Precision current mirror and method for voltage to current conversion in low voltage applications

A voltage-to-current converter circuit comprising:

• (a) an error amplifier amplifying a voltage difference between drains of first and second transistors of a first current mirror, wherein drain current of the first transistor is proportional to an input voltage of the voltage-to-current converter circuit;
• (b) an output of the error amplifier coupled to gates of the first and second transistors;
• (c) a compensation capacitor coupled between a gate and drain of the first transistor;
• (d) a cascode transistor having a source coupled to a drain of the second transistor;
• and
• (e) a second current mirror having an input receiving a drain current of the cascode transistor and including an output transistor which provides a current that is proportional to the input voltage as a bias current for the error amplifier to provide stable operation of the voltage-to-current converter circuit.

A current mirror circuit for converting an input current in a first conductor to an output current, comprising:

• (a) a first current mirror transistor having a drain in which the input current is provided, the first current mirror transistor having a source coupled to a first supply voltage conductor and a gate connected by a second conductor to a gate of a second current mirror transistor having a source coupled to the first supply voltage conductor;
• (b) a cascode transistor having a source coupled by a third conductor to a drain of the second current mirror transistor and a gate coupled to a bias voltage;
• (c) an error amplifier having a first input coupled to the first conductor, a second input coupled to the third conductor, and an output coupled to the second conductor, the error amplifier also having a bias terminal connected to a fourth conductor;
• (d) a current mirror input transistor having a source coupled to a second supply voltage conductor, and a gate and drain connected by a fifth conductor to a drain of the cascode transistor and to a gate of a first current mirror output transistor, the first current mirror output transistor having a source connected to the second supply voltage conductor and a drain coupled to the fourth conductor to provide to the error amplifier a bias current proportional to the output current to cause a transconductance of the error amplifier to vary in the same direction as a transconductance of the first and second current mirror transistors;
• and
• (e) a second current mirror output transistor for conducting the output current, the second current mirror output transistor having a source coupled to one of the first and second supply voltage conductors and also having a gate coupled to one of the second and fifth conductors.

A voltage-to-current conversion circuit for converting an input voltage to an output current, comprising:

• (a) an input voltage-to-input-current converter including an operational amplifier having a first input coupled to receive an input voltage, an output coupled to a gate of a first transistor having a source coupled to a second input of the operational amplifier and to a first terminal of a resistor, a second terminal of the resistor being coupled to a first supply voltage conductor to produce an input current through a drain of the first transistor;
• (b) a first current mirror transistor having a drain coupled by a first conductor to the drain of the first transistor, the first current mirror transistor having a source coupled to a second supply voltage conductor and a gate connected by a second conductor to a gate of a second current mirror transistor having a source coupled to the second supply voltage conductor;
• (c) a cascode transistor having a source coupled by a third conductor to a drain of the second current mirror transistor and a gate coupled to a bias voltage;
• (d) an error amplifier having a first input coupled to the first conductor, a second input coupled to the third conductor, and an output coupled to the second conductor, the error amplifier also having a bias terminal connected to a fourth conductor;
• (e) a current mirror input transistor having a source coupled to the first supply voltage conductor, and a gate and drain connected by a fifth conductor to a drain of the cascode transistor and to a gate of a first current mirror output transistor, the first current mirror output transistor having a source connected to the first supply voltage conductor and a drain coupled to the fourth conductor to provide to the error amplifier a bias current proportional to the output current to cause a transconductance of the error amplifier to vary in the same direction as a transconductance of the first and second current mirror transistors;
• and
• (f) a second current mirror output transistor for conducting the output current, the second current mirror output transistor having a source coupled to one of the first and second supply voltage conductors and also having a gate coupled to one of the second and fifth conductors.

