Fast Unity Gain Opamp

031 COMPONENTS FAST UNITY GAIN OPAMP A number of operational amplifiers can be used only in circuits that have a certain mini- mum amplification, because they have been designed with small internal compensation. The advantage of that is that the amplifier is faster. If we look at two popular opamps, the LF356 and LF357, these characteristics are well illustrated. The LF356 may be used as a unity- gain amplifier with a gain-bandwidth product of 5 MHz and a slew rate of 12 V /gs. The LF357 needs an amplification of not less than 5 and has a gain-bandwidth product of 20 MHz and a slew rate of 50 V /As. It is possible to use the LF357 (and similar opamps) with smaller amplifications by using external compensa- tion, yet retaining most of the bandwidth. Normally, this is achieved with a capacitor, but that is not the only, and certainly not the best, method. An alternative method is illustrated in the dia- gram. Consider the L165 as a summing ampli- fier of which one input is connected...