     
     
         5 . The balun of  claim 1 , wherein the first terminal of the first winding is coupled to an input voltage. 
     
     
         6 . The balun of  claim 1 , wherein the first differential output provides a first differential voltage and the second differential output provides a second differential voltage. The balun of  claim 1 , wherein the balun is a single-ended to a differential balun. 
     
     
         8 . A balun comprising:
 a first winding having a first terminal coupled to a first differential input, a second terminal coupled to a second differential input, and a tap coupled to a reference potential terminal;   a second winding magnetically coupled to the first winding and having a first terminal coupled to a single-ended output and a second terminal coupled to the reference potential terminal; and   a third winding magnetically coupled to the first winding and having a first terminal coupled to the reference potential terminal and a floating second terminal.   
     
     
         9 . The balun of  claim 8 , further comprising:
 a first parasitic capacitor having a first terminal coupled to the first winding and a second terminal coupled to the second winding; and   a second parasitic capacitor having a first terminal coupled to the third winding and a second terminal coupled to the first winding.   
     
     
         10 . The balun of  claim 8 , wherein the tap is coupled to the center of the first winding. 
     
     
         11 . The balun of  claim 8 , wherein the first parasitic capacitor and the second parasitic capacitor have equal capacitances. 
     
     
         12 . The balun of  claim 8 , further comprising an input voltage source coupled between the first and second differential inputs. 
     
     
         13 . The balun of  claim 8 , wherein the output provides a single-ended voltage. 
     
     
         14 . A balun comprising:
 a first winding having a first terminal coupled to an input, and a second terminal coupled to a reference potential terminal;   a second winding magnetically coupled to the first winding and having a first terminal coupled to a first differential output, a second terminal coupled to a second differential output, and a tap coupled to the reference potential terminal;   a first capacitor having a first terminal coupled to the first winding and a second terminal coupled to the second winding;   a third winding magnetically coupled to the first winding and having a first terminal coupled to the reference potential terminal and a floating second terminal; and   a second capacitor having a capacitance approximately equal to the capacitance of the first capacitor having a first terminal coupled to the third winding and a second terminal coupled to the second winding.   
     
     
         15 . The balun of  claim 14 , wherein the tap is coupled to the center of the second winding. 
     
     
         16 . The balun of  claim 14 , wherein the first differential output provides a first differential voltage and the second differential output provides a second differential voltage. 
     
     
         17 . The balun of  claim 14 , wherein the first terminal of the first winding is coupled to an input voltage. 
     
     
         18 . A balun comprising:
 a first winding having a first terminal coupled to a first differential input, a second terminal coupled to a second differential input, and tap coupled to a reference potential terminal;   a second winding magnetically coupled to the first winding and having a first terminal coupled to a single-ended output and a second terminal coupled to the reference potential terminal;   a first capacitor having a first terminal coupled to the first winding and a second terminal coupled to the second winding;   a third winding magnetically coupled to the first winding and having a first terminal coupled to the reference potential terminal and a floating second terminal; and   a second capacitor having a capacitance approximately equal to the capacitance of the first capacitor and having a first terminal coupled to the third winding and a second terminal coupled to the first winding.   
     
     
         19 . The balun of  claim 18 , further comprising an input voltage source coupled between the first and second differential inputs. 
     
     
         20 . The balun of  claim 18 , wherein the output provides a single-ended voltage.