A voltage-to-current conversion circuit for converting an input voltage to an output current, comprising:

• (a) an input voltage-to-input-current converter including an operational amplifier having a first input coupled to receive an input voltage, an output coupled to a gate of a first transistor having a source coupled to a second input of the operational amplifier and to a first terminal of a resistor, a second terminal of the resistor being coupled to a first supply voltage conductor to produce an input current through a drain of the first transistor;
• (b) a first current mirror transistor having a drain coupled by a first conductor to the drain of the first transistor, the first current mirror transistor having a source coupled to a second supply voltage conductor and a gate connected by a second conductor to a gate of a second current mirror transistor having a source coupled to the second supply voltage conductor;
• (c) a cascode transistor having a source coupled by a third conductor to a drain of the second current mirror transistor and a gate coupled to a bias voltage;
• (d) an error amplifier having a first input coupled to the first conductor, a second input coupled to the third conductor, and an output coupled to the second conductor, the error amplifier also having a bias terminal connected to a fourth conductor;
• and
• (e) a current mirror input transistor having a source coupled to the first supply voltage conductor, and a gate and drain connected by a fifth conductor to a drain of the cascode transistor and to gates of a first current mirror output transistor and a second current mirror output transistor, the first and second current mirror output transistors each having a source connected to the first supply voltage conductor, the first current mirror output transistor having a drain coupled to supply the output current, the second current mirror output transistor having a drain coupled to the fourth conductor to provide to the error amplifier a bias current proportional to the output current to cause a transconductance of the error amplifier to vary in the same direction as a transconductance of the first and second current mirror transistors.

A method of operating a current mirror circuit, comprising:

• (a) amplifying a voltage difference between drains of first and second transistors of a first current mirror by means of an error amplifier, wherein a drain current of the first transistor is provided as an input current for the current mirror circuit;
• (b) applying the amplified voltage difference to gates of the first and second transistors;
• (c) providing compensation capacitance between the gate and drain of the first transistor;
• (d) setting the drain voltage of the second transistor by means of a cascode transistor;
• and
• (e) forcing a drain current of the cascode transistor into an input of a second current mirror including an output transistor which provides a current proportional to the input current as a bias current for the error amplifier to provide an output current of the current mirror circuit and to provide stable operation thereof.

A method of operating a voltage-to-current converter, comprising:

• (a) amplifying a voltage difference between drains of first and second transistors of a first current mirror by means of an error amplifier, wherein drain current of the first transistor is proportional to an input voltage of the voltage-to-current converter;
• (b) applying the amplified voltage difference to gates of the first and second transistors;
• (c) providing compensation capacitance between the gate and drain of the first transistor;
• (d) setting the drain voltage of the second transistor by means of a cascode transistor;
• and
• (e) forcing a drain current of the cascode transistor into an input of a second current mirror including an output transistor which provides a current proportional to the input voltage as a bias current for the error amplifier to provide an output current of the voltage-to-current conversion circuit and to provide stable operation thereof.

A current mirror circuit, comprising:

• (a) means for amplifying a voltage difference between drains of first and second transistors of a first current mirror by means of an error amplifier, wherein a drain current of the first transistor is provided as an input current for the current mirror circuit;
• (b) means for applying the amplified voltage difference to gates of the first and second transistors;
• (c) means for providing compensation capacitance between the gate and drain of the first transistor;
• (d) means for setting a drain voltage of the second transistor;
• and
• (e) means for forcing a drain current of the cascode transistor into an input of a second current mirror including an output transistor which provides a current proportional to the input current as a bias current for the error amplifier to provide an output current of the current mirror circuit and to provide stable operation thereof.

A voltage-to-current converter comprising:

• (a) means for amplifying a voltage difference between drains of first and second transistors of a first current mirror by means of an error amplifier, wherein drain current of the first transistor is proportional to an input voltage of the voltage-to-current converter;
• (b) means for applying the amplified voltage difference to gates of the first and second transistors;
• (c) means for providing compensation capacitance between the gate and drain of the first transistor;
• (d) means for setting a drain voltage of the second transistor;
• and
• (e) means for forcing a drain current of the cascode transistor into an input of a second current mirror including an output transistor which provides a current proportional to the input voltage as a bias current for the error amplifier to provide an output current and to provide stable operation of the voltage-to-current conversion circuit.