Integrator distortion correction circuit
A system and method for reducing gain error and distortion in an operational amplifier due to errors in the second or integrator stage. A correction circuit may replicate an error current and insert the current into the signal stream to preempt the induction of an error at the amplifier's input. A capacitor may sample the error voltage at the input of the integrator stage of the amplifier and generate a replica of the error current in the integration capacitor to feed it into the input of the integrator stage. This eliminates any nonlinearity errors created by error currents in the compensation or integration capacitor at the second or integrator stage of the two-stage amplifier. Feeding the error current to the integrator stage may be facilitated with a unity gain buffer and a current mirror.
1. A system comprising:
an operational amplifier having an input transconductance stage driving an integrator stage; and
a correction circuit to cancel an error current in a compensation capacitor created by an error voltage at an input of the integrator stage of the amplifier and to channel a correction current to the compensation capacitor.
2. The system of claim 1 , wherein the correction circuit comprises a unity gain buffer, a correction capacitor, and a current mirror.
3. The system of claim 2 , wherein the correction circuit further comprises a resistor.
4. The system of claim 2 , wherein the value of the compensation capacitor equals the value of the correction capacitor.
5. The system of claim 2 , wherein the unity gain buffer replicates the error voltage at the input of the integrator stage, a current output of the unity gain buffer is input into the current mirror, and an output current of the current mirror is input into the integrator stage of the amplifier.
6. The system of claim 2 , wherein an output of the unity gain buffer is coupled to the correction capacitor.
7. The system of claim 3 , wherein the resistor is coupled to an output of the unity gain buffer and to the correction capacitor.
8. The system of claim 1 , wherein the operational amplifier and the correction circuit are provided in a common integrated circuit.
9. A method comprising:
detecting an error voltage at an input of an integrator unit;
replicating an error current created by the error voltage; and
channeling the replicated current to the input of the integrator unit.
10. The method of claim 9 , wherein the integrator unit comprises:
a transconductance unit; and
an integration capacitor connected across an input of the transconductance unit and an output of the transconductance unit.
11. The method of claim 9 , wherein said replicating further comprises:
sampling an error voltage at the input of the integrator unit; and
inducing the replicated current with the sampled error voltage.
12. The method of claim 11 , wherein said inducing includes applying the error voltage to a capacitor.
13. The method of claim 12 , wherein said inducing further comprises setting a value of the capacitor to replicate the error current.
14. The method of claim 11 , wherein said sampling includes buffering the error voltage with a unity voltage buffer.
15. The method of claim 9 , wherein said channeling further comprises mirroring the replicated current to an input of the integrator unit.
16. A method of amplifying a differential signal comprising:
generating an output current with a received input voltage at an input transconductance stage of an operational amplifier;
generating a correction current based on an input voltage of an integrator stage of the operational amplifier; and
integrating the output current and the correction current at the integrator a stage of the operational amplifier to generate an output voltage.
17. The method of claim 16 , wherein said generating a correction current comprises:
sampling an error voltage at input terminals of the integrator stage;
generating an error current with the error voltage; and
channeling the error current to an input of the integrator stage.