Fast rail-to-rail class AB output stage having stable output bias current and linear performance

A class AB output stage, comprising:

• (a) an amplifying stage adapted to produce first and second output signals which incrementally increase and decrease in response to an incremental increase and decrease, respectively, of a first input signal, and which also incrementally increase and decrease in response to an incremental increase and decrease, respectively, of a second input signal, the amplifying stage including first and second signal input conductors on which the first and second input signals are applied, respectively, first and second output conductors on which the first and second output signals are applied, respectively, first and second control conductors, and first and second reference conductors;
• (b) a pull-up transistor having a source coupled to a first supply voltage conductor, a gate coupled to receive the second output signal, and a drain coupled to an output terminal, and a pull-down transistor having a source coupled to a second supply voltage conductor, a gate coupled to receive the first output signal, and a drain coupled to the output terminal;
• (c) a first feedback circuit including a first current sensing transistor having a gate and source connected to the gate and source, respectively, of the pull-down transistor and a drain coupled to the first control conductor and operative to increase the second output signal only until the drain current of the first current sensing transistor increases to a first predetermined value representative of a minimum desired quiescent current in the pull-down transistor, and a second feedback circuit including a second current sensing transistor having a gate and source connected to the gate and source, respectively, of the pull-up transistor, and a drain coupled to the second control conductor and operative to decrease the first output signal only until the drain current of the second current sensing transistor increases to a second predetermined value representative of a minimum desired quiescent current in the pull-up transistor.

A class AB output stage, comprising:

• (a) amplifying means for producing first and second output signals which incrementally increase and decrease in response to an incremental increase and decrease, respectively, of a first input signal, and which also incrementally increase and decrease in response to an incremental increase and decrease, respectively, of a second input signal;
• (b) a pull-up transistor having a source coupled to a first supply voltage conductor, a gate coupled to receive the second output signal, and a drain coupled to an output terminal, and a pull-down transistor having a source coupled to a second supply voltage conductor, a gate coupled to receive the first output signal, and a drain coupled to the output terminal;
• (c) a first feedback circuit including a first current sensing transistor having a gate and source connected to the gate and source, respectively, of the pull-down transistor and a drain coupled to a first control input of the amplifying means and operative to increase the gate voltage of the first current sensing transistor only until its drain current increases to a first predetermined value representative of a minimum desired quiescent current in the pull-down transistor;
• and
• (d) a second feedback circuit including a second current sensing transistor having a gate and source connected to the gate and source, respectively, of the pull-up transistor, and a drain coupled to a second control input of the amplifying means and operative to decrease the gate voltage of the second current sensing transistor only until its drain current increases to a second predetermined value representative of a minimum desired quiescent current in the pull-up transistor.

A rail-to-rail class AB output circuit, comprising:

• (a) an amplifying stage including first and second signal input conductors, first and second feedback input conductors, and first and second output conductors the amplifier stage including
• i. a first transistor having a source coupled by the first input conductor to a first constant current source, a drain coupled to the first output conductor, and a gate coupled to a first reference voltage,
• ii. a second transistor having a source coupled to the first input conductor, a drain coupled to the second output conductor, and a gate coupled to a first terminal of a first current control circuit,
• iii. a first V/I converter circuit coupled between the gates of the first and second transistors,
• iv. a third transistor having a source coupled by the second input conductor to a second constant current source, a drain coupled to the second output conductor, and a gate coupled to a second reference voltage,
• v. a fourth transistor having a source coupled to the second input conductor, a drain coupled to the first output conductor, and a gate coupled to a first terminal of a second current control circuit,
• vi. a second V/I converter circuit coupled between the gates of the third and fourth transistors;
• (b) output circuitry including an output terminal, a pull-up transistor coupled between a first supply voltage conductor and the output terminal, a pull-down transistor coupled between the output terminal and a second supply voltage conductor, the pull-down transistor having a gate coupled to the first output conductor, the pull-up transistor having a gate coupled to the second output terminal;
• (c) feedback circuitry coupled to the first and second feedback input conductors and the first and second output conductors to stabilize quiescent currents in the pull-up transistor and the pull-down transistor, the feedback circuitry including
• i. a first sensor transistor having a gate connected to the gate of the pull-down transistor, a source connected to the source of the pull-down transistor,
• ii. a first reference current source supplying a first reference current, the first reference current source having a terminal coupled to the drain of the first sensor transistor,
• iii. a first current control circuit having a terminal coupled to the drain of the first sensor transistor,
• iv. a second sensor transistor having a gate connected to the gate of the pull-up transistor, a source connected to the source of the pull-up transistor,
• v. a second reference current source supplying a second reference current, the second reference current source having a terminal coupled to the drain of the second sensor transistor,
• vi. a second current control circuit having a terminal coupled to the drain of the second sensor transistor,
• wherein the first current control circuit controls feedback from the first sensor transistor to reduce the turn-on voltage of the second transistor relative to the turn-on voltage of the first transistor if the drain current of the first sensor transistor is less than the first reference current to increase the turn-on voltage of the first sensor transistor until its drain current equals the first reference current, and
• wherein the second current control circuit controls feedback from the second sensor transistor to reduce the turn-on voltage of the fourth transistor relative to the turn-on voltage of the third transistor if the drain current of the second sensor transistor is less than the second reference current to increase the turn-on voltage of the second sensor transistor until its drain current equals the second reference current.

