TABLE OF CONTENTS Whether constructed from discrete components or opamps all amplifier systems can be described in terms of two frequency-dependent noise sources, a voltage noise generator e n and a current noise generator i n (Fig. 3.4). Both these quantities are usually specified in manufacturers' data sheets for opamps and are either specified or can be calculated from discrete transistor data sheets. The current noise generator is essentially just the shot noise of the bias currents in the amplifier input stages. Depending on bias current there are large differences in current noise spectral densities among opamps, especially between bipolar devices, which run at relatively high base bias currents, and FET types, which require much smaller gate bias currents. Table 3.4 compares the noise generators of a typical bipolar and an FET opamp. Current noise is clearly much higher in the bipolar device than in the FET device, whereas voltage noise is somewhat smaller. Nevertheless, both these devices can be labeled "low noise." Whether one or the other performs better in your measurement is a function of the circuit configuration. Another important difference to note between bipolar and FET types is that FET bias currents, and with them current noise magnitudes, are very dependent on temperature. Hence if high operating temperatures are expected, these parameters in particular should be calculated at the temperature extremes. It may be that a conventional or "super- ?" bipolar design has smaller bias currents at high temperature than...
