Analog Devices Inc./Maxim Integrated

The OP275 is the first amplifier to feature the Butler Amplifier front-end. This new front-end design combines both bipolar and JFET transistors to attain amplifiers with the accuracy and low noise performance of bipolar transistors, and the speed and sound quality of JFET's. Total Harmonic Distortion plus Noise equals that of previous audio amplifiers, but at much lower supply currents.A very low l/f corner of below 6 Hz maintains a flat noise density response. Whether noise is measured at either 30 Hz or 1kHz, it is only 6 nV/(root)Hz. The JFET portion of the input stage gives the OP275 its high slew rates to keep distortion low, even when large output swings are required, and the 22 V/µs slew rate of the OP275 is the fastest of any standard audio amplifier. Best of all, this low noise and high speed are accomplished using less than 5mA of supply current, lower than any standard audio amplifier.Improved dc performance is also provided with bias and offset currents greatly reduced over purely bipolar designs. Input offset voltage is guaranteed at 1mV and is typically less than 200µV. This allows the OP275 to be used in many dc coupled or summing applications without the need for special selections or the added noise of additional offset adjustment circuitry.The output os capable of driving 600 Ohm loads to 10 V rms while maintaining low distortion. THD + Noise at 3 V rms is a low 0.0006%.The OP275 is specified over the extended industrial (-40°C to +85°C) temperature range. OP275s are availale in both plastic DIP and SOIC-8 packages. SOIC-8 packages are available in 2500 piece reels. Many audio amplifiers are not offered in SOIC-8 surface mount packages for a variety of reasons; however, the OP275 was designed so that it would offer full performance in surface mount packaging.

The OP279 is a dual rail-to-rail, high output current, single supply amplifier. It is designed for low voltage applications that require either current or capacitive load drive capability. The OP279 can sink and source currents of ±80 mA (typ) and is stable with capacitive loads to 10 nF.Applications with the benefit from the OP279's high output current include driving headphones, displays, transformers, and power transistors. The powerful output is combined with a unique input stage that maintains very low distortion with wide common-mode range, even in single supply designs.The OP279 can be used as a buffer to provide much greater drive capability than can usually be provided by CMOS outputs. CMOS ASICs and DACs often have outputs that can swing to both the positive supply and ground, but are incapable of driving greater than a few milliamps.Bandwith is typically 5 MHz and the slew rate is 3 V/µs, making these amplifiers well suited for single supply applications that require audio bandwidths when used in high gain configurations. Operation is guaranteed from voltages as low as 4.5 V, up to 12 V.When using the OP279 in +5 volt systems, very good audio performance can be attained. THD is below 0.01% with a 600 Ohm load, and noise is a respectable 21 nV/Hz. Supply current is less than 3.5 mA per amplifier.The OP279 is available in 8-pin plastic DIP and SO-8 surface mount packages. They are specified over the industrial (-40°C to +80°C) temperature range.