A rail-to-rail class AB output stage, comprising:

• (a) an amplifying stage producing first and second output signals which both incrementally change in a common direction in response to an incremental change of a first input signal and which both also change in the common direction in response to an incremental change of a second input signal;
• (b) a pull-up transistor having a first electrode coupled to a first supply voltage conductor, a control electrode coupled to receive the second output signal, and a second electrode coupled to an output terminal, and a pull-down transistor having a first electrode coupled to a second supply voltage conductor, a control electrode coupled to receive the first output signal, and a second electrode coupled to the output terminal;
• and
• (c) a first feedback circuit including a first current sensing transistor having a control electrode and first electrode connected to the control electrode and first electrode, respectively, of the pull-down transistor and a second electrode coupled to a first control input of the amplifying stage and operative to increase the control electrode voltage of the first current sensing transistor only until its second electrode current increases to a first predetermined value representative of a minimum desired quiescent current in the pull-down transistor, and a second feedback circuit including a second current sensing transistor having a control electrode and first electrode connected to the control electrode and first electrode, respectively, of the pull-up transistor, and a second electrode coupled to a second control input of the amplifying stage and operative to decrease the control electrode voltage of the second current sensing transistor only until its second electrode current increases to a second predetermined value representative of a minimum desired quiescent current in the pull-up transistor.

An operational amplifier comprising:

• (a) a front end circuit including an input stage receiving an input signal and producing complementary first and second output signals in response to the input signal;
• and
• (b) a rail-to-rail class AB output stage including
• i. an amplifying stage producing third and fourth output signals which incrementally increase and decrease in response to an incremental increase and decrease, respectively, of the first output signal, and which also incrementally increase and decrease in response to an incremental increase and decrease, respectively, of the second output signal,
• ii. a pull-up transistor having a first electrode coupled to a first supply voltage conductor, a control electrode coupled to receive the fourth output signal, and a second electrode coupled to an output terminal, and a pull-down transistor having a first electrode coupled to a second supply voltage conductor, a control electrode coupled to receive the third output signal, and a second electrode coupled to the output terminal, and
• iii. a first feedback circuit including a first current sensing transistor having a control electrode and first electrode connected to the control electrode and first electrode, respectively, of the pull-down transistor and a second electrode coupled to a first control input of the amplifying stage and operative to increase the control electrode voltage of the first current sensing transistor only until its second electrode current increases to a first predetermined value representative of a minimum desired quiescent current in the pull-down transistor, and a second feedback circuit including a second current sensing transistor having a control electrode and first electrode connected to the control electrode and first electrode, respectively, of the pull-up transistor, and a second electrode coupled to a second control input of the amplifying stage and operative to decrease the control electrode voltage of the second current sensing transistor only until its second electrode current increases to a second predetermined value representative of a minimum desired quiescent current in the pull-up transistor.

A method of providing rail-to-rail operations of a class AB output stage, comprising:

• (a) producing first and second output signals which both incrementally change in a common direction by amplifying an incremental change of a first input signal and/or amplifying an incremental change of a second input signal;
• (b) applying the second output signal to a control electrode of a pull-up transistor having a first electrode coupled to a first supply voltage conductor and a second electrode coupled to an output terminal, and applying the second output signal to a control electrode of a pull-down transistor having a first electrode coupled to a second supply voltage conductor and a second electrode coupled to the output terminal;
• and
• (c) operating a first feedback circuit including a first current sensing transistor having a control electrode and first electrode connected to the control electrode and first electrode, respectively, of the pull-down transistor and a second electrode coupled to a first control input of the amplifying stage to increase the control electrode voltage of the first current sensing transistor only until its second electrode current increases to a first predetermined value representative of a minimum desired quiescent current in the pull-down transistor, and operating a second feedback circuit including a second current sensing transistor having a control electrode and first electrode connected to the control electrode and first electrode, respectively, of the pull-up transistor, and a second electrode coupled to a second control input of the amplifying stage to decrease the control electrode voltage of the second current sensing transistor only until its second electrode current increases to a second predetermined value representative of a minimum desired quiescent current in the pull-up transistor.