Introduction
Noise is one of the more important aspects of circuit design. Lower noise in audio amplifier design means better quality audio. Low-noise in instrument design means more reliable and accurate measurements at lower voltage and current levels. And low-noise in current source designs means less noise injected into the electronic system. One of the most basic approaches to reduce noise in an amplifier is to use low-noise components. This would include low-noise bipolar transistors and low-valued resistors. Although one can use low-noise operational amplifiers in the design of low-noise circuits or low-noise current sources it is often not done. This is because lower noise can be often be obtained with a well-thought out discrete design. And that’s because discrete transistors offer much more design customization capability for specific circuit applications – like optimized collector biasing. One must remember that monolithic chip designs are built for the most part for general purpose applications. It is hard to tailor a specific op amp or current source chip for a specific source resistance, specific power consumption requirement and a specific bandwidth that will lower the overall noise – that is unless you add-on more compensation components.