Telescopic operational amplifier and reference buffer utilizing the same

A reference buffer, comprising:

• a telescopic operational amplifier (OPAMP) comprising:
• a first current source;
• an input stage, coupled to the first current source and comprising a group of input transistors, for receiving an input voltage, wherein the group of input transistors comprises a first input transistor and a second input transistor connected to the first input transistor, and the first and second input transistors are coupled between the first current source and the load stage;
• and
• a load stage, coupled to the input stage and comprising a group of load transistors, for generating an intermediate voltage, wherein the threshold voltages of the group of input transistors are larger than that of the group of the load transistors;
• and
• an output stage comprising an output transistor for receiving the intermediate voltage to generate an output voltage and returning the output voltage to the input stage of the telescopic OPAMP,
• wherein the group of load transistors comprises:
• a plurality of first load transistors connected to the first input transistor in series between a voltage source and the first current source;
• and
• a plurality of second load transistors connected to the second input transistor in series between the voltage source and the first current source.

A reference buffer, comprising:

• a first telescopic operational amplifier (OPAMP) comprising:
• a first current source;
• a first input stage, coupled to the first current source and comprising a first group of input transistors, wherein the first group of input transistors comprises a first input transistor and a second input transistor connected to the first input transistor, and the first and second input transistors are coupled between the first current source and the first load stage;
• and
• a first load stage, coupled to the first input stage and comprising a first group of load transistors, for generating a first intermediate voltage, wherein the threshold voltages of the first group of input transistors are larger than that of the first group of the load transistors;
• and
• a second telescopic OPAMP comprising:
• a second current source;
• a second input stage, coupled to the second current source and comprising a second group of input transistors, wherein the second group of input transistors comprises a third input transistor and a fourth input transistor connected to the third input transistor, and the third and fourth input transistors are coupled between the second current source and the second load stage;
• and
• a second load stage, coupled to the second input stage and comprising a second group of load transistors, for generating a second intermediate voltage, wherein the threshold voltages of the second group of input transistors are larger than that of the second group of load transistors;
• and
• an output stage comprising a first output transistor and a second output transistor, wherein the first output transistor is arranged to receive the first intermediate voltage to generate a first output voltage, the second output transistor is arranged to receive the second intermediate voltage to generate a second output voltage, and the output stage is arranged to return the first and second output voltages to the first and the second input stages, respectively,
• wherein the first group of load transistors comprises a plurality of first load transistors and a plurality of second load transistors, the second group of load transistors comprises a plurality of third load transistors and a plurality of fourth load transistors, the first load transistors serially connects to the first input transistor between a first voltage source and the first current source, the second load transistors serially connects to the second input transistor between the first voltage source and the first current source, the third load transistors serially connects to the third input transistor between a second voltage source and the second current source, and the fourth load transistors serially connects to the fourth input transistor in series between the second voltage source and the second current source.