Gain enhancement technique for operational amplifiers

An operational amplifier circuit having improved common-mode rejection comprising:

• a differential input section including a differential input pair for receiving a differential input signal and a differential output pair including first and second differential outputs;
• a cascode current mirror section having a first mirror current source coupled to it first output of the operational amplifier circuit and a second mirror current source coupled to a second output of the operational amplifier circuit;
• and
• a cascode current source section having a first cascode current source coupled to the first output of the operational amplifier circuit and a second cascode current source coupled to the second output of the operational amplifier circuit, the first cascode current source having a feedback node coupled to the second differential output of the input section and having a control node, the second cascode current source having a feedback node coupled to the first differential output of the input section and having a control node, and
• a fully-differential operational amplifier having a negative input coupled to the first cascode current source feedback node, a positive input coupled to the second cascode current source feedback node, a positive output coupled to the first cascode current source control node, and a negative output coupled to the second cascode current source control node, wherein the second fully-differential amplifier is constructed and arranged to amplify a differential feedback signal appearing on the feedback nodes of the first and second cascode current sources while remaining substantially unresponsive to a common-mode feedback signal appearing thereon when said operational amplified circuit is in operative condition.

An operational amplifier circuit having improved common-mode rejection comprising:

• a differential input section having a differential input pair for receiving a differential input signal and a differential output pair including first and second differential outputs;
• a cascode current source section having a first cascode current source coupled to the first output of the operational amplifier circuit and a second cascode current source coupled to the second output of the operational amplifier circuit, the first cascode current source having a feedback node coupled to the second differential output of the input section and having a control node, the second cascode current source having a feedback node coupled to the first differential output of the input section and having a control node;
• and
• a first fully-differential operational amplifier having a negative input coupled to the first cascode current source feedback node, a positive input coupled to the second cascode current source feedback node, a positive output coupled to the first cascode current source control node, and a negative output coupled to the second cascode current source control node, wherein the first fully-differential amplifier is constructed and arranged to amplify a differential feedback signal appearing on the feedback nodes of the first and second cascode current sources while remaining substantially unresponsive to a common-mode feedback signal appearing thereon when said operational amplified circuit is in operative condition;
• a cascode current mirror section having a first mirror current source coupled to the first output and a second mirror current source coupled to the second output, the first mirror current source having a feedback node and a control node and the second mirror current source having a feedback node and a control node, and
• a second fully-differential operational amplifier having a negative input coupled to the first mirror current source feedback node, a positive input coupled to the second mirror current source feedback node, a positive output coupled to the first mirror current source control node, and a negative output coupled to the second mirror current source control node, wherein the second fully-differential amplifier is constructed and arranged to amplify a differential feedback signal appearing on the feedback nodes of the first and second mirror current sources while remaining substantially unresponsive to a common-mode feedback signal appearing thereon when said operational amplified circuit is in operative condition.

An operational amplifier circuit having improved common-mode rejection comprising:

• a differential input section having a differential input pair for receiving a differential input signal and a differential output pair including first and second differential outputs.
• a cascode current source section having a first cascode current source coupled to the first output of the operational amplifier circuit and a second cascode current source coupled to the second output of the operational amplifier circuit, the first cascode current source having a feedback node coupled to the second differential output of the input section and having a control node, the second cascode current source having a feedback node coupled to the first differential output of the input section and having a control node, and
• a first fully-differential operational amplifier having a negative input coupled to the first cascode current source feedback node, a positive input coupled to the second cascode current source feedback node, a positive output coupled to the first cascode current source control node, and a negative output coupled to the second cascode current source control node, wherein the first fully-differential amplifier amplifies a differential feedback signal appearing on the feedback nodes of the first and second cascode current sources while remaining substantially unresponsive to a common-mode feedback signal appearing thereon;
• and
• a cascode current mirror section having a first mirror current source coupled to the first output and a second mirror current source coupled to the second output, the first mirror current source having a feedback node and a control node and the second mirror Current source having a feedback node and a control node, and
• a second fully-differential operational amplifier having a negative input coupled to the first mirror current source feedback node, a positive input coupled to the second mirror current source feedback node, a positive output coupled to the first mirror current source control node, and a negative output coupled to the second mirror current source control node, wherein the second fully-differential amplifier amplifies a differential feedback signal appearing on the feedback nodes of the first and second mirror current sources while remaining substantially unresponsive to a common-mode feedback signal appearing thereon:
• wherein the differential input section includes a P-channel input section including P-channel field-effect-transistors (FET), the P-channel input section having a P-channel differential input pair coupled to the differential input pair of the input section for receiving the differential input signal and having a P-channel differential output pair coupled to the cascode current source, and
• wherein the differential input section further includes an N-channel input section including N-channel field-effect-transistors (FET) having an N-channel differential input pair coupled to the differential input pair of the P-channel input section and having an N-channel differential output pair coupled to the cascode current mirror section.