US Patent 7215153 Fully differential input buffer with wide signal swing range

An input buffer for use in a differential operational amplifier is disclosed that regulates current through a main input differential pair while preventing output distortion and allowing high linearity. The input buffer includes a main input transistor pair that receives a voltage input, a tail current source, and a squeezable tail current source circuit including a single-ended self-biased folded feedback loop. These are configured such that current through the main input transistor pair is maintained as the voltage input varies. The folded feedback loop includes a folding transistor and a biasing current source that biases the folding transistor. The squeezable tail current source circuit also includes a replica transistor pair, a bias transistor, and a tail transistor pair. The biasing current source and folding transistor isolate the bias transistor and tail transistor pair from a drain voltage of the replica transistor pair, preventing output distortion and allowing high linearity.