Diamond buffer

The diamond buffer or diamond follower is a four-transistor, two-stage, push-pull, translinear emitter follower, or less commonly source follower, in which the input transistors are folded, or placed upside-down with respect to the output transistors. Like any unity buffer, the diamond buffer does not alter the phase and magnitude of input voltage signal; its primary purpose is to interface a high-impedance voltage source with a low-impedance, high-current load. Unlike the more common compound emitter follower (a "double" in audio engineering terms), where each input transistor drives the output transistor of the same polarity, each input transistor of a diamond buffer drives the output transistor of the opposite polarity. When the transistors operate in close thermal contact, the input transistors stabilize the idle current of the output pair, eliminating the need for a bias spreader.

The diamond buffer is used primarily in the input and output stages of high-speed current-feedback operational amplifiers. The circuit is also the essential building block of bipolar current conveyors, and has seen limited use in line-level audio preamplifiers and in the output stages of audio power amplifiers.

Origins and applications

The diamond follower was a natural development of the complementary emitter follower with diode biasing. In 1971 Harris Corporation used it in the output stage of the HA-2600 monolithic operational amplifier. American press of the 1970s analyzed the HA-2600 in detail, but did not give its output stage a specific name The circuit remained uncommon, because early fabrication processes could not produce high-quality pnp transistors. The first dedicated diamond buffer circuit, the 30-MHz LH0002, was introduced by National Semiconductor in the late 1970s, and then described merely as "wide band, high current, unity gain buffer amplifier".