Buffers are often called voltage followers because they have largely unity voltage gain, thus the name has generally come to mean a device that supplies current gain but no voltage gain. Buffers serve in applications requiring isolation of source and load, for example, high input impedance and low output impedance (high output current drive). In addition, they offer gain flatness and wide bandwidth.

Most operational amplifiers that meet the other given requirements in a particular application can be configured as buffers, though they are generally more complex and are, for the most part, not optimized for unity gain operation. The commercial buffer is a cost effective substitute for an op amp. Buffers serve several useful functions, either in tandem with op amps or in standalone applications. As mentioned, their primary function is to isolate a high impedance source from a low impedance load, since a high Z source cannott supply the needed current to the load. For example, in the case where the signal source to an analog to digital converter is a sensor, it is recommended that the sensor be isolated from the A/D converter. The use of a buffer ensures a low output impedance and delivery of a stable output to the converter. In A/D converter applications buffers need to drive varying and complex reactive loads.

Buffers come in two flavors: Open Loop and Closed Loop. While sacrificing the precision of some DC characteristics, and generally displaying poorer gain linearity, open loop buffers offer lower cost and increased bandwidth, along with less phase shift and propagation delay than do closed loop buffers. The LMH6321 is of the open loop variety.

Figure 6-1 shows a simplified diagram of the LMH6321 topology, revealing the open loop complementary follower design approach. Figure 6-2 shows the LMH6321 in a typical application, in this case, a 50 Ω coaxial cable driver.

Figure 6-1 Simplified Schematic