Slew rate and settling time improvement circuitry and method for 3-stage amplifier

An operational amplifier comprising:

• a 3-stage amplifier having first and second input conductors conducting a differential input signal and an output conducting an output signal. wherein the 3-stage amplifier includes:
• a first amplifier stage having an input coupled to the input signal;
• a second amplifier stage having an input coupled to an output of the first amplifier stage;
• a third amplifier stage having an input coupled to an output of the second amplifier stage and an output conducting the output signal of the 3-stage amplifier;
• and
• an input chopper circuit coupling the input signal to the input of the first amplifier stage and an output chopper circuit coupled between the output of the first amplifier stage and the input of the second amplifier stage;
• and
• a slew rate and settling time improvement circuit having an input coupled to the input signal and an output, the slew rate and settling time improvement circuit including:
• first circuitry coupled to the input signal for producing a slew detection current when the magnitude of the input signal exceeds a predetermined level;
• second circuitry for converting the slew detection current to a control signal on the output of the slew rate and settling time improvement circuit;
• and
• a first switch controlled by the control signal coupled to prevent signal charge from building up on capacitances associated with the output of the first amplifier stage during slewing of the operational amplifier.

An operational amplifier comprising:

• a 3-stage amplifier having first and second input conductors conducting a differential input signal and an output conducting an output signal, wherein the 3-stage amplifier includes:
• a first amplifier stage having an input coupled to the input signal;
• a second amplifier stage having an input coupled to an output of the first amplifier stage;
• and
• a third amplifier stage having an input coupled to an output of the second amplifier stage and an output conducting the output signal of the 3-stage amplifier;
• and
• a slew rate and settling time improvement circuit having an input coupled to the input signal and an output, the slew rate and settling time improvement circuit including:
• first circuitry coupled to the input signal for producing a slew detection current when the magnitude of the input signal exceeds a predetermined level, wherein the first circuit includes a first transistor having a gate coupled to the first input conductor and a source coupled to the second input conductor and a second transistor having a gate coupled to the second input conductor and a source coupled to the first input conductor, drains of the first and second transistors being coupled to a first conductor to cause the slew detection current to flow in the first conductor if the magnitude of the input signal exceeds the predetermined level;
• second circuitry for converting the slew detection current to a control signal on the output of the slew rate and settling time improvement circuit;
• and
• a first switch controlled by the control signal coupled to prevent signal charge from building up on capacitances associated with the output of the first amplifier stage during slewing of the operational amplifier.

A method of amplifying an input signal to produce an output signal, comprising:

• providing a 3-stage amplifier having first and second input conductors conducting a differential input signal and an output conducting the output signal, wherein the 3-stage amplifier including includes:
• a first amplifier stage having an input coupled to the input signal;
• a second amplifier stage having an input coupled to an output of the first amplifier stage;
• a third amplifier stage having an input coupled to an output of the second amplifier stage and an output conducting the output signal;
• and
• a feed-forward amplifier stage having an input coupled to the input signal and an output coupled to the input of the third amplifier stage;
• and
• producing a slew detection current when the magnitude of the input signal exceeds a predetermined level;
• converting the slew detection current to a control signal;
• and
• short-circuiting conductors used in coupling the output of the first amplifier stage to the input of the second amplifier stage by means of a switch controlled by the control signal to prevent signal charge from building up on capacitances associated with the output of the first amplifier stage during slewing of the 3-stage amplifier.

An apparatus comprising:

• a multi-stage amplifier having a first input terminal and a second input terminal that receive a differential input signal, wherein the multi-stage amplifier includes:
• an first chopper that is coupled to the first input terminal and the second input terminal;
• a first amplifier that is coupled to the input chopper;
• a second chopper that is coupled to the first amplifier;
• a notch filter that is coupled to the second chopper;
• a second amplifier that is coupled to the notch filter;
• a third amplifier that is coupled to the first input terminal and the second input terminal;
• and
• a fourth amplifier that is coupled to the second amplifier and the third amplifier;
• and
• detection circuitry having:
• a first slew detecting transistor that is coupled to the first input terminal;
• a second slew detecting transistor that is coupled to the second input terminal;
• a current mirror that is coupled to each of the first and second slew detecting transistors;
• an inverter that is coupled to the current mirror;
• a first switch transistor that is coupled to the notch filter and the inverter, wherein the inverter controls the first switch;
• and
• a second switch transistor that is coupled the notch filter and the inverter, wherein the inverter controls the switch transistor.