Portable analog design needs rail-to-rail op amps

摘要

Portable-equipment design dictates single-supply, low-voltage design techniques. Rail-to-rail op amps are mandatory in portable designs because only they satisfy the design criteria of low noise, high dynamic range, signal sensing at the input rails, and rail-to- rail output-voltage swing. They satisfy the design criteria, but why do they cost more than conventional op amps? Transducers are invariably connected to a supply rail rather than to a V_(CC)/2 bias voltage; thus, the transducer output voltage swings above and below a supply rail. Conventional op amps have a differential-amplifier input structure, a "long-tailed pair," and they use either npn or pnp input transistors. The pnp transistors can function at the lower but not the upper rail and vice versa for the npn transistors.