Folded cascode operational amplifier
A folded cascode operational amplifier is disclosed. The folded cascode operational amplifier includes a first current source, a second current source, and a first voltage terminal connected to the first current source and the second current source. The folded cascode operational amplifier also includes a first input-transistor connected to the first current source in series, and a second input-transistor connected to the second current source in series. Further, the folded cascode operational amplifier includes a tail current source connected to a connection point between the first input-transistor and the second input-transistor, a load current source, and a second voltage terminal connected to the tail current source and the load current source. The folded cascode operational amplifier also includes an output-transistor connected to the load current source, and an output-terminal arranged between the second current source and the second input-transistor and connected to the output-transistor. The second current source is a mirroring current source of the first current source, and a ratio of a current passing through the second current source to a current passing through the first current source is greater than one.
1. A folded cascode operational amplifier, comprising:
a first current source;
a second current source;
a first voltage terminal connected to the first current source and the second current source;
a first input-transistor connected to the first current source in series;
a first metal-oxide-semiconductor field-effect transistor (MOSFET) connected in series between the first current source and the first input-transistor;
a second input-transistor connected to the second current source in series;
a tail current source connected to a connection point between the first input-transistor and the second input-transistor;
a load current source;
a second voltage terminal connected to the tail current source and the load current source;
an output-transistor connected to the load current source; and
an output-terminal arranged between the second current source and the second input-transistor and connected to the output-transistor,
wherein the second current source is a mirroring current source of the first current source, and a ratio of a current passing through the second current source to a current passing through the first current source is greater than one.
2. The folded cascode operational amplifier according to claim 1 , wherein:
the output-transistor is a P-type MOSFET;
the source of the output-transistor is connected to the output-terminal; the drain of the output-transistor is connected to the load current source;
the drain of the first MOSFET is connected to the first input-transistor; the gate of the first MOSFET is connected to the output-transistor; the first MOSFET and the output-transistor form a symmetrical circuit structure; and
a gate voltage of the first MOSFET is equal to a gate voltage of the output-transistor.
3. The folded cascode operational amplifier according to claim 1 , wherein:
the first input-transistor and the second input-transistor are MOSFETs;
a gate voltage of the first input-transistor and a gate voltage of the second input-transistor have opposite directions but equal absolute values; and
the first input-transistor and the second input-transistor compose a differential pair.
4. The folded cascode operational amplifier according to claim 2 , wherein:
the first current source and the second current source are P-type MOSFETs;
both the source of the first current source and the source the second current source are connected to the first voltage terminal; and
the gate of the first current source is connected to the gate of the second current source and also between the first MOSFET and the first input-transistor.
5. The folded cascode operational amplifier according to claim 1 , further including:
a buffer module connected to the output-terminal, wherein:
the buffer module includes a Miller capacitor, a resistor and a buffer unit;
one terminal of the Miller capacitor is connected to the output-terminal;
the other terminal of the Miller capacitor is connected to the resistor;
a V in terminal of the buffer unit is connected between the output-transistor and the load current source; and
a V out terminal of the buffer unit is connected to the resistor.
6. The folded cascode operational amplifier according to claim 5 , wherein:
the buffer unit is one of a common-source amplifier structure, a push-pull output structure, and a source follower push-pull output structure.
7. The folded cascode operational amplifier according to claim 1 , wherein:
the tail current source is an N-type MOSFET;
the drain of the tail current source is connected to both the first input-transistor and the second input-transistor; and
the source of the tail current source is connected to the load current source.
8. The folded cascode operational amplifier according to claim 1 , wherein:
the load current source is an N-type MOSFET;
the source of the load current source is connected to both the tail current source and the second voltage terminal; and
the drain of the load current source is connected to the output-transistor.
9. The folded cascode operational amplifier according to claim 1 , wherein:
the first input-transistor and the second input-transistor are N-type MOSFETs;
the source of the first input-transistor and the source of the second input-transistor are connected together;
the drain of the first input-transistor is connected to the first MOSFET; and
the drain of the second input-transistor is connected to the output-terminal, the second current source, and the output-transistor.
10. A folded cascode operational amplifier, comprising:
a first current source;
a second current source;
a first input-transistor connected to the first current source in series;
a first metal-oxide-semiconductor field-effect transistor (MOSFET) connected in series between the first current source and the first input-transistor such that size of the first current source can be reduced;
a second input-transistor connected to the second current source in series;
a tail current source connected to a connection point between the first input-transistor and the second input-transistor;
a load current source;
an output-transistor connected to the load current source; and
an output-terminal arranged between the second current source and the second input-transistor and connected to the output-transistor, wherein the folded cascode operational amplifier is configured to increase a slew rate by adjusting a width-to-length ratio of the second current source and a width-to-length ratio of the first current source.
11. The folded cascode operational amplifier according to claim 10 , wherein:
the slew rate is increased without changing a current I passing through the tail current source.
12. The folded cascode operational amplifier according to claim 11 , wherein:
the width-to-length ratio of the second current source and the width-to-length ratio of the first current source are adjusted such that the width-to-length ratio of the second current source is X times of the width-to-length ratio of the first current source;
a bias current passing through the load current source is also adjusted to (X−1)I/2; and
the slew rate is increased to (X+1)I/2C, wherein X is greater than 1, and C is capacitance of a corresponding load capacitor.