Completely differential filter with switched condensers using CMOS operational amplifiers with no common-mode feedback

Abstract

The filter comprises at least one completely differential operational amplifier having two inputs and two outputs and at least one pair of feedback circuits connecting said outputs with respective inputs of said amplifier outside of same. The operational amplifier has no common-mode feedback circuit, whose functions are performed by said feedback circuits external to the amplifier.

I claim: 
     
       1. Completely differential filter with switched condensers, comprising at least one completely differential operational amplifier having two inputs and two outputs and at least one pair of feedback circuits arranged to connect said outputs with respective inputs of said amplifier outside of same, characterized in that said operational amplifier has no common-mode circuit and in that the latter's function is performed by said pair of feedback circuits external to said amplifier and further characterized in that said operational amplifier is formed by two series of CMOS transistors connected between a voltage supply and ground and having an intermediate branch point connected to respective outputs of said amplifier, one of the transistors of each series having its gate connected to a respective input of said amplifier. .Iadd. 
     
     
       2.  A completely differential filter, comprising: at least one differential operational amplifier having two inputs, two outputs, and no internal common-mode feedback circuit;   first and second feedback circuits coupled between a respective output and input of the differential operational amplifier;   wherein the operational amplifier includes at least two amplifying means, connected in parallel, between a voltage supply and ground; and   wherein the first and second feedback circuits provide common-mode feedback between the respective inputs and the respective outputs. .Iaddend..Iadd.   
     
     
       3.  The filter of claim 2, wherein each amplifying means includes at least a first and a second series connected transistor element, the first transistor element including a control connection coupled to a respective input of the operational amplifier. .Iaddend..Iadd.4. The filter of claim 3, wherein an intermediate node between each of the first and second series transistor elements is coupled to a respective output of the operational amplifier. .Iaddend..Iadd.5. The filter of claim 2, wherein each of the first and second feedback circuits includes a first condenser coupled between the respective input and the respective output of the 
     
     
        operational amplifier. .Iaddend..Iadd.6.  The filter of claim 5, wherein the first and second feedback circuits further comprise a second condenser coupled between a pole of a first single pole double throw switch and a pole of a second single pole double throw switch. .Iaddend..Iadd.7. The filter of claim 6, wherein a first throw of the first single pole double throw switch is coupled to the respective operational amplifier input and a second throw of the first single pole double throw switch is coupled to a polarization voltage. .Iaddend..Iadd.8. The filter of claim 7, wherein a first throw of the second single pole double throw switch is coupled to the respective operational amplifier output and the second throw of the second single pole double throw switch is coupled to a reference voltage. .Iaddend..Iadd.9. The filter of claim 8, wherein the reference voltage is one half of the polarization voltage. .Iaddend..Iadd.10. The filter of claim 4, wherein the first transistor element is an N-channel CMOS 
     
     
        transistor. .Iaddend..Iadd.11.  The filter of claim 10, wherein the second transistor is a P-channel CMOS transistor. .Iaddend..Iadd.12. A completely differential band-pass filter, comprising: at least two differential operational amplifiers, each differential operational amplifier having two inputs, two outputs and no internal common-mode feedback circuit;   a first and second feedback condenser coupled between a respective input and a respective output of the respective operational amplifier;   a first and second pair of switched feedback circuits, the first pair of feedback circuits being coupled between the outputs of a first differential operational amplifier and the inputs of the second differential operational amplifier, the second pair of switched feedback circuits being coupled between the outputs of the second differential operational amplifier and the inputs of the first differential operational amplifier; and   wherein the operational amplifier includes at least two amplifying means, connected in parallel between a voltage supply and ground.   
     
     
        .Iaddend..Iadd. 3.  The filter of claim 12, wherein each amplifying means includes at least a first and a second series connected transistor element, the first transistor element including a control connection coupled to a respective input of the operational amplifier. .Iaddend..Iadd.14. The filter of claim 13, wherein an intermediate node between each of the first and second series transistor elements is coupled to a respective output of the operational amplifier. .Iaddend..Iadd.15. The filter of claim 14, wherein the first transistor element is an N-channel CMOS transistor. .Iaddend..Iadd.16. The filter of claim 15, wherein the second transistor is a P-channel CMOS transistor. .Iaddend..Iadd.17. The band pass filter of claim 12, further comprising a multiplier circuit coupled between the outputs of the first operational amplifier and the inputs of the second operational amplifier. 
     
     
        .Iaddend..Iadd.18.  A completely differential filter, comprising: at least one differential operational amplifier having two inputs, two outputs, and no internal common-mode feedback circuit;   first and second feedback circuits coupled between a respective output and input of the differential operational amplifier; and   wherein the first and second feedback circuits provide common-mode feedback between the respective inputs and the respective outputs.   
     
     
        .Iaddend..Iadd.  .  A completely differential band-pass filter, comprising: at least two differential operational amplifiers, each differential operational amplifier having two inputs, two outputs and no internal common-mode feedback circuit;   a first and second feedback condenser coupled between a respective input and a respective output of the respective operational amplifier; and   a first and second pair of switched feedback circuits, the first pair of feedback circuits being coupled between the outputs of a first differential operational amplifier and the inputs of the second differential operational amplifier, a second pair of switched feedback circuits being coupled between the outputs of the second differential operational amplifier and the inputs of the first differential operational   
     
     
        amplifier. .Iaddend..Iadd.20.  The filter of claim 18, wherein each of the first and second feedback circuits includes a first condenser coupled between the respective input and the respective output of the operational amplifier. .Iaddend.