Rail-to-rail comparator with hysteresis

1. A comparator, comprising:

at least one current stage for providing a first current proportional to a difference between first and second comparator inputs, the first current being provided to an amplifier input;

an amplifier for amplifying a current provided to the amplifier input and providing a comparator output;

an apparatus for introducing hysteresis, comprising at least one of a current source and a current sink, the current source being arranged to selectively source a source current to the amplifier input such that the comparator output changes from a first state to a second state when a difference between the first and second inputs rises above a first value, and the current sink being arranged to selectively sink a sink current from the amplifier input such that the comparator output changes from the second state to the first state when the difference between the first and second inputs falls below a second value; and

apparatus for controlling at least one of the source current and the sink current to be proportional to a bias current of the current stage.

2. A comparator as claimed in claim 1 , wherein the current source comprises at least one first input transistor for producing a first input current in response to the first input, and at least one second transistor for producing a second input current in response to the second input, the bias current of the current stage substantially comprising a sum of the first and second input currents.

3. A comparator as claimed in claim 2 , comprising control apparatus for receiving at least one of the first and second input currents and producing control signals proportional to the at least one of the first and second input currents, the control signals being used to control the at least one of the current source and the current sink.

4. A comparator as claimed in claim 1 , wherein the apparatus for controlling comprises an apparatus for generating at least one control signal for the at least one of the source current and the sink current such that the at least one control signal is proportional to the bias current of the current stage.

5. A method of comparing two signals, comprising:

producing a first current proportional to a difference between the two signals;

providing the first current to an amplifier input;

amplifying the current provided to the amplifier input to produce a comparator output;

sourcing a source current to the amplifier input when the comparator output is a first logic level;

sinking a sink current from the amplifier input when the comparator output is a second logic level; and

controlling at least one of the source current and the sink current to be proportional to a bias current of apparatus used to produce the first current.

6. An electronic device including a comparator as claimed in claim 1 .