Differential amplifier having extended common mode input voltage range

A differential amplifier comprising:

• a first current mirror having an input and having first and second outputs;
• a first active device having a first node, having a second node coupled to said first current mirror first output, and having a control node;
• a second active device having a first node, having a second node coupled to said first current mirror first output, and having a control node;
• a third active device having a first node, having a second node coupled to said first current mirror second output, and having a control node coupled to a first one of said first and second active device control nodes;
• a second current mirror having an input coupled to said second active device first node and an output coupled to said first active device first node;
• a third current mirror having an input coupled to said third active device first node and having an output;
• and
• a fourth active device having a control node coupled to said third current mirror output and a first node coupled to said first current mirror input.

A differential amplifier comprising:

• a first current mirror having first, second, and third outputs;
• a second current mirror having an input coupled to said first current mirror first output and having an output;
• first and second active devices, each having first, second, and control nodes, said first and second active device second nodes being coupled together and to said first current mirror second output;
• third and fourth active devices, each having first, second, and control nodes, said third and fourth active device control nodes being coupled to said first and second active device control nodes, respectively, and said third and fourth active device second nodes being coupled together and to said second current mirror output;
• a fifth active device having a first node coupled to said second current mirror output, a second node coupled to said first current mirror third output, and a control node coupled to a first one of said first and second active device control nodes;
• a third current mirror having an input and an output coupled to the first nodes of said first and second active devices, respectively;
• a fourth current mirror having an input coupled to said third active device first node and an output coupled to said third current mirror output;
• and
• a fifth current mirror having an input coupled to said fourth active device first node and an output coupled to said third current mirror input.

A differential amplifier circuit for extending common mode range, said amplifier comprising:

• a differential input stage of said amplifier, said input stage having a signal input;
• means for supplying a bias current in said differential input stage of said amplifier;
• means for generating an image current which simulates said bias current in said differential input stage, said generating means being separate from said differential input stage and having an input coupled to said signal input of said differential input stage;
• and
• means for controlling the bias current of said supplying means in response to the image current of said generating means to maintain said bias current at a substantially constant level.

A method of extending common mode range of a differential amplifier comprising the steps of:

• supplying a bias current in a differential input stage of the differential amplifier;
• generating an image current which simulates the bias current in the differential input stage, said generating occurring in a stage which is remote from said differential input stage and has an input coupled to an input of the differential input stage;
• and
• controlling the bias current of said supplying step in response to the image current of said generating step to maintain the bias current at a substantially constant level.