The AD743/745 Low Noise JFET IC Op Amps


Prior to about 1990, input voltage noise performance in JFET IC op amps had never been competitive with the best bipolar devices, many of which achieved noise densities of 3nV/√Hz (see earlier OP27 discussions).

In 1990, ADI introduced an answer to applications such as hydrophone amplifiers, which require simultaneously low voltage and current noise from an amplifier. The new amplifier was the AD743 and AD745, designed by Scott Wurcer (Reference 55: “FET-Input Op Amp Has Lowest Combined V and I Noise," Analog Dialogue, Vol. 25, No. 1, 1991, pp. 12 (The AD743 and AD745 low noise IC op amps)). The design of these amplifiers attacked the voltage noise issue by the use of a quad array of very large input transistors, as described in Reference 56(Reference 56: Scott Wurcer, "A 3nV/√Hz, DC-Precise, JFET Operational Amplifier," Proceedings of BCTM, 1989, pp. 116-119. (Design principles of the AD743 and AD745 low noise IC op amps))(Of course, "very large" is a relative description. Nevertheless, Fig. 6 of Reference 56 shows the four input stage transistors consuming about ½ the chip area).

The result was an input referred noise of 2.9nV/√Hz (at 10kHz) for the two devices, and precision DC amplifier performance specifications. The basic AD743 is a unity-gain stable part, while the faster AD745 is stable at noise gains of 5 or more.

The AD820/822/824 and AD823 Series JFET IC Op Amps

In the early 1990's, the first of a series of JFET op amps on the ADI CB process began to appear. This process featured comparable speed and gain NPN and PNP bipolars, designed by Jody Lapham and Brad Scharf (References 57 and 58: “Op Amps Combine Superb DC Precision and Fast Settling," Analog Dialogue, Vol. 22, No. 2, 1988 (The AD846 IC op amp, the AD840 series, and the high speed CB process used) and Jerome F. Lapham, Brad W. Scharf, "Integrated Circuit with Complementary Junction-Isolated Transistors and Method of Making Same," US Patent 4.969,823, filed May 5, 1988, issued Nov. 13, 1990. (Design of the ADI CB IC process)). It also had an Nchannel FET structure, and a neat feature of this FET was that the pinchoff voltage allowed it to be used as a differential pair at the op amp front end, and the two gates could operate linearly to the negative rail. Thus with a common-emitter complementary bipolar output, a rail-to-rail output stage could be built. The combination of these two key features created a single (or dual) supply op amp with a low-current JFET input stage.

The first op amp of this type to appear was the AD820, a single low-power op amp, released in 1993 (Reference 59: “Single-Supply FET," Analog Dialogue, Vol. 27, No. 2, 1993 pp. 26 (The AD820 single-supply railrail output IC op amp)). The AD820 was designed by JoAnn Close and Francisco dos Santos. The device architecture was very flexible, and it could be operated from single supplies as low as 3V, or from dual supplies of up to ±18V. The input bias current was 10pA(max) for the AD820B, and the quiescent current was 800μA (typical).

With the success of the AD820, a dual version was the obvious next step, and the AD822 appeared in 1994, with specs similar to the AD820 (Reference 60: “Dual FET, 3 to ±18V," Analog Dialogue, Vol. 28, No. 1, 1994, pp. 24 (The AD822 single-supply rail-rail output dual IC op amp)). Rounding out this family next was the AD824, which appeared in 1995 (Reference 61: “Quad JFET, Single-Supply Op Amp," Analog Dialogue, Vol. 29, No. 1 (The AD824 single-supply, rail-rail output quad IC op amp)).

The AD820/822/824 were relatively low power parts, with moderate speed. In 1995 a higher speed dual using the same general topology appeared, the AD823 (Reference 62: “Dual 16MHz Rail-Rail FET,” Analog Dialogue, Vol. 29, No. 2 (The AD823 single-supply, rail-rail output dual IC op amp)). Designed by Jeff Townsend, this amplifier had a 16MHz bandwidth, and a 22V/μs slew rate. It also operated from a wide supply range, ±1.5 to ±18V dual supplies, or single supplies of +3 to +36V.

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